Alcohols Phenols and Ethers

Multiple Choice Questions (MCQs)

1. Monochlorination of toluene in sunlight followed by hydrolysis with aq. $NaOH$ yields

(a) o-cresol

(b) $m$-cresol

(c) 2, 4- dihydroxytoluene

(d) benzyl alcohol

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Answer

(d) Monochlorination of toluene in sunlight gives benzyl chloride. On hydrolysis with aq. $NaOH$, benzyl chloride shows nucleophilic substitution reaction to give benzyl alcohol.

So, the correct option is (d).

  • (a) o-cresol: Monochlorination of toluene in sunlight does not lead to the formation of o-cresol. o-Cresol is an ortho-substituted hydroxytoluene, which would require a hydroxyl group to be directly attached to the benzene ring in the ortho position relative to the methyl group. This is not achieved through monochlorination followed by hydrolysis.

  • (b) m-cresol: Similar to o-cresol, m-cresol is a meta-substituted hydroxytoluene. The monochlorination of toluene followed by hydrolysis does not introduce a hydroxyl group directly onto the benzene ring in the meta position relative to the methyl group.

  • (c) 2, 4-dihydroxytoluene: This compound has two hydroxyl groups attached to the benzene ring in the 2 and 4 positions relative to the methyl group. The monochlorination of toluene followed by hydrolysis does not introduce multiple hydroxyl groups onto the benzene ring.

2. How many alcohols with molecular formula $C_4 H_{10} O$ are chiral in nature?

(a) 1

(b) 2

(c) 3

(d) 4

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Thinking Process

To solve this question, it should keep in mind that, if all four groups or atoms attached to the carbon atom are different, such a carbon is called asymmetric carbon or chiral carbon or stereocentre.

Answer

(a) The three isomers of butanol are possible. Structural formula of these isomers are given below

(i) $\underset{\text { Butan-1-ol }}{CH_3 CH_2 -CH_2-CH_2 OH}$

No carbon is chiral in this compound as none of the four carbon is bonded to four different substituents. (ii)

alt text

In this compound, asterisk marked carbon is chiral carbon as all four substituents, attached to it are different.

(iii)

alt text

Here, again carbon is not chiral in nature.

So, only one alcohol is chiral in nature and the correct option is (a).

  • Option (b) is incorrect because there is only one chiral alcohol with the molecular formula $C_4 H_{10} O$, not two. The other isomers do not have a carbon atom bonded to four different substituents.

  • Option (c) is incorrect because there are not three chiral alcohols with the molecular formula $C_4 H_{10} O$. Only one of the isomers has a chiral center.

  • Option (d) is incorrect because there are not four chiral alcohols with the molecular formula $C_4 H_{10} O$. Only one isomer has a chiral center.

3. What is the correct order of reactivity of alcohols in the following reaction?

$$ R-OH+HCl \xrightarrow{ZnCl_2} R-Cl+H_2 O $$

(a) $1^{\circ}>2^{\circ}>3^{\circ}$

(b) $1^{\circ}<2^{\circ}>3^{\circ}$

(c) $3^{\circ}>2^{\circ}>1^{\circ}$

(d) $3^{\circ}>1^{\circ}>2^{\circ}$

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Answer

(c) The given reaction is nucleophilic substitution reaction in which $-OH$ group is replaced by $-Cl$. Tertiary alcohols, when react with $HCl$ in presence of $ZnCl_2$, form tertiary carbocation.

This intermediate $3^{\circ}$ carbocation is more stable than $2^{\circ}$ carbocation as well as $1^{\circ}$ carbocation. Higher the stability of intermediate, higher will be the reactivity of reactant molecule.

So, the order of reactivity of alcohols in the given reaction is $3^{\circ}>2^{\circ}>1^{\circ}$ and correct option is (c).

  • Option (a) $1^{\circ}>2^{\circ}>3^{\circ}$: This option is incorrect because it suggests that primary alcohols are more reactive than secondary and tertiary alcohols. However, in the presence of $ZnCl_2$, tertiary alcohols form more stable tertiary carbocations, which makes them more reactive than secondary and primary alcohols.

  • Option (b) $1^{\circ}<2^{\circ}>3^{\circ}$: This option is incorrect because it implies that secondary alcohols are more reactive than tertiary alcohols. In reality, tertiary alcohols form more stable carbocations compared to secondary alcohols, making tertiary alcohols more reactive in this reaction.

  • Option (d) $3^{\circ}>1^{\circ}>2^{\circ}$: This option is incorrect because it suggests that primary alcohols are more reactive than secondary alcohols. However, secondary alcohols form more stable carbocations than primary alcohols, making secondary alcohols more reactive than primary alcohols in this reaction.

Q.4 $ CH_3 CH_2 OH$ can be converted into $CH_3 CH$ by…… .

(a) catalytic hydrogenation

(b) treatment with $LiAlH_4$

(c) treatment with pyridinium chlorochromate

(d) treatment with $KMnO_4$

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Answer

(c) Ethanal $(CH_3 CHO)$ is an oxidised product of ethanol. Pyridinium chlorochromate $(C _5 H _5 \stackrel{+}{N} HCl \overline CrO _3)$ oxidises primary alcohols to aldehydes. Strong oxidising agents such as $KMnO_4$ are used for getting carboxylic acid from alcohols.

The oxidation process can be stopped at the aldehyde stage if $Cr(VI)$ reagents such as Collin’s reagent $(CrO_3 \cdot 2 C_5 H_5 N)$, Corey’s reagent or pyridinium chlorochromate and pyridinium dichromate $[(C_5 H_5 NH_2)^{2+} Cr_2 O_7^{2-}]$ in anhydrous medium are used as the oxidising agent. So, the correct option is (c).

$\underset{\text{Ethanol}}{\mathrm{CH_3CH_2OH}} \xrightarrow{\mathrm{PCC}} \underset{\text{Ethanal}}{\mathrm{CH_3CHO}}$

  • (a) Catalytic hydrogenation: This process involves the addition of hydrogen to a molecule, typically reducing double or triple bonds. It does not oxidize ethanol to ethanal; instead, it would reduce any present double or triple bonds, which is not applicable in this context.

  • (b) Treatment with $LiAlH_4$: Lithium aluminium hydride is a strong reducing agent, which is used to reduce aldehydes, ketones, and carboxylic acids to alcohols. It does not oxidize ethanol to ethanal; rather, it would reduce any oxidized form back to ethanol or even further to ethane.

  • (d) Treatment with $KMnO_4$: Potassium permanganate is a strong oxidizing agent that typically oxidizes primary alcohols all the way to carboxylic acids. It would not stop at the aldehyde stage, thus converting ethanol to acetic acid rather than ethanal.

5. The process of converting alkyl halides into alcohols involves…… .

(a) addition reaction

(b) substitution reaction

(c) dehydrohalogenation reaction

(d) rearrangement reaction

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Answer

(b) An addition reaction is a reaction where two or more molecules combine to form a larger one. These reactions occur to change the unsaturated compound to saturated compound. In dehydrohalogenation reaction, alkyl halides give alkenes.

Rearrangement gives the structural isomers of the reactant while in substitution reaction one of the group or an atom is replaced by other group. Therefore, the process of converting alkyl halides into alcohols involves substitution reaction.

Reaction of alkyl halides

$$ \underset{\text { Alkyl halide }}{R-X} \xrightarrow{OH^{-}} \underset{\text { Alcohol }}{R-OH} $$

Reaction of alkyl halides

Primary alkyl halides $-S_N^{2}$

Secondary alkyl halides- $S_{N}{ }^{1}$

Tertiary alkyl halides $-S_{N}{ }^{1}$

  • (a) Addition reaction: An addition reaction involves the combination of two or more molecules to form a larger molecule, typically converting unsaturated compounds to saturated compounds. This is not applicable to the conversion of alkyl halides to alcohols, which involves replacing a halogen atom with a hydroxyl group.

  • (c) Dehydrohalogenation reaction: Dehydrohalogenation is a reaction where an alkyl halide loses a hydrogen atom and a halogen atom, resulting in the formation of an alkene. This process does not produce alcohols but rather alkenes.

  • (d) Rearrangement reaction: A rearrangement reaction involves the reorganization of the molecular structure to form isomers of the original compound. This does not apply to the conversion of alkyl halides to alcohols, which involves the substitution of a halogen atom with a hydroxyl group rather than a structural rearrangement.

6. Which of the following compounds is aromatic alcohol?

(a) $A, B, C, D$

(b) $A, D$

(c) $B, C$

(d) $A$

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Answer

(c) Phenol is also known as, carbolic acid’ cannot be considered as aromatic alcohol. It is quite separate branch of compound called phenols. So, compound (A) i.e., phenol and compound $(D)$ i.e., a derivative of phenol cannot be considered as aromatic alcohol.

