SATHEE: Unit 12 Organic Chemistry Some Basic Principal And Technique (Part-B)

Unit 12 Organic Chemistry Some Basic Principal And Technique (Part-B)

Tetravalency of carbon : shapes of simple molecules

Carbon forms covalent bonds with other carbon atoms and also with other elements like hydrogen, oxygen, halogens, nitrogen, sulphur etc. The formation of these covalent bonds and the tetravalence of carbon is because of its electronic configuration and the hybridization of $s$ and $p$ orbitals. The simplest compound of the alkane series is methane in which carbon is $\mathrm{sp}^{3}$ hybridised and forms bonds with four hydrogen atoms to form a tetrahedron where carbon is in the centre and the hydrogens at the four corners. Similarly the shapes of other molecules like ethane $\left(\mathrm{C} _{2} \mathrm{H} _{6}\right)$,ethene $\left(\mathrm{C} _{2} \mathrm{H} _{4}\right)$ and ethyne $\left(\mathrm{C} _{2} \mathrm{H} _{2}\right)$ can be explained by using $\mathrm{sp}^{3}, \mathrm{sp}^{2}$ and $\mathrm{sp}$ hybridization.

As we more from $\mathrm{sp}^{3}$ to $\mathrm{sp}^{2}$ to $\mathrm{sp}$ hybridization, the $\mathrm{s}$ character increases from $25 %$ to $33 %$ to $50 %$ respectively. Greater the s character, closer the electrons are to the nucleus, more effective the overlap with other orbitals and thus shorter and stronger bonds are formed. Thus bond length decreases and bond strength increases with increase in s character. In other words, electronegativity increases as s character increases. Thus a carbon having sp hybridization is more electronegative than a carbon in $s p^{2}$ hybridization which is more electronegative than a carbon having $\mathrm{sp}^{3}$ hybridization.

Classification

All the known organic compounds have been broadly divided into the following classes on the basis of their structure.

I. Acyclic or aliphatic compounds are the ones which consist of straight chain or branched chain compounds, for example,

II. Cyclic or ring compounds are the ones which contain atoms joined in the form of a ring. These rings may be (a) Homocyclic and (b) Heterocyclic.

(a) Homocyclic compounds are those in which only carbon atoms are joined to form a ring.

They are of two types:

Alicyclic compounds

The aliphatic cyclic compounds are called alicyclic compounds. For example.

Aromatic compounds

Completely conjugated cyclic planar polyenes which obey Huckel’s rule, i.e. contain $(4 n+2) \pi$ electrons, where $n=0,1,2,3, \ldots$. are called aromatic compounds. These are of two types:

(i) Benzenoids

Aromatic compounds and their alkyl, alkenyl and alkynyl derivatives which contain one or more benzene rings either fused or isolated in their molecules are called benzenoids. For example

These are also called arenes.

(ii) Non-benzenoids

Aromatic compounds which do not contain a benzene ring but instead contain other highly unsaturated rings are called non-benzenoids. For example,

(b) Heterocyclic compounds are those in which one or more heteroatom (e. g, 0, N, S) is also present in the ring. They are of two types:

Non-aromatic heterocyclic compounds

Heterocyclic compounds which resemble aliphatic compounds in their properties are called non-aromatic heterocyclic compounds. For example,

Aromatic heterocyclic compounds

Heterocyclic compounds which resemble benzene and other aromatic compounds in most of their properties are called aromatic heterocyclic compounds. For example,

Another classification of organic compounds is on the basis of the functional group that they may contain.

Functional Group

A functional group is an atom or a group of atoms present in a molecule which determines its chemical properties. For example,

$-\mathrm{OH}$ (hydroxyl), $-\mathrm{CHO}$ (aldehydic), $-\mathrm{COOH}$ (carboxylic), etc.

Homologous Series

Homologous series is a family of chemically similar organic compounds, all the members of which contain the same functional group and any two adjacent (consecutive) members of which differ by a $-\mathrm{CH} _{2}$ group.

The members of homologous series show gradation in physical properties and similarity in chemical properties.

Each homologous series can be represented by a general formula. For example, $\mathrm{ROH}$ (alcohols), $\mathrm{R}-\mathrm{X}$ (alkyl halides), $\mathrm{RCOOH}$ (carboxylic acids), etc. where $\mathrm{R}$ is any alkyl group.