On the other hand, compound $(B)$ and $(C),-OH$ group is bonded to $s p^{3}$ hybridised carbon which inturn is bonded to benzene ring. Hence, the correct option is (c).

  • Option (a) is incorrect because it includes compounds (A) and (D), which are phenols and not aromatic alcohols. Phenols have the -OH group directly attached to the benzene ring, making them a separate class of compounds.

  • Option (b) is incorrect because it includes compound (A), which is phenol and not an aromatic alcohol. Phenol has the -OH group directly attached to the benzene ring, making it a phenol rather than an aromatic alcohol.

  • Option (d) is incorrect because it includes only compound (A), which is phenol and not an aromatic alcohol. Phenol has the -OH group directly attached to the benzene ring, making it a phenol rather than an aromatic alcohol.

7. Give IUPAC name of the compound given below.

alt text

(a) 2-chloro-5-hydroxyhexane

(b) 2-hydroxy-5-chlorohexane

(c) 5-chlorohexan-2-ol

(d) 2-chlorohexan-5-ol

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Answer

(c)

alt text

The correct IUPAC name of the compound is 5-chlorohexan-2-ol. Hence, option (c) is the correct answer.

  • Option (a) 2-chloro-5-hydroxyhexane: This name is incorrect because the hydroxyl group (-OH) should be given priority over the chloro group (Cl) in the numbering of the carbon chain. The hydroxyl group should be assigned the lowest possible number, which is not the case here.

  • Option (b) 2-hydroxy-5-chlorohexane: This name is incorrect for the same reason as option (a). The hydroxyl group should be given priority and assigned the lowest possible number, which is not reflected in this name.

  • Option (d) 2-chlorohexan-5-ol: This name is incorrect because the numbering of the carbon chain should prioritize the hydroxyl group, resulting in the hydroxyl group being at position 2 and the chloro group at position 5, not the other way around.

8. IUPAC name of $m$ - cresol is…… .

(a) 3-methylphenol

(b) 3-chlorophenol

(c) 3-methoxyphenol

(d) benzene-1, 3-diol

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Answer

(a) The structure of $m$-cresol is

IUPAC name is 3-methylphenol because - $OH$ is the functional group and the methyl is substituent.

  • (b) 3-chlorophenol: This option is incorrect because it suggests the presence of a chlorine substituent at the 3-position on the benzene ring, whereas $m$-cresol has a methyl group at the 3-position, not a chlorine atom.

  • (c) 3-methoxyphenol: This option is incorrect because it suggests the presence of a methoxy group (-OCH₃) at the 3-position on the benzene ring, whereas $m$-cresol has a methyl group at the 3-position, not a methoxy group.

  • (d) benzene-1, 3-diol: This option is incorrect because it suggests the presence of two hydroxyl groups (-OH) at the 1 and 3 positions on the benzene ring, whereas $m$-cresol has only one hydroxyl group at the 1-position and a methyl group at the 3-position.

9. IUPAC name of the compound

alt text

(a) 1-methoxy-1-methylethane

(b) 2-methoxy-2-methylethane

(c) 2-methoxypropane

(d) isopropylmethyl ether

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Answer

(c)

IUPAC name of the above compound is 2-methoxypropane and correct option is (c).

  • Option (a) 1-methoxy-1-methylethane: This name suggests that the methoxy group (-OCH3) is attached to the first carbon of a two-carbon chain (ethane) with a methyl group also attached to the same carbon. However, the compound in question has a three-carbon chain (propane) with the methoxy group attached to the second carbon, not the first.

  • Option (b) 2-methoxy-2-methylethane: This name implies that the methoxy group and a methyl group are both attached to the second carbon of a two-carbon chain (ethane). This is incorrect because the compound has a three-carbon chain (propane) with the methoxy group attached to the second carbon, and there is no additional methyl group attached to the second carbon.

  • Option (d) isopropylmethyl ether: This name describes an ether with an isopropyl group (CH3-CH-CH3) and a methyl group (CH3) attached to the oxygen atom. However, the compound in question is an ether with a methoxy group (-OCH3) attached to the second carbon of a propane chain, not an isopropyl group.

10. Which of the following species can act as the strongest base?

(a) ${ }^{\ominus} OH$

(b) ${ }^{\ominus} OR$

(c) ${ }^{\ominus} OC_6 H_5$

alt text

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Thinking Process

To compare the species, the point to be noted that weak acid has strong conjugate base and vice-versa. Write the corresponding acid of the given base and choose the weakest acid among these.

Answer

(b) Weakest acid has the strongest conjugate base. $ROH$ is the acid of $RO^-$ conjugate base, HOH is the acid of $^-OH,C_6H_5OH$ is the acid of $C_6H_5O^-$ and is the acid of . Among all these acids, ROH is the weakest acid.

Therefore, the strongest base is $RO^{-}$and the correct option is (b).

  • Option (a) ${ }^{\ominus} OH$: The conjugate acid of ${ }^{\ominus} OH$ is water (HOH), which is a stronger acid compared to ROH. Therefore, ${ }^{\ominus} OH$ is a weaker base compared to ${ }^{\ominus} OR$.

  • Option (c) ${ }^{\ominus} OC_6 H_5$: The conjugate acid of ${ }^{\ominus} OC_6 H_5$ is phenol ($C_6H_5OH$), which is a stronger acid compared to ROH. Therefore, ${ }^{\ominus} OC_6 H_5$ is a weaker base compared to ${ }^{\ominus} OR$.

11. Which of the following compounds will react with sodium hydroxide solution in water?

(a) $C_6 H_5 OH$

(b) $C_6 H_5 CH_2 OH$

(c) $(CH_3)_3 COH$

(d) $C_2 H_5 OH$

Show Answer

Answer

(a) Phenol is more acidic in nature because by the loss of one proton, it gives phenoxide ion. This phenoxide ion is resonance stabilised. As phenoxide ion is a stable intermediate so, the tendency to give proton is more in phenol than the others. Phenols being more acidic than alcohols, dissolves in $NaOH$.

Therefore, option (a) is the correct.

  • (b) $C_6 H_5 CH_2 OH$: Benzyl alcohol ($C_6 H_5 CH_2 OH$) is not acidic enough to react with sodium hydroxide. It does not form a stable ion like phenoxide ion, and thus does not dissolve in $NaOH$.

  • (c) $(CH_3)_3 COH$: Tertiary butyl alcohol ($(CH_3)_3 COH$) is also not acidic enough to react with sodium hydroxide. It does not have the ability to form a stable ion upon losing a proton, and therefore does not dissolve in $NaOH$.

  • (d) $C_2 H_5 OH$: Ethanol ($C_2 H_5 OH$) is a typical alcohol and is not acidic enough to react with sodium hydroxide. It does not form a stable ion upon losing a proton, and thus does not dissolve in $NaOH$.

12. Phenol is less acidic than…… .

(a) ethanol

(b) 0 -nitrophenol

(c) $O$-methylphenol

(d) $O$-methoxyphenol

Show Answer

Thinking Process

This question is based on the acidic character of phenol. Electron withdrawing group at o - and p-position w.r. t. - OH group of phenol, increases the acidic strength.

Answer

(b) In o-nitrophenol, nitro group is present at ortho position. Presence of electron withdrawing group at ortho position increases the acidic strength. On the other hand, in o-methylphenol and in o-methoxyphenol, electron releasing group $(-CH_3,-OCH_3)$ are present.

Presence of these groups at ortho or para positions of phenol decreases the acidic strength of phenols. So, phenol is less acidic than o - nitrophenol.

  • Ethanol: Ethanol is less acidic than phenol because the hydroxyl group in ethanol is attached to an alkyl group, which is electron-donating. This decreases the ability of the oxygen to stabilize the negative charge on the conjugate base, making ethanol less acidic than phenol.

  • o-Methylphenol: In o-methylphenol, the methyl group is an electron-donating group. The presence of this group at the ortho position increases the electron density on the phenol ring, making it less able to stabilize the negative charge on the conjugate base, thus decreasing its acidity compared to phenol.

  • o-Methoxyphenol: In o-methoxyphenol, the methoxy group is also an electron-donating group. The presence of this group at the ortho position increases the electron density on the phenol ring, making it less able to stabilize the negative charge on the conjugate base, thus decreasing its acidity compared to phenol.

13. Which of the following is most acidic?

(a) Benzyl alcohol

(b) Cyclohexanol

(c) Phenol

(d) $m$ - chlorophenol

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Answer

(d) Alpha carbon of benzyl alcohol and cyclohexanol is $s p^{3}$ hybridised while in phenol and $m$-chlorophenol, it is $s p^{2}$ hybridised. In $m$-chlorophenol electron withdrawing group $(-Cl)$ is present at meta position.

Presence of electron withdrawing group increases the acidic strength. So, $m$-chlorophenol is most acidic among all the given compounds. The correct option is (d).

  • Benzyl alcohol: The alpha carbon of benzyl alcohol is $sp^3$ hybridized, which makes it less capable of stabilizing the negative charge on the conjugate base after deprotonation. Additionally, benzyl alcohol lacks any electron-withdrawing groups that could enhance its acidity.

  • Cyclohexanol: Similar to benzyl alcohol, the alpha carbon of cyclohexanol is $sp^3$ hybridized, which does not favor the stabilization of the conjugate base. Cyclohexanol also lacks any electron-withdrawing groups that could increase its acidic strength.

  • Phenol: Although phenol has an $sp^2$ hybridized carbon, which is better at stabilizing the negative charge on the conjugate base compared to $sp^3$ hybridized carbons, it does not have an additional electron-withdrawing group like $m$-chlorophenol. This makes phenol less acidic than $m$-chlorophenol.

14. Mark the correct order of decreasing acid strength of the following compounds.

alt text

(a) V $>$ IV $>$ II $>$ I $>$ III

(b) II $>$ IV $>$ I $>$ III $>$ V

(c) IV $>$ V $>$ III $>$ II $>$ I

(d) V $>$ IV $>$ III $>$ II $>$ I

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Answer

(b) Presence of electron withdrawing group on phenols, increases its acidic strength. So, both compounds i.e., $p$-nitrophenol (II) and $m$-nitrophenol (IV) are stronger acid than (I). If this $-NO_2$ group is present at $p$-position, then it exerts both $-I$ and $-R$ effect but if it is present at meta position, then it exerts only $-I$ effect. Therefore, $p$-nitrophenol is much stronger acid then $m$-nitrophenol.

On the other hand, presence of electron releasing group on phenol, decreases its acidic strength. If $-OCH_3$ group is present at meta position, it will not exert $+R$ effect but exert $-I$ effect.

But, if it is present at para position, then it will exert $+R$ effect. Therefore, $m$-methoxy phenol is more acidic than $p$-methoxy phenol.

Hence, the correct option is (b).

  • Option (a) V $>$ IV $>$ II $>$ I $>$ III:

    • This option incorrectly places $m$-methoxy phenol (V) as the strongest acid. The $-OCH_3$ group is an electron-donating group, which decreases the acidity of phenol. Therefore, $m$-methoxy phenol cannot be stronger than $p$-nitrophenol (II) and $m$-nitrophenol (IV), which have electron-withdrawing $-NO_2$ groups.
  • Option (c) IV $>$ V $>$ III $>$ II $>$ I:

    • This option incorrectly places $m$-methoxy phenol (V) as stronger than $p$-nitrophenol (II). The $-NO_2$ group in $p$-nitrophenol exerts both $-I$ and $-R$ effects, making it a stronger acid than $m$-methoxy phenol, which only has a $-I$ effect and no $+R$ effect.
  • Option (d) V $>$ IV $>$ III $>$ II $>$ I:

    • This option incorrectly places $m$-methoxy phenol (V) as the strongest acid. As mentioned, the $-OCH_3$ group is an electron-donating group, which decreases the acidity of phenol. Therefore, $m$-methoxy phenol cannot be stronger than $p$-nitrophenol (II) and $m$-nitrophenol (IV), which have electron-withdrawing $-NO_2$ groups.

15. Mark the correct increasing order of reactivity of the following compounds with $HBr / HCl$.

(a) I $<$ II $<$ III

(b) II $<$ I $<$ III

(c) II $<$ III $<$ I

(d) III $<$ II $<$ I

Show Answer

Thinking Process

This question is based on conceptual mixing of substitution reaction carbocation and reactivity. Nucleophilic substitution reactions depend upon the stability of carbocation. Higher the stability of carbocation (intermediate), higher will be the reactivity of reactant molecule.

Answer

(c) Reaction of the given compounds with $HBr / HCl$ is a nucleophilic substitution reaction. It follows $S_{N}{ }^{1}$ mechanism. $S_{N}{ }^{1}$ mechanism depends upon the stability of carbocation. Presence of electron withdrawing group decreases the stability of carbocation. In compound (II) and (III) EWG is present at para position.

Since, $-NO_2$ group is a stronger EWG than $-Cl$. So, $NO_2-C_6 H_5-\stackrel{+}{C} H_2$ carbocation will be less stable than $Cl-C_6 H_5-\stackrel{+}{C} H_2$ carbocation.

Thus, the order of stability of carbocation is

alt text

Therefore, compound (II) is least reactive and correct option is(III).

  • Option (a) I < II < III:

    • This option suggests that compound I is the least reactive and compound III is the most reactive. However, compound I has no electron-withdrawing group, making its carbocation the most stable and thus the most reactive. Therefore, this order is incorrect.
  • Option (b) II < I < III:

    • This option suggests that compound II is the least reactive and compound III is the most reactive, with compound I in between. However, compound I should be the most reactive due to the absence of electron-withdrawing groups, making its carbocation the most stable. Therefore, this order is incorrect.
  • Option (d) III < II < I:

    • This option suggests that compound III is the least reactive and compound I is the most reactive, with compound II in between. While it correctly places compound I as the most reactive, it incorrectly places compound III as the least reactive. Compound II should be the least reactive due to the stronger electron-withdrawing effect of the $-NO_2$ group compared to the $-Cl$ group in compound III. Therefore, this order is incorrect.

16. Arrange the following compounds in increasing order of boiling point. Propan-1-ol, butan-1-ol, butan-2-ol, pentan-1-ol

(a) Propan-1-ol, butan-2-ol, butan-1-ol, pentan-1-ol

(b) Propan-1-ol, butan-1-ol, butan-2-ol, pentan-1-ol

(c) Propan-1-ol, butan-2-ol, butan-1-ol, pentan-1-ol

(d) Propan-1-ol, butan-1-ol, butan-2-ol, pentan-1-ol

Show Answer

Thinking Process

This question is based on the boiling point of the compound. Boiling point increases with increase in the number of carbon atoms and decreases with increase of brancing in carbon chain.

Answer

(a) Boiling point increases with increase in the number of carbon atoms because of increase in molecular mass. So, the boiling point of pentan-1-ol is more than that of all other given compounds. Further, among isomeric alcohols, $1^{\circ}$ alcohols have higher boiling points than $2^{\circ}$ alcohols because of higher surface area in $1^{\circ}$ alcohols. Therefore, boiling point increase in the order

  • Option (b) is incorrect because it places butan-1-ol before butan-2-ol. However, butan-1-ol has a higher boiling point than butan-2-ol due to its primary alcohol structure, which has a higher surface area and stronger intermolecular hydrogen bonding compared to the secondary alcohol structure of butan-2-ol.

  • Option (c) is incorrect because it repeats the same order as option (a), which is actually the correct order. Therefore, it does not provide a different incorrect order to analyze.

  • Option (d) is incorrect because it places butan-2-ol after butan-1-ol. However, butan-2-ol should come before butan-1-ol in the order of increasing boiling points due to the reasons mentioned above regarding primary and secondary alcohols.

Multiple Choice Questions (More Than One Options)

17. Which of the following are used to convert $RCH 0$ into $RCH_2 OH$ ?

(a) $H_2 / Pd$

(b) $LiAlH_4$

(c) $NaBH_4$

(d) Reaction with $R M g X$ followed by hydrolysis

Show Answer

Answer

$(a, b, c)$

Conversion of aldehyde into alcohol is a reduction reaction. This reduction can be carried out by adding hydrogen in presence of finely divided metal catalyst such as platinum, palladium or nickel.

$$ RCHO+\xrightarrow{H_2 / Pd} R CH_2 OH $$

It can also be prepared by using $NaBH_4$ and $LiAlH_4$ as a reducing agent.

$$ \begin{aligned} & RCHO \xrightarrow{NaBH_4} RCH_2 OH \\ & RCHO \xrightarrow{LiAlH_4} RCH_2 OH \end{aligned} $$

Reaction of $R MgX$ with any aldehyde other than methanal gives secondary alcohols not the primary alcohols.

(Where, $R=-C_2 H_5, C_3 H_7$ etc.)

  • Option (d) Reaction with $R M g X$ followed by hydrolysis:
    • This option is incorrect because the reaction of $R MgX$ (Grignard reagent) with an aldehyde other than methanal (formaldehyde) results in the formation of secondary alcohols, not primary alcohols. The question specifically asks for the conversion of $RCHO$ (an aldehyde) into $RCH_2OH$ (a primary alcohol).