The general formula, the functional group, the common name and IUPAC names of some important homologous series are given below :

Chain length	Word root	Chain length	Word root	Chain length	Word root
$\mathrm{C} _{1}$	Math	$\mathrm{C} _{5}$	Peot (a)	$\mathrm{C} _{9}$	Non
$\mathrm{C} _{2}$	Eth	$\mathrm{C} _{6}$	$\operatorname{Hek}(\mathrm{a})$	$\mathrm{C} _{10}$	$\operatorname{Dec}(\mathrm{a})$
$\mathrm{C} _{3}$	Prop (a)	$\mathrm{C} _{7}$	$\operatorname{Hept}(\mathrm{d})$	$\mathrm{C} _{11}$	$\operatorname{Updec}(\mathrm{a})$
$\mathrm{C} _{4}$	But (a)	$\mathrm{C} _{8}$	Oct (a)	$\mathrm{C} _{12}$	$\operatorname{Dodec}(\mathrm{a})$

Isomerism

Compounds having same molecular formula but different structural formula.

Types of Isomerism

I. Constitutional isomerism

Same molecular formula but different connectivity of atoms, that means atoms are bonded in a different sequence in the molecule. It is of five types.

1. Chain

2. Position

3. Functional

4. Metamerism

5. Tautomerism

6. Ring chain

1.Chain isomers

Isomers having different length of parent chain.

2.Position isomers

Isomers having different position of functional group on the same chain of carbonatoms.

3.Functional isomers

Isomers having different functional groups.

(a) Alcohols and ethers

$\mathrm{CH} _{3} \mathrm{CH} _{2} \mathrm{CH} _{2} \mathrm{OH}$ $\qquad$ $\mathrm{CH} _{3}-\mathrm{O}-\mathrm{CH} _{2}-\mathrm{CH} _{3}$

Propanol $\qquad \qquad \qquad $ Methoxyethane

(b) Aldehydes, ketones and unsaturated alcohols

$$ \begin{array}{lll} & & \\ \mathrm{CH} _{3} \mathrm{CH} _{2} \mathrm{CHO} \qquad & \mathrm{CH} _{3}-\mathrm{C}-\mathrm{CH} _{3} & \qquad \mathrm{CH} _{2}=\mathrm{CH}-\mathrm{CH} _{2} \mathrm{OH} \\ \text { propanal } & \text { propanone } & \text { \qquad prop-2-en-1-ol } \end{array} $$

$\mathrm{CH} _{3} \mathrm{CH} _{2} \mathrm{COOH} \quad \mathrm{CH} _{3} \mathrm{COOCH} _{3}$

propanoic acid $\qquad$ methyl ethanoate

(d) Cyanides and isocyanides

$\mathrm{CH} _{3} \mathrm{CH} _{2} \mathrm{CN} \qquad \mathrm{CH} _{3} \mathrm{CH} _{2} \mathrm{NC}$

ethylcyanide $\qquad$ ethyl isocyanide

e. Nitro and nitrite

4.Metamerism

Isomerism in which number of carbon atoms on either side of functional group are different. Metamerism can be considered as a special case of position isomerism.

5. Tautomerism

(a) Arises due to migration of a hydrogen atom from one atom to other within same molecule with necessary rearrangement.

(b) Isomers exist in dynamic equilibrium and are readily interconvertible.

(c) A compound can show tautomerism only if it has electron withdrawing groups like-CO, $-\mathrm{NO} _{2}$ and $-\mathrm{CN}$. It should have atleast one acidic hydrogen on $\alpha$-carbon.

(d) Compound having quinonoid structure do not show tautomerism.

Tautomerism is further classified by naming the functional groups. For example keto-enol tautomerism

6.Ring chain isomerism

Ring-chain isomerism is shown by compounds which have different chain length and ring size but the same molecular formula. For example

Practice Questions

1. The enolic form of acetone contains

(a) 9 sigma bonds, 1 pi bond and 2 lone pairs

(b) 8 sigma bonds, 2 pi bonds and 2 lone pairs

(d) 9 sigma bonds, 2 pi bonds and 1 lone pair

Show Answer

Answer: (a)

2. Amongst the following, a pair representing tautomerism is:

Show Answer

Answer: (c)

3. Number of constitutional isomers of the formula $\mathrm{C} _{5} \mathrm{H} _{11} \mathrm{Br}$ is

(a) 4

(b) 8

(d) 6

Show Answer

Answer: (b)

4. Methoxymethane and ethanol are

(a) functional isomers

(b) optical isomers

(d) chain isomers

Show Answer

Answer: (a)

5. Metamers of ethyl propionate are

(a) $\mathrm{C} _{4} \mathrm{H} _{9} \mathrm{COOH}$ and $\mathrm{HCOOC} _{4} \mathrm{H} _{9}$

(b) $\mathrm{C} _{4} \mathrm{H} _{9} \mathrm{COOH}$ and $\mathrm{CH} _{3} \mathrm{COOC} _{3} \mathrm{H} _{7}$

(d) $\mathrm{CH} _{3} \mathrm{COOC} _{3} \mathrm{H} _{7}$ and $\mathrm{C} _{3} \mathrm{H} _{7} \mathrm{COOCH} _{3}$

Show Answer

Answer: (d)

II Stereoisomerism

Isomers having same connectivity but different arrangement of atoms / groups in space.