18. Which of the following reactions will yield phenol?

Show Answer

Answer

$(a, b, c)$

(a)

Haloarenes are less reactive towards nucleophilic substitution reaction. Therefore, it requires very high temperature and pressure. Chlorobenzene does not undergo hydrolysis on treatment with aq. $NaOH$ at $298 K$ and $1 atm$.

Therefore, correct options are (a), (b) and (c).

  • Option (d): The reaction conditions are not specified, and without high temperature and pressure, chlorobenzene will not undergo hydrolysis to yield phenol. Haloarenes are less reactive towards nucleophilic substitution reactions under standard conditions.

  • Option (e): The reaction involves a different mechanism or reagent that does not lead to the formation of phenol. The specific conditions or reagents required to convert the starting material to phenol are not met in this option.

  • Option (f): Similar to option (d), the reaction conditions are not conducive for the nucleophilic substitution required to form phenol. Haloarenes need specific conditions (high temperature and pressure) to undergo hydrolysis, which are not provided here.

19. Which of the following reagents can be used to oxidise primary alcohols to aldehydes?

(a) $CrO_3$ in anhydrous medium

(b) $KMnO_4$ in acidic medium

(c) Pyridinium chlorochromate

(d) Heat in the presence of $Cu$ at $573 K$

Show Answer

Answer

$(a, c, d)$

$CrO_3$ in anhydrous medium is used to oxidise primary alcohols to aldehydes.

$$ RCH_2 OH \xrightarrow{CrO_3 \text { (anhydrous) }} RCHO $$

Acidic $KMnO_4$ is a very strong oxidising agent. It oxidises primary alcohols into carboxylic acid.

$$ RCH_2 OH \xrightarrow{KMnO_4} RCOOH $$

Pyridinium chlorochromate $(C_6 H_5 \stackrel{+}{N} HClCr O_3^{-})$is a very good reagent for the oxidation of primary alcohols to aldehydes.

$$ RCH_2 OH \xrightarrow{PCC} RCHO $$

When the vapours of primary alcohols are passed over heated copper at $573 K$, dehydrogenation takes place and an aldehyde is formed.

$$ RCH_2 OH \xrightarrow[573 K]{Cu} RCHO $$

  • Acidic $KMnO_4$ is a very strong oxidising agent. It oxidises primary alcohols into carboxylic acid, not aldehydes.

$$ RCH_2 OH \xrightarrow{KMnO_4} RCOOH $$

20. Phenol can be distinguished from ethanol by the reactions with…… .

(a) $Br_2$ /water

(b) $Na$

(c) neutral $FeCl_3$

(d) All of these

Show Answer

Answer

(a, c)

Ethanol does not react with $Br_2$ /water while phenol gives 2, 4, 6-tribromophenol as white precipitate.

Ethanol and phenol both react with sodium, so phenol cannot be distinguished from ethanol by the reaction with sodium.

Ethanol does not give any reaction with neutral $FeCl_3$ solution while phenol gives purple, blue or red colour when treated with neutral $FeCl_3$.

Therefore, correct options are (a) and (c).

  • Option (b) $Na$: Ethanol and phenol both react with sodium, producing hydrogen gas and forming their respective sodium alkoxides. Therefore, phenol cannot be distinguished from ethanol by the reaction with sodium.

  • Option (d) All of these: This option is incorrect because ethanol reacts with sodium, so it cannot be used to distinguish phenol from ethanol. Only reactions with $Br_2$/water and neutral $FeCl_3$ can be used to distinguish phenol from ethanol.

21. Which of the following are benzylic alcohols?

alt text

Show Answer

Answer

$(b, c)$

In benzylic alcohols, the $-OH$ group is attached to a $s p^{3}$ hybridised carbon atom next to an aromatic ring. In compound (a) and (d), the $-OH$ group is attached to a $s p^{3}$ hybridised carbon atom but this carbon is not attached to the benzene ring.

On the other hand, in compound (a) and (c), the $-OH$ group is attached to a $s p^{3}$ hybridised carbon atom next to an aromatic ring.

  • In compound (a), the $-OH$ group is attached to a $s p^{3}$ hybridised carbon atom, but this carbon is not attached to the benzene ring.
  • In compound (d), the $-OH$ group is attached to a $s p^{3}$ hybridised carbon atom, but this carbon is not attached to the benzene ring.

Short Answer Type Questions

22. What is the structure and IUPAC name of glycerol?

Show Answer

Answer

Glycerol is the trihydric alcohol. Structure of glycerol is

So, IUPAC name is - propan- 1, 2, 3-triol.

23. Write the IUPAC name of the following compounds.

Show Answer

Answer

The correct IUPAC name of the following compounds are given below

(a)

IUPAC name $\rightarrow$ 3-ethyl-5-methylhexan-2, 4-diol. (b)

IUPAC name $\rightarrow$ 1-methoxy-3-nitrocyclohexane.

24. Write the IUPAC name of the compound given below.

alt text

Show Answer

Answer

The IUPAC name of the compound given below is

3-methylpent-2-ene-1, 2-diol.

25. Name the factors responsible for the solubility of alcohols in water.

Show Answer

Answer

Solubility of alcohols in water depends upon the two factors.

(i) Hydrogen bonding Higher the extent of hydrogen bonding, higher is the solubility. The hydrogen group of alcohol form hydrogen bonding. Solubility increases with increase in the number of $-OH$ groups in alcohols of comparable molecular masses.

alt text

(I) Compound (II) will form stronger $H$-bond due to two — $OH$ group and hence has higher boiling point.

(ii) Size of alkyl/aryl group Higher the size of alkyl/aryl group (hydrocarbon part), lower is the extent of hydrogen bonding and lower is the solubility. Hence,

$$ H_3 C-CH_2-OH>CH_3 CH_2-CH_2 OH>CH_3 CH_2 CH_2-CH_2 OH $$

26. What is denatured alcohol?

Show Answer

Answer

Alcohol is used in large quantities in the manufacture of alcoholic liquors. Its continuous use damages the various vital organs. Therefore, to refrain the people from drinking alcohol, heavy excise duty is levied on the sale of alcoholic beverages. But, it is used in various industries as it is a very good solvent.

Therefore, industrial alcohol must be cheap. Thus, to provide cheaper alcohol to industries and to refrain people from drinking alcohol, it is mixed with some copper sulphate, pyridine, methyl alcohol or acetone.

Alcohol is made unfit for drinking by mixing some quantity of any of these substances in it. This is called denatured alcohol.

27. Suggest a reagent for the following conversion.

alt text

Show Answer

Answer

The given reactant is $H_3 C-CH=CH-\stackrel{C}{C}-CH_3$. It is a secondary alcohol. Secondary alcohol $(>CHOH)$ gives ketone when oxidises by $CrO_3$ or pyridinium chlorochromate without carrying out oxidation at the double bond.

alt text

28. Out of 2-chloroethanol and ethanol which is more acidic and why?

Show Answer

Answer

2-chloroethanol is more acidic than ethanol. Due to $-I$ effect (electron withdrawing group) of the $Cl$-atom electron density in $O-H$ bond decreases. So, $O-H$ bond of 2-chloroethanol becomes weaker than $O-H$ bond of ethanol. Thus, 2-chloroethanol is more acidic than ethanol.

$$ Cl \leftarrow CH_2 \leftarrow CH_2 \leftarrow O \leftarrow H>CH_3 \rightarrow CH_2 \rightarrow O \rightarrow H $$

Stronger acid due to $-I$ effect of $Cl$.

29. Suggest a reagent for conversion of ethanol to ethanal.

Show Answer

Answer

Ethanol can be oxidises into ethanal by using pyridinium chlorochromate.

$(C_6 H_5 \stackrel{+}{N} HCl \overline{C} CrO_3)$ in $CH_2 Cl_2$.

$$ H_3 C-\underset{\text { Ethanol }}{CH_2}-OH \xrightarrow{PCC / CH_2 Cl_2} \underset{\text { Ethanal }}{CH_3}-CHO \text {. } $$

30. Suggest a reagent for conversion of ethanol to ethanoic acid.

Show Answer

Answer

Ethanol can be converted into ethanoic acid by using acidified $KMnO_4$ or $K_2 Cr_2 O_7$. Both $KMnO_4$ and $K_2 Cr_2 O_7$ are strong oxidising agents.

$ \underset{\text{Ethanol}}{\mathrm{CH_3CH_2OH}} \xrightarrow{\substack{\text{Acidified}\\ \mathrm{KMnO_4}}} \underset{\text{Ethanoic acid}}{\mathrm{CH_3COOH}} $

31. Out of o-nitrophenol and p-nitrophenol, which is more volatile? Explain.

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Answer

o-nitrophenol is more volatile than $p$-nitrophenol due to presence of intramolecular hydrogen bonding. In para nitrophenol intermolecular hydrogen bonding is present. This intermolecular hydrogen bonding causes the association of molecules.

32. Out of o-nitrophenol and o-cresol which is more acidic?

Show Answer

Answer

The acidic character of alcohols is due to the polar nature of $O-H$ bond. Higher the polarity of $O-H$ bond, more will be the acidic strength. Due to $-I$ and $-R$ effect of nitro group, electron density decreases in the $O-H$ bond of $o$-nitrophenol and thus polarity increases.

alt text

Whereas due to $+I$ effect of $-CH_3$ group, electron density increases in the $O-H$ bond of $o$-cresol. Thus, $O-H$ bond of $o$-nitrophenol is weaker than $O-H$ bond of $o$-cresol and o-nitrophenol is more acidic than o-cresol.

33. When phenol is treated with bromine water, white precipitate is obtained. Give the structure and the name of the compound formed.

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Answer

When phenol is treated with bromine water, white ppt. of 2, 4,6-tribromophenol is obtained.

34. Arrange the following compounds in increasing order of acidity and give a suitable explanation.

Phenol, o-nitrophenol, o-cresol

Show Answer

Answer

Nitro group shows $-I$ and $-R$ effect as follows

Due to this $-I$ and $-R$ effect of o-nitrophenol, it is a stronger acid than phenol. On the other hand, $-CH_3$ group produces $+I$ effect $-I$ and $-R$ effect increases the acidic strength by increasing the polarity of $-OH$ bond while $+I$ effect decreases the polarity due to increase in electron density on $-OH$ bond. So, o-cresol is a weaker acid than phenol. Thus, the correct order is o-cresol $<$ phenol $<0$-nitrophenol.

35. Alcohols react with active metals e.g., $Na, K$ etc., to give corresponding alkoxides. Write down the decreasing order of reactivity of sodium metal towards primary, secondary and tertiary alcohols.

Show Answer

Thinking Process

This question is based on the concept of the acidity and reactivity. The acidic character of alcohols is due to the polar nature of the $O-H$ bond.

Answer

An electron releasing group produces $+I$ effect so it increases the electron density on oxygen and decreases the polarity of $O-H$ bond. As the number of alkyl group increases from $1^{\circ}$ to $3^{\circ}$ alcohols, the electron density on the $O-H$ bond increases.

It will finally decrease the polarity and increase the strength of $O-H$ bond in going from $1^{\circ}$ to $3^{\circ}$ alcohols. Thus, acidic strength decreases in the following order

As we know that, sodium metal is basic in nature and alcohols are acidic in nature. Thus, reactivity of alcohol with sodium metal decreases with decrease in acidic strength. Therefore, the correct order is $1^{\circ}>2^{\circ}>3^{\circ}$.

36. What happens when benzene diazonium chloride is heated with water?

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Answer

When benzene diazonium chloride is heated with water then phenol is formed.

37. Arrange the following compounds in decreasing order of acidity.

$$ H_2 O, ROH, HC \equiv CH $$

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Answer

A stronger acid displaces a weaker acid from its salt. When $RONa$ is treated with $H_2 O$, it forms $R O H$. So, water is a stronger acid than $R O H$.

$$ \underset{\text { Stronger acid }}{HOH}+RONa \longrightarrow NaOH+\underset{\text { Weaker acid }}{ROH} $$

Similarly, when sodium ethynide is treated with water and alcohol, then acetylene is obtained.

$$ \begin{aligned} & \underset{\substack{\text { Stronger } \\ \text { acid }}}{HOH}+CH \equiv CNa \longrightarrow \underset{\text { Weak acid }}{HC} \equiv CH+NaOH \\ & \underset{\substack{\text { Stronger } \\ \text { acid }}}{ROH}+HC \equiv CNa \longrightarrow \underset{\text { Weak acid }}{HC} \equiv CH+RONa \end{aligned} $$

Thus, water and alcohol are stronger acid than ethyne and the decreasing order of acidity of given compounds are

$$ HOH>ROH>HC \equiv CH $$

38. Name the enzymes and write the reactions involved in the preparation of ethanol from sucrose by fermentation.

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Answer

Sucrose is converted to glucose and fructose in the presence of an enzyme, invertase or sucrase. Glucose and fructose undergo fermentation in the presence of another enzyme, zymase. Both these enzymes are present in yeast.

$$ \begin{aligned} & \underset{\text { Sucrose }}{\mathrm{C}_{12} \mathrm{H} _{22} \mathrm{O} _{11}}+\mathrm{H} _{2} \mathrm{O} \xrightarrow[\text { Invertase/sucrose }]{\mathrm{C} _{6} \mathrm{H} _{12} \mathrm{O} _{6}}+\underset{\text { Glucose }}{\mathrm{C} _{6} \mathrm{H} _{12} \mathrm{O} _{6}} \\ & \underset{\text { Glucose/Fructose }}{\mathrm{C} _{6} \mathrm{H} _{12} \mathrm{O} _{6}} \xrightarrow[\substack{\text { Ethanol }}]{\text { Zymase }} \underset{2}{2 \mathrm{CH} _{3} \mathrm{CH} _{2} \mathrm{OH}}+2 \mathrm{CO} _{2} \end{aligned} $$

39. How can propan-2-one be converted into tert-butyl alcohol?

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Answer

Propan-2-one is a ketone. Its structural formula is $\mathrm{H}_{3} \mathrm{C}>\mathrm{C}=\mathrm{O}$ ketones when treated with Grignard reagent give tertiary alcohols.

40. Write the structures of the isomers of alcohols with molecular formula $\mathrm{C} _{4} \mathrm{H} _{10} \mathrm{O}$. Which of these exhibits optical activity?

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Answer

Some compounds can rotate the plane polarised light, when it is passed through their solution. Such compounds are called optically active compounds. The structures of the isomers of alcohols with molecular formula $\mathrm{C} _{4} \mathrm{H} _{10} \mathrm{O}$ are as follows

alt text

The asymmetry of the molecule is responsible for the optical activity in a molecule. If all the four substituents attached to the carbon are different then the carbon is called asymmetric or chiral carbon and such a molecule is called asymmetric molecule.

In the above explained structure, it is only butan-2-ol which contains a chiral carbon and hence it is optically active.

41. Explain why is $\mathrm{OH}$ group in phenols more strongly held as compared to $\mathrm{OH}$ group in alcohols?

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Answer

In phenols, the electron pairs on oxygen atom of $-\mathrm{OH}$ group are in conjugation (or resonance) with $\pi$ electrons of the ring and the following resonating structures are possible

Out of these five resonating structures, II, III and IV structures contain a carbon-oxygen double bond character. In other words, carbon-oxygen bond in phenol acquires a partial double bond character due to resonance.

But in alcohols carbon-oxygen bond in alcohols is purely single bond. Therefore, $-\mathrm{OH}$ group in phenols is more strongly held as compared to $-\mathrm{OH}$ group in alcohols.

42. Explain why nucleophilic substitution reactions are not very common in phenols?

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Answer

Resonance is an important factor in phenols. During resonance $-\mathrm{OH}$ group in phenol gives its electrons to the benzene ring. As a result of this, the electron density on benzene ring is very high. This increased electron density repels nucleophiles.

Therefore, nucleophiles cannot attack the benzene ring and phenols usually do not give nucleophilic substitution reaction.

43. Preparation of alcohols from alkenes involves the electrophilic attack on alkene carbon atom. Explain its mechanism.

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Answer

Preparation of alcohols from alkene by the hydration of alkenes in presence of sulphuric acid.

$$ \mathrm{CH} _{2}=\mathrm{CH} _{2}+\mathrm{H} _{2} \mathrm{O} \xrightarrow{\mathrm{H}^{+}} \mathrm{CH} _{3} \mathrm{CH} _{2} \mathrm{OH} $$

This addition reaction takes place in accordance with Markownikoff’s rule.

Mechanism The mechanism of the reaction involves the following three steps

Step 1 Protonation of alkene to form carbocation by electrophilic attack of $\mathrm{H} _{3}^{+} \mathrm{O}$

alt text

Step 2 Attack of water molecule to the secondary carbocation.