1.Configurational isomerism

Different spatial arrangements of atoms in a molecule which are readily interconvertible without breaking and formation of bonds.

(a) Geometrical isomerism

(cis-trans isomerism) : arises because of restricted rotation around the double bond.

For cis-trans isomerism, two identical groups should not be present on same $\mathrm{sp}^{2}$ carbon.

Physical properties of geometrical isomers.

1.Melting point

Trans isomer being symmetrical fits well into the crystal lattice and greater energy is required to break the lattice.

2. Boiling point

Since cis isomer has higher dipole moment than trans isomer so it has higher boiling point.

3. Dipole moment

Cis is more polar than trans thus $\mu$ cis $>\mu$ trans.

Geometrical isomerism in nitrogen compounds aldoxime. $\left(\mathrm{C} _{6} \mathrm{H} _{5} \mathrm{CH}=\mathrm{NOH}\right)$

(b) Optical isomerism

Compounds that rotate plane polarized light and are non superimposable on its mirror image.

Polarimeter

An instrument to measure the angle of rotation of plane polarized light.

Laevorotatory

Optical isomers which rotate plane polarized light to left and are indicated by placing a $(-)$ sign before the degree of rotation.

Dextrorotatory

Optical isomers which rotate plane polarized light to right and are indicated by placing a $(+)$ sign before the degree of rotation.

Chiral carbon

Carbon atom bonded to four different atoms or groups (Asymmetric carbon)

An asymmetric or chiral compound rotates the plane of polarized light and is the simplest source of optical activity.

Enantiomers

Stereoisomers which are non superimposable mirror images. They have same physical properties except direction of rotation of plane polarized light and thus they cannot be separated by fractional distillation / crystallization. They have same chemical properties towards optically inactive reagents.

Diastereomers

Configurational isomers which are not mirror images. They have different physical and chemical properties and can be separated by physical techniques. They exist only when a compound has two or more assymetric carbon atoms.

Meso form

Compounds containing chiral carbon but superimposable on its mirror image. They are optically inactive due to presence of plane of symmetry.

Racemic mixture (or racemic modification)

When both the enantiomers are mixed together in the same ratio, the resulting mixture will have zero optical rotation, since the rotation values of the two isomers cancel each other (having the same absolute value of rotation but in opposite directions). The racemic mixture is thus optically inactive. Racemic mixture is represented by placing ( $\pm$ ) before the name, as ( $\pm$ ) tartaric acid.

Racemisation

Process of conversion of an enantiomer into a racemic mixture.

Points to Remember

Calculation of number of optical isomers: (a) If the molecule is not divisible into two identical halves and has $\mathrm{n}$ asymmetric carbon atoms, number of optically active forms $=2^{n}$.

(b) If the molecule is divisible into two identical halves and has even number of chiral carbon atoms i.e. $n=2,4,6 \ldots$, number of optically active forms $=2^{n-1}$.

2.Conformational isomerism

Different spatial arrangements of atoms that result from rotation about a C-C single bond in alkanes. Infinite number of spatial arrangements are thus possible. A small amount of energy is required for this rotation and thus the rotation is not completely free. Each spatial arrangement hence obtained is known as conformation or conformer or rotamer.

Conformations of ethane

Staggered conformation

Conformation with $60^{\circ}$ dihedral angle. Here atoms attached to the two carbons are as far apart as possible.

Eclipsed conformation

Conformation of $0^{\circ}$ dihedral angle. Here atoms attached to the two carbons are as close together as possible. All other intermediate conformations may be called as skew or gauche conformations.

Torsional strain

Repulsions between the bonding electrons of the substituents along the axis of rotation.

Conformations of butane

Relative stability of conformers of butane

anti > gauche or skew >eclipsed > fully eclipsed

Due to steric hinderance in gauche, anti is more stable. But sometimes gauche becomes more stable as in the example below.

Practice Questions

1. Which of the following compounds are chiral?

Show Answer

Answer: (d)

2. Which of the following compounds possesses a chiral centre?

Show Answer

Answer: (b)

3. Which of the following compounds will exhibit geometrical isomerism?

(a) 2-butene

(b) 2-butyne

(d) butanal

Show Answer

Answer: (a)

4. Isomers which can be interconverted through rotation around a single bond are

(a) conformers

(b) diastereomers

(d) positional isomers

Show Answer

Answer: (a)

5. How many optically active stereoisomers are possible for butane-2,3-diol?

(a) 1

(b) 2

(d) 4

Show Answer

Answer: (b)

6. Which of the following compounds will show geometrical isomerism.

(a) 1-phenyl-2-butene

(b) 3-phenyl-1-butene

(d) 1,1-diphenyl-1-propene

Show Answer

Answer: (a)

7. $\mathrm{C}-2$ is rotated anti clockwise $120^{\circ}$ about $\mathrm{C} 2-\mathrm{C} 3$ bond. The resulting conformer is

(a) partially eclipsed

(b) eclipsed

(d) staggered

Show Answer

Answer: (c)

8. Which among the following pairs are diastereomers?

(a) only 2

(b) only 1

(d) 1,2 and 4

Show Answer

Answer: (a)

9. Geometrical isomerism is possible in

Show Answer

Answer: (c)

10. Number of stereocentres and stereoisomers of the given compound

(a) 1 and 2

(b) $\quad 2$ and 4

(d) 3 and 6

Show Answer

Answer: (d)

IUPAC Nomenclature of Simple Monofunctional Organic Compounds

Organic chemistry deals with a large number of compounds. A systematic method has been developed so that each compound could be methodically named and is known as the IUPAC nomenclature (International Union of Pure and Applied Chemistry).

The IUPAC name of any organic compound essentially consists of three parts, i.e.,

(a) Word root (b) Suffix and (c) Prefix

Word root

Select the word root to denote the longest possible continuous chain of carbon atoms containing the functional group. The word roots for chains containing 1 to 12 carbon atoms are given below:

General Formula	Functional group	Common < name	IUPAC name
$\mathrm{R}-\mathrm{X}$	$-\mathrm{XCX}=\mathrm{P}, \mathrm{Cl}, \mathrm{Br}, \mathrm{D}$	Alkyl puldes	Holo
$\mathrm{R}-\mathrm{OX}$	$-\mathrm{OX}$	Alcohols
$\mathrm{R}-0-\mathrm{R}$	$-0-$	Eithers
$\mathrm{R}-\mathrm{CH}$ -	$-\mathrm{CH} 0$
$\mathrm{R}>\mathrm{C}=0$	$-\mathrm{CH} 0$
$\mathrm{R}$	$-\mathrm{COOH}$
$\mathrm{RCO0H}$	$-\mathrm{COCl}$
$\mathrm{R}-\mathrm{COCl}$	$-\mathrm{COOR}$
$\mathrm{R}-\mathrm{COOR}$	$-\mathrm{CO}>0$
$(\mathrm{RCO}) _{2} \mathrm{O}$	$-\mathrm{CO}$
$\mathrm{R}-\mathrm{NH} \mathrm{H} _{2}$	$-\mathrm{NH} 2$
$\mathrm{R}-\mathrm{C}=\mathrm{N}$	$-\mathrm{C}=\mathrm{N}$
$\mathrm{R}-\mathrm{N}=\mathrm{N}$	$-\mathrm{N}=\mathrm{C}$
$\mathrm{R}-\mathrm{NO} \mathrm{N} _{2}$	$-\mathrm{NO}$
$\mathrm{R}-0-\mathrm{N}=0$	$-0-\mathrm{N}=0$

The extra ‘a’ given in parenthesis is used only if the suffix to be added to the word root begins with a consonant. For example,

$\mathrm{CH} _{2}=\mathrm{CH}-\mathrm{CH} _{2}-\mathrm{CH} _{2}-\mathrm{CH} _{3}$ is pent-I-ene whereas $\mathrm{CH} _{2}=\mathrm{CH}-\mathrm{CH} _{2}-\mathrm{CH}=\mathrm{CH} _{2}$ is penta-1, 4-diene.

Suffix

These are of two types:

(i) Primary suffix

A primary suffix is added to the word root to indicate whether the carbon chain is saturated or unsaturated. The three basic primary suffixes are :

Type of carbon chain	Primary suffix	General name
Saturated	-ane	Alkane
Unsaturated	-ene	Alkene
Unsaturated	-yne	Alkyne

If the carbon chain contains two, three, four or more double or triple bonds, numerical prefixes such as di, tri, tetra, etc. is added before the primary suffix. For example, diene, triyne, tetraene, etc.

(ii) Secondary suffix

A secondary suffix is then added to the word root after the primary suffix to indicate the functional group present in the organic compound. Some important secondary suffixes are :

Class of organic compound	Functional group	Secondary suffix
1. Alcohols	$-\mathrm{OH}$	ol
2. Aldehydes	$-\mathrm{CHO}$	al
3. Ketones	$>\mathrm{C}=0$	one
4. Carboxylic acids	$-\mathrm{COOH}$	oic acid
5. Esters	$-\mathrm{COOR}$	oate
6. Amides	$-\mathrm{CONH} 2$	amide
7. Acid chlorides	$-\mathrm{COCl}$	oyl chloride
8. Nitriles	$-\mathrm{CN}$	nitrile
9. Thiol	$-\mathrm{SH} _{2}$	thiol
10. Amines	$-\mathrm{NH} _{2}$	amine

It may be noted that while adding the secondary suffix to the primary suffix, the terminal ’ $e$ ’ of the primary suffix (i.e., ane, ene oryne) is dropped if the secondary suffix begins with a vowel, but is retained if the secondary suffix begins with a consonant. For example,