Step 3 Loss of the hydrogen from the protonated alcohol.

alt text

44. Explain why is $0=\mathrm{C}=0$ non-polar while $\mathrm{R}-0-\mathrm{R}$ is polar?

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Answer

$\mathrm{CO} _{2}$ is a linear molecule. The dipole moment of two $\mathrm{C}=\mathrm{O}$ bonds are equal and opposite and they cancel each other and hence the dipole moment of $\mathrm{CO} _{2}$ is zero and it is a non-polar molecule.

$$ \begin{gathered} \mathrm{O} \xlongequal{\leftarrow+} \mathrm{C} \xlongequal{+\rightarrow} \mathrm{O} \\ \mu=0 \end{gathered} $$

While for ethers, two dipoles are pointing in the same direction. These two dipoles do not cancel the effect of each other. Therefore, there is a finite resultant dipoles and hence $R-\mathrm{O}-\mathrm{R}$ is a polar molecule.

alt text

45. Why is the reactivity of all the three classes of alcohols with conc. $\mathrm{HCl}$ and $\mathrm{ZnCl} _{2}$ (Lucas reagent) different?

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Answer

The reaction of alcohols with Lucas reagent (conc. $\mathrm{HCl}$ and $\mathrm{ZnCl} _{2}$ ) follow $\mathrm{S} _{\mathrm{N}}{ }^{1}$ mechanism. $\mathrm{S} _{\mathrm{N}}{ }^{1}$ mechanism depends upon the stability of carbocations (intermediate). More stable the intermediate carbocation, more reactive is the alcohol.

Tertiary carbocations are most stable among the three classes of carbocations and the order of the stability of carbocation is $3^{\circ}>2^{\circ}>1^{\circ}$. This order, inturn, reflects the order of reactivity of three classes of alcohols i.e., $3^{\circ}>2^{\circ}>1^{\circ}$.

Thus, as the stability of carbocations are different so the reactivity of all the three classes of alcohols with Lucas reagent is different.

46. Write steps to carry out the conversion of phenol to aspirin.

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Answer

Aspirin can be prepared by the reaction of salicylic acid with acetic anhydride. Salicylic acid is prepared by the reaction of phenol with $\mathrm{CO} _{2}$ and $\mathrm{NaOH}$.

This process is known as Kolbe’s reaction. The product salicylic acid is used in the preparation of aspirin. After wards, when salicylic acid is treated with acetic anhydride then acetyl group replaces the hydrogen of $-\mathrm{OH}$ group i.e., acetylation occurs at $-\mathrm{OH}$ group of salicylic acid. Reaction is as follows

47. Nitration is an example of aromatic electrophilic substitution and its rate depends upon the group already present in the benzene ring. Out of benzene and phenol, which one is more easily nitrated and why?

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Answer

Nitration of benzene and phenol is an electrophilic substitution reaction. During nitration $\stackrel{+}{\mathrm{N}} \mathrm{O} _{2}$ (nitronium ion) is produced as an intermediate as follows

This nitronium ion (electrophile) attacks on benzene or on phenol. Phenol is more easily nitrated than benzene as the presence of $-\mathrm{OH}$ group in phenol increases the electron density at ortho and para positions in benzene ring by $+R$ effect.

Since, the electron density is more in phenol than in benzene, therefore, phenol is more easily nitrated than benzene.

48. In Kolbe’s reaction, instead of phenol, phenoxide ion is treated with carbon dioxide. Why?

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Answer

In phenoxide ion, the ability to give lone pair of electrons to the benzene ring is more in comparison to phenols. Therefore, the reactivity of phenoxide ion towards electrophilic substitution reaction is more in comparison to phenols.

Thus, phenoxide ion being a stronger nucleophile reacts easily with $\mathrm{CO} _{2}$ (weak electrophile) than phenols in Kolbe’s reaction.

alt text

Note Kolbe’s process is also known as Kolbe - Schmitt reaction. This reaction is precursor to aspirin.

49. Dipole moment of phenol is smaller than that of methanol. Why?

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Answer

Dipole moment depends upon the polarity of bonds. Higher the polarity of bonds in molecule, higher will be its dipole moment. In phenol carbon is $s p^{2}$ hybridised and due to this reason benzene ring is producing electron withdrawing effect.

On the other hand, carbon of methanol is $s p^{3}$ hybridised and produces electron releasing effect (+ I effect). Thus, $\mathrm{C}-\mathrm{O}$ bond in phenol is less polar than $\mathrm{C}-\mathrm{O}$ bond in methanol and therefore, the dipole moment of phenol is smaller than that of methanol.

50. Ethers can be prepared by Williamson synthesis in which an alkyl halide is reacted with sodium alkoxide. Di-tert -butyl ether can’t be prepared by this method. Explain.

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Answer

In order to prepare di-tert-butyl ether, sodium tert-butoxide must be reacted with tert-butyl bromide. Alkoxides are not only nucleophiles but they are strong base as well. They react with $3^{\circ}$ alkyl halides leading to the elimination reaction.

When tert-butyl-bromide reacts with sodium tert-butoxide instead of substitution, elimination takes place. As a result of this elimination reaction, Iso butylene is formed instead of di-tert butyl ether.

51. Why is the $\mathrm{C}-\mathrm{O}-\mathrm{H}$ bond angle in alcohols slightly less than the tetrahedral angle whereas the $\mathrm{C}-0-\mathrm{C}$ bond angle in ether is slightly greater?

Show Answer

Thinking Process

To solve this question, it should keep in mind that the order of repulsion between electron pairs is $\rightarrow|p-| p>\mid p-b p>b p-b p$.

Answer

The bond angle in $\mathrm{C}-\underset{..}{\ddot{\mathrm{O}}}-\mathrm{H}$ in alcohols is slightly less than tetrahedral angle $\left(109^{\circ} 28^{\prime}\right)$. It is due to the repulsion between the unshared electron pairs of oxygen. In alcohols, two lone pair of electrons are present. Therefore, there is comparatively more repulsion and less bond angle.

The $\mathrm{C}-\mathrm{O}-\mathrm{C}$ bond angle in ether is slightly greater than the tetrahedral angle due to the repulsive interaction between the two bulky $(-R)$ groups.

52. Explain why low molecular mass alcohols are soluble in water?

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Answer

Solubility of alcohols and phenols in water is due to their ability to form hydrogen bonds with water molecule. The hydrocarbon part methoxy methane (i.e., $R$ group) tends to prevent the formation of hydrogen bonds.

Alcohols with lower molar mass will have smaller hydrocarbon part and therefore tendency to form hydrogen bonding is more and they are more soluble in water.

$\underset{\text{(More soluble)}}{CH_3 CH_2 Oh} > \underset{\text{(Less soluble)}}{CH_3 CH_2 CH_2 Oh} $

53. Explain why p-nitrophenol is more acidic than phenol?

Show Answer

Answer

Nitro group of phenol produces $-I$ and $-R$ effect. Because of these two effects $-\mathrm{NO} _{2}$ group is electron withdrawing in nature. So, the electron density in the $\mathrm{O}-\mathrm{H}$ bond of $p$-nitrophenol decreases relative to the $\mathrm{O}-\mathrm{H}$ bond of phenol.

The decrease in electron density of the $\mathrm{O}-\mathrm{H}$ bond of $p$-nitrophenol, the polarity of $\mathrm{O}-\mathrm{H}$ bond is decrease and in turn make it more acidic than phenol.

54. Explain why alcohols and ethers of comparable molecular mass have different boiling points?

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Answer

Boiling point depends upon the strength of intermolecular forces of attraction. Higher these forces of attraction, more will be the boiling point. Alcohols undergo intermolecular hydrogen bonding. So, the molecules of alcohols are held together by strong intermolecular forces of attraction.

But in ethers no hydrogen atom is bonded to oxygen. Therefore, ethers are held together by weak dipole-dipole forces, not by strong hydrogen bond.

Since, lesser amount of energy is required than to break weak dipole-dipole forces in ethers than to break strong hydrogen bonds in alcohol.

alt text

55. The carbon-oxygen bond in phenol is slightly stronger than that in methanol. Why?

Show Answer

Answer

Positive resonance effect is observed in phenols. Due to this $+R$ effect, lone pair of electrons of $-\mathrm{OH}$ group are in conjugation with $\pi$ electrons of the ring and the following resonance hybrid are obtained.

From the above resonating structure, it is very clear that $\mathrm{C}-\mathrm{O}$ bond of phenol acquires some partial double bond character while the $\mathrm{C}-\mathrm{O}$ bond of methanol is purely single bond. Therefore, the carbon-oxygen bond in phenol is slightly stronger than that in methanol.