Organic compound	Word root	Primary suffix	Secondary suffix	IUPAC name
$\mathrm{CH} _{3} \mathrm{CH} _{2} \mathrm{OH}$	Eth	an $(\mathrm{e})^{\star}$	0	Ethanol
$\mathrm{CH} _{3} \mathrm{CH} _{2} \mathrm{CN}$	Prop	ane	nitrile	Propanenitrile

Prefix

Like suffixes, prefixes are also of two types:

(i) Primary prefix

A primary prefix is used simply to distinguish cyclic from acyclic compounds. For example, in case of carbocyclic compounds, a primary suffix cyclo is used immediately before the word root. Thus,

(ii) Secondary prefix

In IUPAC system of nomenclature, certain groups are not considered as functional groups but instead are treated as substituents. These are called secondary prefixes and are added immediately before the word root (or the primary suffix in case of carbocyclic compounds) in alphabetical order to denote the substituent groups or the side chains.

The secondary prefixes for some groups which are always treated as substituent groups (regardless of the fact whether the organic compound is monofunctional or polyfunctional) are given below :

Subsitituent group	Secondary prefix	Substituent group	Secondary prefix
$-\mathrm{F}$	Fluoro	$-\mathrm{OCH} _{3}$ (or-OMe)	Methoxy
$-\mathrm{Cl}$	Chloro	$-\mathrm{OC} _{2} \mathrm{H} _{5}$ (orOEt)	Ethoxy
$-\mathrm{Br}$	Bromo	$-\mathrm{CH} _{3}$ (or-Me)	Methyl


-1	Iodo	$-\mathrm{C}_2 \mathrm{H}_5$ (or-Et)	Ethyl
$-\mathrm{NO}_2$	Nitro	$-\mathrm{CH}_2 \mathrm{CH}_2 \mathrm{CH}_3$ (or-Pr)	n-Propyl
$-\stackrel{+}{N} \equiv N$	Diazo	$-\mathrm{CH}\left(\mathrm{CH}_3\right)_2$	Isopropyl
$-0 \mathrm{R}$	Alkoxy	$-\mathrm{C}\left(\mathrm{CH}_3\right)_3$	t-Butyl or tert-Butyl

2-Fluoro-4-methoxypentane

IUPAC Nomenclature of Branched Chain and Complex Organic Compounds

Longest continuous chain or the parent chain.

Select the longest continuous chain of carbon atoms in the molecule. This is called the longest chain or the parent chain while the rest of the carbon chains are called the side chains or substituents.

Rule for larger number of side chains

If two chains contain the same number of carbon atoms then the one containing a larger number of side chains is considered the parent chain. For example,

Parent chain contains one alkyl substituent (wrong).

Lowest locant rule.

If only one side chain is present, the carbon atoms of the parent chain are numbered in such a way that the side chain gets the lowest possible number called the locant. For example, the correct locant for the $\mathrm{CH} _{3}$ side chain in structure (I) is 3 and not 4 .

If more than one side chains are present then numbering is done so as to give smallest possible number to all the substituents.

Alphabetical order of the simple substituents

When two or more simple substituents (i.e., unbranched alkyl groups) are present on the parent chain, each alkyl group prefixed by its locant is arranged in alphabetical order (irrespective of its locant) before the name of the parent alkane. For example:

Numbering of different substituents at equivalent positions

When two different substituents are present at equivalent positions, the numbering of the parent chain is done in such a way that the substituent which comes first in the alphabetical order (written first in the name) gets the lower number.

Numerical prefixes-Naming same substituent at different positions

When the same substituent occurs more than once on the parent chain at the same or different positions, the locants of each substituent are separated by commas and suitable numerical prefixes such as ‘di’ (for two), ’tri’ (for three), tetra (for four), etc. are attached to the name of the substituents. However, the prefixes di, tri, etc. are not considered while deciding the alphabetical order of the alkyl groups.

Naming the complex substituent (or branched substituent)

In case the substituent on the parent chain is complex (i.e., it has branched chain), it is named as a substituted alkyl group by separately numbering the carbon atoms of this substituted alkyl group with the carbon atom attached to the parent chain being counted as 1 . The name of such a substituent is always enclosed in brackets to avoid confusion with the numbers of the parent chain.

The prefix iso-and other related common ones are allowed by the IUPAC nomenclature for naming alkyl groups as long as they are not further substituted, whereas the prefixes sec- and tert-are not allowed.

If there are two chains of equal length, then the chain containing more number of side chains is selected.

Nomenclature of Unsaturated Hydrocarbons (Alkenes and Alkynes)

The parent chain must contain the multiple bond regardless of the fact whether it also denotes the longest continuous chain of carbon atoms or not. For example, in structure (I), the parent chain consists of five carbon atoms while the longest continuous chain contains six carbon atoms.