56. Arrange water, ethanol and phenol in increasing order of acidity and give reason for your answer.

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Answer

The phenoxide ion obtained after the removal of a proton is resonance stabilised as follows

Whereas, the ethoxide ion obtained after the removal of a proton is not stabilised but destabilised due to $+I$ effect of $-\mathrm{C} _{2} \mathrm{H} _{5}$ group. Therefore, phenol is a stronger acid than ethanol. $+\mathrm{I}$ effect of $\mathrm{CH} _{3}-\mathrm{CH} _{2}$ group increase the electron density on $\mathrm{O}-\mathrm{H}$ bond in ethanol. As the electron density on the $\mathrm{O}-\mathrm{H}$ bond of ethanol is more than that of water. So, ethanol is a weaker acid than water. Thus, the increasing order of acidity is

ethanol $<$ water $<$ phenol

Matching The Columns

57. Match the structures of the compounds given in Column I with the name of the compounds given in Column II.

Show Answer

Answer

A. $\rightarrow(4)$

B. $\rightarrow$ (3)

C. $\rightarrow(6)$

D. $\rightarrow(1)$

E. $\rightarrow(7)$

F. $\rightarrow$ (2)

A. Cresols are organic compounds which are methyl phenols. There are three forms of cresol-o-cresol, $p$ - cresol and $m$ - cresol.

B. Catechol is also known as pyrocatechol. Its IUPAC name is 1,2-dihydrobenzene. It is used in the production of pesticides, perfumes and pharmaceuticals.

C. Its IUPAC name is 1, 3-dihydroxybenzene. Resorcinol is used to treat acne, seborrheic dermatitis and other skin disorder.

D. Hydroquinone is also known as quinol. Its IUPAC name is 1, 4- dihydroxybenzene. It is a white granular solid. It is a good reducing agent.

E. Anisole or methoxy benzene, is a colourless liquid with a smell reminiscent of anise seed.

F. Phenetole is an organic compound. It is also known as ethylphenyl ether. It is volatile in nature and its vapours are explosive in nature.

58. Match the starting material given in Column I with the products formed by these (Column II) in the reaction with $\mathrm{HI}$.

alt text

Show Answer

Answer

A. $\rightarrow(4)$

B. $\rightarrow(5)$

C. $\rightarrow(2)$

D. $\rightarrow(1)$

A. $\mathrm{CH} _{3}-\mathrm{O}-\mathrm{CH} _{3}$ is a symmetrical ether so the products are $\mathrm{CH} _{3}$ land $\mathrm{CH} _{3} \mathrm{OH}$.

B. In $\left(\mathrm{CH} _{3}\right) _{2} \mathrm{CH}-\mathrm{O}-\mathrm{CH} _{3}$ unsymmetrical ether, one alkyl group is primary while another is secondary. So, it follows $\mathrm{S} _{\mathrm{N}}{ }^{2}$ mechanism. Thus, the halide ion attacks the smaller alkyl group and the products are

$$ \begin{aligned} & \mathrm{H} _{3} \mathrm{C} \\ & \mathrm{H} _{3} \mathrm{C} \end{aligned}>\mathrm{CHOH}+\mathrm{CH} _{3} \mathrm{I} $$

C. In this case, one of the alkyl group is tertiary and the other is primary. It follows $\mathrm{S} _{\mathrm{N}}{ }^{1}$ mechanism and halide ion attacks the tertiary alkyl group and the products are $\left(\mathrm{CH} _{3}\right) _{3} \mathrm{C}-\mathrm{I}$ and $\mathrm{CH} _{3} \mathrm{OH}$.

D. Here, the unsymmetrical ether is alkyl aryl ether. In this ether $\mathrm{O}-\mathrm{CH} _{3}$ bond is weaker than $\mathrm{O}-\mathrm{C} _{6} \mathrm{H} _{5}$ bond which has partial double bond character due to resonance. So, the halide ion attacks on alkyl group and the products are $\mathrm{C} _{6} \mathrm{H} _{5}-\mathrm{OH}$ and $\mathrm{CH} _{3} \mathrm{I}$.

59. Match the items of Column I with items of Column II.

Column I Column II
A. Antifreeze used in car engine 1. Neutral ferric chloride
B. Solvent used in perfumes 2. Glycerol
C. Starting material for picric acid 3. Methanol
D. Wood spirit 4. Phenol
E. Reagent used for detection of phenolic group 5. Ethylene glycol
F. By product of soap industry used in cosmetics 6. Ethanol

Show Answer

Answer

A. $\rightarrow(5)$

B. $\rightarrow(6)$

C. $\rightarrow(4)$

D. $\rightarrow(3)$

E. $\rightarrow(1)$

F. $\rightarrow(2)$

A. IUPAC name of ethylene glycol is ethane $-1,2$ - diol. It is primarily used as raw material in the manufacturing of polyester fibers and fabric industry. A small percentage of it is used in antifreeze formulations.

B. Ethanol is a good solvent for fatty and waxy substances. Fats and waxes provide odour to the perfumes. Apart from being a good solvent, it is less irritating to the skin. So, it is used in perfumes.

C. Phenol is converted into picric acid (2, 4, 6-trinitro-phenol) by the reaction of phenol with conc. $\mathrm{HNO} _{3}$.

D. Methanol, $\mathrm{CH} _{3} \mathrm{OH}$ is also known as ‘wood spirit’ as it was produced by the destructive distillation of wood.

E. Neutral ferric chloride give purple/red colour when treated with phenols. It is the reagent used for detection of phenolic group.

F. Soaps are prepared by the reactions of fatty acid with $\mathrm{NaOH}$.

This glycerol (propan -1, 2, 3 - triol) is the by product of soap industry and used in cosmetics.

60. Match the items of Column I with items of Column II.

Column I Column II
A. Methanol 1. Conversion of phenol to
o-hydroxysalicylic acid
B. Kolbe’s reaction 2. Ethyl alcohol
C. Williamson’s synthesis 3. Conversion of phenol to
salicylaldehyde
D. Conversion of $2^{\circ}$ alcohol to
ketone
4. Wood spirit
E. Reimer-Tiemann reaction 5. Heated copper at $573 \mathrm{~K}$
F. Fermentation 6. Reaction of alkyl halide
with sodium alkoxide
Show Answer

Answer

A. $\rightarrow(4)$

B. $\rightarrow(1)$

C. $\rightarrow(6)$

D. $\rightarrow(5)$

E. $\rightarrow(3)$

F. $\rightarrow(2)$

A. Methanol is also known as ‘wood spirit’ as it was produced by the destructive distillation of wood.

B. In Kolbe’s reaction, 2 - hydroxy benzoic acid (salicylic acid) is prepared by the reaction of phenol with $\mathrm{CO} _{2}$ gas.

C. Williamson synthesis is an important method for the preparation of ethers. In this method, an alkyl halide is allowed to react with sodium alkoxide.

$$ R-\mathrm{X}+\mathrm{R}-\mathrm{ONa} \longrightarrow \mathrm{ROR}+\mathrm{NaX} $$

D. When a $2^{\circ}$ alcohol is allowed to pass over heated copper at $573 \mathrm{~K}$, dehydrogenation takes place and an ketone is formed.

$$ R-\underset{OH}{\underset{|}{CH}} -R^{\prime} \xrightarrow[{ 573 K }]{\mathrm{Cu}} R-\underset{O}{\underset{||}{C}}-\mathrm{R} $$

E. On treating phenol with chloroform in the presence of $\mathrm{NaOH}$, an aldehydic group is introduced at ortho position of benzene ring

F. Ethanol is prepared by the fermentation of sugars.

$$ \begin{array}{r} \mathrm{C} _{12} \mathrm{H} _{22} \mathrm{O} _{11}+\mathrm{H} _{2} \mathrm{O} \xrightarrow{\text { Invertase }} \mathrm{C} _{6} \mathrm{H} _{12} \mathrm{O} _{6}+\mathrm{C} _{6} \mathrm{H} _{12} \mathrm{O} _{6} \\ \mathrm{C} _{6} \mathrm{H} _{12} \mathrm{O} _{6} \xrightarrow{\text { Zymase }} 2 \mathrm{C} _{2} \mathrm{H} _{5} \mathrm{OH}+2 \mathrm{CO} _{2} \end{array} $$

Assertion and Reason

In the following questions a statement of Assertion (A) followed by a statement of Reason $(\mathrm{R})$ is given. Choose the correct answer out of the following choices.

(a) Assertion and reason both are correct and reason is correct explanation of assertion.

(b) Assertion and reason both are wrong statements.

(c) Assertion is correct statement but reason is wrong statement.

(d) Assertion is wrong statement but reason is correct statement.

(e) Both assertion and reason are correct statements but reason is not correct explanation of assertion.

61. Assertion (A) Addition reaction of water to but-1-ene in acidic medium yields butan-1-ol.

Reason ( $R$ ) Addition of water in acidic medium proceeds through the formation of primary carbocation.

Show Answer

Answer

(b) Assertion and reason both are wrong statements.

Correct Assertion Addition reaction of water to but-1-ene in acidic medium yields butan-2-ol.

Correct Reason Addition of water in acidic medium proceeds through the formation of secondary carbocation.