If both double and triple bonds are present, the numbering of the parent chain should always be done from that end which is nearer to the double or the triple bond, i.e., the lowest locant rule for the multiple bonds must be followed. For example,

If, however, there is a choice in numbering, the double bond is always given preference over the triple bond. For example,

According to the latest convention (1993 recommendations) while naming unsaturated hydrocarbons, the locant of the double bond or that of the triple bond is placed immediately before the suffix ’ene’ or the ‘yne’ and not before the word root as was the practice being followed earlier. For example,

If however, both double and triple bonds are present in the compound, their locants are placed immediately before their respective suffixes and the terminal ${ }^{*} \mathrm{e}^{\star}$ from the suffix ’ene’ is omitted while writing its complete name.

Practice Questions

1. The name of

according to IUPAC nomenclature system is

(a) 2,3-dibromo-1, 4-dichlorobut-2-ene

(b) 1,4-dichloro-2, 3-dibromobut-2-ene

(d) dichlorodibromobutane

Show Answer

Answer: (a)

2. The IUPAC name of $\mathrm{BrCH} _{2} \mathrm{CH} _{2} \mathrm{OCH}=\mathrm{CH} _{2}$ is

(a) 1-bromo-2-ethoxy ethene

(b) (2-bromoethoxy) ethene

(d) 2-bromoethyl ethenyl ether

Show Answer

Answer: (b)

3. The IUPAC name of $\mathrm{CH} _{2}=\mathrm{CH}-\mathrm{C} \equiv \mathrm{C}-\mathrm{CH} _{2} \mathrm{OH}$ is

(a) 2-pentyn-4-en-1-0l

(b) pent-4-en-2-yn-1-0

(d) 5-hydroxy-1-penten-3-yne

Show Answer

Answer: (b)

4. The IUPAC name of is

(a) 3-oxo-2-heptyne

(b) hept-3-yn-4-0xone

(d) hept-3-en-4-one

Show Answer

Answer: (c)

5. The IUPAC name of $\mathrm{HC} \equiv \mathrm{C}-\mathrm{CH} _{2} \mathrm{CN}$ is

(a) 3-cyanopropyne

(b) but-3-yne-1-nitrile

(d) but-1-yne-3-nitrile

Show Answer

Answer: (b)

6. Indicate the wrongly named compound

Show Answer

Answer: (d)

7. Which of the following compound has wrong IUPAC name?

Show Answer

Answer: (c)

8. The IUPAC name of

(a) 2-(1, 1-dimethylethyl) prop-1-en-3-0l

(b) 2-(1, 1-dimethylethyl) prop-2-en-1-0l

(d) 2,2-dimethyl-3-hydroxymethylbut-3-ene

Show Answer

Answer: (b)

9. IUPAC name of $\mathrm{CH} \equiv \mathrm{C}-\mathrm{CH} _{2}-\mathrm{CH}=\mathrm{CH} _{2}$

(a) pent-2-en-4-yne

(b) pent-1-yn-3-ene

(d) pent-2-en-5-yne

Show Answer

Answer: (c)

10. IUPAC name of neopentane is

(a) 2, 2-dimethylpropane

(b) 2-methlypropane

(d) 2-methylbutane

Show Answer

Answer: (a)

11. The IUPAC name of is

(a) 2-Ethyl-3-methylhex-1-en-4-yne

(b) 5-Ethyl-4-methyl-hex-2-yn-5-ene

(d) 5-Methylene-5-ethyl-4-methylhept-2-yne

Show Answer

Answer: (a)

12. The IUPAC name of the compound is

(a) 2-lodo-3-chloro-4-pentanoic acid

(b) 4-0xo-3-chloro-2-iodo pentanoic acid

(d) 3-Chloro-2-iodo-4-oxopentanoic acid

Show Answer

Answer: (d)

Nomenclature of Polyfunctional Organic Compounds

Organic compounds which contain two or more functional groups are called polyfunctional compounds. Their IUPAC names are obtained as follows:

Principal functional group

If the organic compound contains two or more functional groups, one of the functional groups is selected as the principal functional group while all the remaining functional groups (also called the secondary functional groups) are treated as substituents. The following order of preference is used while selecting the principal functional group.

Amine salts > carboxylic acids > sulphonic acids > anhydrides > esters > acid chlorides > acid amides $>$ nitriles $>$ isonitriles $>$ aldehydes $>$ ketones $>$ alcohols $>$ phenols $>$ thiols $>$ amines.

All the remaining functional groups such as halo (fluoro, chloro, bromo, iodo), nitroso (-NO), nitro (- $\left.\mathrm{NO} _{2}\right)$, alkoxy (-OR), alkyl (R), phenyl $\left(-\mathrm{C} _{6} \mathrm{H} _{5}\right)$, etc, are always treated as substituent groups.

The prefixes for some secondary functional groups are given below:

Secondary functional group	Prefix	Secondary functional group	Prefix
$-\mathrm{X} \mathrm{(F,} \mathrm{Cl,} \mathrm{Br,} \mathrm{I)}$	Halo	$-\mathrm{NHR}$	N-Alkyl amino
$-\mathrm{OH}$	Hydroxy	$-\mathrm{NR} _{2}$	$\mathrm{~N}, \mathrm{~N}-$ Dialkylamino
$-\mathrm{SH}$	Sulphanyl $\mathrm{I}^{*}$	$-\mathrm{C} \equiv \mathrm{N}$	Cyano
$-\mathrm{OR}$	Alkoxy	$-\mathrm{COOH}$	Carboxy
$-\mathrm{CHO}$	Formyl	$-\mathrm{COOR}$	Alkoxycarbonyl or
			Carb alkoxy
$\mathrm{C}=0$	Oxo	$-\mathrm{COX}$	Halocarbonyl
$-\mathrm{NH} _{2}$	Amino	$-\mathrm{CONH} _{2}$	Carbamoyl

The rules of IUPAC nomenclature for these compounds are,

Selecting the principal chain

Select the longest continuous chain of carbon atoms containing the principal functional group and maximum number of secondary functional groups and multiple bonds.

Numbering the principal chain

Number the principal chain in such a way that the principal functional group gets the lowest possible locant followed by double/triple bond.

Alphabetical order.

Identify the prefixes and the locants for the secondary functional groups and other substituents and place them in alphabetical order before the word root:

Organic compounds may be sometimes represented by bond-line notation.

In this notation, bonds are represented by lines and carbon atoms by line ends and intersections. It is assumed that required number of $\mathrm{H}$-atoms are present wherever they are necessary to satisfy tetracovalency of carbon. For example,

Rules for IUPAC Nomenclature of Alicyclic Compounds

The following rules are generally followed:

The names of alicyclic compounds are obtained by adding the prefix ‘cyclo’ to the name of the corresponding straight chain hydrocarbon (alkane, alkene or alkyne).

If two or more alkyl groups or other substituent groups are present in the ring, their positions are indicated by numerals, such as $1,2,3,4 \ldots .$. etc. While numbering the carbon atoms of the ring, the substituent which comes first in the alphabetical order is given the lowest number provided it does not violate the lowest locant rule. For example,

If the ring contains more or equal number of carbon atoms as the alkyl group attached to it, it is named as a derivative of cycloalkane and the alkyl group is treated as a substituent group, otherwise it is named as a derivative of alkane and the cycloalkyl group is considered as a substituent group. For example,

If, however, the side chain contains a multiple bond or a functional group, the alicyclic ring is treated as the substituent irrespective of the size of the ring. For example,

If a multiple (double or triple) bond and some other substituents are present in the ring, the numbering is done in such a way that the carbon atoms of the multiple bond get lowest locants 1 and 2 and also the substituents get the lower locants at the first point of difference. For example,

1, 5-Dimethylcyclopent-1-ene

If the ring contains a multiple bond and the side chain contains a functional group, then the ring is treated as the substituent and the compound is named as a derivative of the side chain. For example,

2-(Cyclopent-3-en-1-yl) propan-1-ol

If the ring as well as the side chain contain functional groups, the compound is named as a derivative of the side chain or the alicyclic ring depending on wheather the side chain or the ring contains, the principal functional group. For example,

3-(4-Nitrocyclohex-1-en-1-yl)prop-2-en-1-oic acid

2-(2-Hydroxypropyl) cyclohexan-1-one

If, however, the alicyclic ring and the side chain contain the same functional group, the compound is named as a derivative of the side chain or the ring according as the side chain or the ring contains higher number of carbon atoms. For example,

2-(2-Hydroxylbut-1-yl)cyclohexan-1-ol

If a compound contains an alicyclic ring directly linked to the benzene ring, it is named as a derivative of benzene, i.e. the compound having lowest state of hydrogenation. For example,

Cyclohexylbenzene

If an alicyclic ring is directly attached to a carbon containing functional group, the carbon atom of the functional group is not included in the parent name of the alicyclic system. Therefore, for such systems, the following prefixes and suffixes for the functional group are commonly used

Functional group	Prefix	Suffix
$-\mathrm{CHO}$	Formyl	Carbaldehyde
$-\mathrm{COOH}$	Carboxy	Carboxylic acid
$-\mathrm{COX}(\mathrm{X}=\mathrm{F}, \mathrm{Cl}, \mathrm{Br}, \mathrm{I})$	Halocarbonyl	Carbonyl halide
$-\mathrm{COOR}$	Alkoxycarbonyl or	Alkyl carboxylate
	Carbalkoxy
$-\mathrm{CONH} _{2}$	Carbamoyl	Carboxamide
$-\mathrm{CN}$	Cyano	Carbonitrile