62. Assertion (A) p-nitrophenol is more acidic than phenol.

Reason (R) Nitro group helps in the stabilisation of the phenoxide ion by dispersal of negative charge due to resonance.

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Answer

(a) Assertion and reason both are correct and reason is correct explanation of assertion. $p$-nitrophenol is more acidic than phenol because nitro group helps in the stabilisation of the phenoxide ion by dispersal of negative charge due to resonance.

63. Assertion (A) IUPAC name of the compund

alt text

Reason (R) In IUPAC nomenclature, ether is regarded as hydrocarbon derivative in which a hydrogen atom is replaced by $-\mathrm{OR}$ or $-\mathrm{OAr}$ group [where, $\mathrm{R}=$ alkyl group and $\mathrm{Ar}=$ aryl group].

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Answer

(d) Assertion is wrong statement but reason is correct statement.

Correct Assertion The IUPAC name of the given compound is 1-(2-propoxy) propane.

64. Assertion (A) Bond angle in ethers is slightly less than the tetrahedral angle.

Reason (R) There is a repulsion between the two bulky $(-R)$ groups.

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Answer

(d) Assertion is wrong statement but reason is correct statement.

Correct Assertion Bond angle in ether is slightly more than the tetrahedral angle.

65. Assertion (A) Boiling points of alcohols and ethers are high.

Reason (R) They can form intermolecular hydrogen-bonding.

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Answer

(b) Assertion and reason both are wrong statements.

Correct Assertion Boiling points of alcohols are higher than that of ethers of comparable molecular mass.

Correct Reason Alcohols can form intermolecular hydrogen bonding while ethers cannot.

66. Assertion (A) Like bromination of benzene, bromination of phenol is also carried out in the presence of Lewis acid.

Reason (R) Lewis acid polarises the bromine molecule.

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Answer

(d) Assertion is wrong statement but reason is correct statement.

Correct Assertion Bromination of benzene but not of phenol is carried out in presence of a Lewis acid.

67. Assertion (A) o-nitrophenol is less soluble in water than the $m$ and p-isomers.

Reason ( $R$ ) $m$ and p-nitrophenols exist as associated molecules.

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Answer

(d) Both assertion and reason are correct statements but reason is not correct explanation of assertion.

Correct Explanation Due to the presence of intramolecular hydrogen bonding, o-nitrophenol does not form hydrogen bonds with $\mathrm{H} _{2} \mathrm{O}$ but $m$ and $p$-nitrophenol form hydrogen bonds with water.

68. Assertion (A) Ethanol is a weaker acid than phenol.

Reason (R) Sodium ethoxide may be prepared by the reaction of ethanol with aqueous $\mathrm{NaOH}$.

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Answer

(c) Assertion is correct statement but reason is wrong statement.

Correct Reason Phenoxide ion is stabilised by resonance but ethoxide ion is not stabilised by resonance.

Resonance in phenoxide ion

69. Assertion (A) Phenol forms 2, 4, 6-tribromophenol on treatment with $\mathrm{Br} _{2}$ in carbon disulphide at $273 \mathrm{~K}$.

Reason (R) Bromine polarises in carbon disulphide.

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Answer

(b) Assertion and reason both are wrong statements.

Correct Assertion Phenol form 2, 4, 6-tribromophenol on treatment with bromine in water.

Correct Reason In water, phenol gives phenoxide ion. This phenoxide ion activates the ring towards electrophilic substitution reaction.

70. Assertion $(A)$ Phenols give o-and $p$-nitrophenol on nitration with conc. $\mathrm{HNO} _{3}$ and $\mathrm{H} _{2} \mathrm{SO} _{4}$ mixture.

Reason $(\mathrm{R})-\mathrm{OH}$ group in phenol is $0-, \mathrm{p}$-directing.

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Answer

(d) Assertion is wrong statement but reason is correct statement.

Correct Assertion Phenols give $\mathrm{o}$ and $p$-nitrophenol on nitration with dil. $\mathrm{HNO} _{3}$ at $298 \mathrm{~K}$.

Long Answer Type Questions

71. Write the mechanism of the reaction of HI with methoxybenzene.

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Answer

In case of alkyl aryl ethers, the products are always phenol and an alkyl halide because due to resonance $\mathrm{C} _{6} \mathrm{H} _{5}-\mathrm{O}$ bond has partial double bond character. The mechanism is given below

Mechanism Protonation of anisole gives methylphenyl oxonium ion.

In this ion, the bond between $\mathrm{O}-\mathrm{CH} _{3}$ is weaker than the bond between $\mathrm{O}-\mathrm{C} _{6} \mathrm{H} _{5}$ which has partial double bond character. This partial double bond character is due to the resonance between the lone pair of electrons on the $\mathrm{O}$-atom and the $s p^{2}$ hybridised carbon atom of the phenyl group. Therefore, attack by $I^{-}$ion exclusively breaks the weaker $\mathrm{O}-\mathrm{CH} _{3}$ bond forming methyl iodide and phenol.

72. (a) Name the starting material used in the industrial preparation of phenol.

(b) Write complete reaction for the bromination of phenol in aqueous and non-aqueous medium.

(c) Explain why Lewis acid is not required in bromination of phenol?

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Answer

(a) The starting material used in the industrial preparation of phenol is cumene.

(b) Phenols when treated with bromine water gives polyhalogen derivatives in which all the hydrogen atoms present at ortho and para positions with respect to $-\mathrm{OH}$ group are replaced by bromine atoms.

However, in non-aqueous medium such as $\mathrm{CS} _{2}, \mathrm{CCl} _{4}, \mathrm{CHCl} _{3}$, monobromophenols are obtained.

In aqueous solution, phenol ionises to form phenoxide ion. This ion activates the benzene ring to a very large extent and hence the substitution of halogen takes place at all three positions.

On the other hand, in non-aqueous solution ionisation of phenol is greatly suppressed. Therefore, ring is activated slightly and hence monosubstitution occur.

(c) Lewis acid is an electron deficient molecule. In bromination of benzene, Lewis acid is used to polarise $\mathrm{Br} _{2}$ to form $\mathrm{Br}^{+}$electrophile.

In case of phenol, oxygen atom of phenol itself polarises the bromine molecule to form $\mathrm{Br}^{+}$ion (electrophile). So, Lewis acid is not required in the bromination of phenol.

Mechanism of bromination of phenol

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Mechanism of bromination of benzene

$$ \mathrm{AlBr} _{3}+\mathrm{Br}-\mathrm{Br} \longrightarrow\left[\mathrm{AlBr} _{4}\right]^{-}+\mathrm{Br}^{+} $$

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73. How can phenol be converted to aspirin?

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Answer

Phenol is converted into salicylic acid. This salicylic acid is treated with acetic anhydride. On acetylation of salicylic acid, aspirin (acetyl salicylic acid) is formed.

74. Explain a process in which a biocatalyst is used in industrial preparation of a compound known to you.

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Answer

Enzymes are biocatalyst. These biocatalysts (enzymes) are used in the industrial preparation of ethanol. Ethanol is prepared by the fermentation of molasses - a dark brown coloured syrup left after crystallisation of sugar which still contains about $40 %$ of sugar.

The process of fermentation actually involves breaking down of large molecules into simple ones in the presence of enzymes. The source of these enzymes is yeast. The various reactions taking place during fermentation of carbohydrates are

$$ \underset{\text { Sucrose }}{\mathrm{C} _{12} \mathrm{H} _{22} \mathrm{O} _{11}}+\mathrm{H} _2 \mathrm{O} \xrightarrow{\text { Invertase }} \underset{\text { Glucose }}{\mathrm{C} _6 \mathrm{H} _{12} \mathrm{O} _6}+\underset{\text { Fructose }}{\mathrm{C} _6 \mathrm{H} _{12} \mathrm{O} _6} $$

$$ \underset{\substack{\text { Glucose }}}{\mathrm{C} _6 \mathrm{H} _{12} \mathrm{O} _6} \xrightarrow{\text { Zymase }} \underset{\text { Ethyl alcohol }}{2 \mathrm{C} _2 \mathrm{H} _5 \mathrm{OH}}+2 \mathrm{CO} _2 $$

In wine making, grapes are the source of sugars and yeast. As grapes ripen, the quantity of sugar increases and yeast grows on the outer skin. When grapes are crushed, sugar and the enzyme come in contact and fermentation starts. Fermentation takes place in anaerobic conditions i.e., in absence of air. $\mathrm{CO} _{2}$ gas is released during fermentation.

The action of zymase is inhibited once the percentage of alcohol formed exceeds 14 per cent. If air gets into fermentation mixture, the oxygen of air oxidises ethanol to ethanoic acid which in turn destroys the taste of alcoholic drinks.



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