For example,

Nomenclature of Aromatic Compounds

When giving IUPAC names to substituted benzene compounds, the prefix of the substituent is used before the word benzene. Also, the common names of many substituted benzene compounds are also accepted, for example,

When an aromatic compound contains two or more functional groups, it is named as a derivative of the compound with the principal functional group at position 1 . For example,

In case three or more functional groups are present they are named in alphabetical order. For example,

If all the functional groups present in the benzene ring are such which are normally treated as substituent groups, the various groups are arranged in alphabetical order with the group named first in the alphabetical order getting the lowest locant provided it does not violate the lowest locant rule for all the substituents. for example,

When a substituent is such which when taken together with the benzene ring gives a special name to the molecule, then it is named as a derivative of that molecule with the substituent at position I. for example,

When a benzene ring is attached to an aliphatic compound having a functional group, it is named as a phenyl derivative of that aliphatic compound. For example,

If, however, the benzene ring is attached to an alkane, it is named as a derivative of the bigger structural unit. For example,

However, simple phenyl substituted alkenes and alkynes are named either as derivative of benzene or the alkene / alkyne. For example,

Practice Questions

1. The IUPAC name of $\mathrm{CH} _{3}-\mathrm{CH}-\mathrm{CH}=\mathrm{C}-\mathrm{CHO}$ is

(a) 4-hydroxy-1-methylpentanal b. 4-hydroxy-2-methylpent-2-en-1-al

(d) 2-hydroxy-3-methylpent-2-en-5-al

Show Answer

Answer: (b)

2. The IUPAC name of the compound

(a) 2-cyano-2-methyl-4-oxopentane

(b) 4-cyano-4-methyl-2-pentanone

(d) 2,2 dimethyl-4-0x0-pentanenitrile

Show Answer

Answer: (d)

3. Name of the compound given below is

(a) 4-ethyl-3-methyloctane

(b) 3-methyl-4-ethyloctane

(d) 5-ethyl-6-methyloctane

Show Answer

Answer: (a)

4. The IUPAC name of

(a) 4-acetylhex-5-en-1-yne

(b) 2–Methylpentan–1–ol

(d) 4 -acetylhex-2-en-6-yne

Show Answer

Answer: (b)

5. The IUPAC name of $\left(\mathrm{C} _{2} \mathrm{H} _{5}\right) _{2} \mathrm{CHCH} _{2} \mathrm{OH}$ is

(a) 2-Ethylbutan-1-0l

(b) 2-Methylpentan-1-0l

(d) 3-Ethylbutan-1-0l

Show Answer

Answer: (a)

6. The IUPAC name of

(a) 6-carboxy-2,5-dimethylpent-3-enoic acid

(b) 6-carboxy-2, 3-dimethylhept-4-enoic acid

(d) 2,5,6-trimethylhept-3-ene-1, 7-dioic acid

Show Answer

Answer: (d)

7. The IUPAC name of

(a) 2,3-dimethylpentanoyl chloride

(b) 3,4-dimethylpentanoyl chloride

(d) 2-ethyl-3-methylbutanoyl chloride

Show Answer

Answer: (a)

8. The IUPAC name of the given compound,

(a) 2-cyclohexylbutane

(b) 1-cyclohexyl-1-ethylethane

(d) 1-cyclohexyl-1-methylpropane

Show Answer

Answer: (c)

9. The IUPAC name of the compound,

(a) 3-cyclopentylhexane

(b) 4-cyclopentylhexane

(d) 4-hexylcyclopentane

Show Answer

Answer: (a)

10. The IUPAC name of

(a) acetylcyclohexadiene

(b) 1-cyclohexa-2, 4-dienylethanone

(d) none of these

Show Answer

Answer: (b)

11. The correct decreasing order of priority for the functional groups of organic compounds in the IUPAC system of nomenclature is

(a) $-\mathrm{SO} _{3} \mathrm{H},-\mathrm{COOH},-\mathrm{CONH} _{2},-\mathrm{CHO}$

(b) $-\mathrm{CHO},-\mathrm{COOH},-\mathrm{SO} _{3} \mathrm{H},-\mathrm{CONH} _{2}$

(d) $-\mathrm{COOH},-\mathrm{SO} _{3} \mathrm{H},-\mathrm{CONH} _{2}, \mathrm{CHO}$

Show Answer

Answer: (d)

12. The IUPAC name of

(a) 2-formyl-2-methyl-1-propanol

(b) 2,2-dimethyl-3-hydroxypropanal

(d) 2-(hydroxymethyl) -2-methylpropanal

Show Answer

Answer: (b)

13. The IUPAC name of the compound

(a) Ethoxybutane

(b) 1-Ethoxy-2-propanone

(d) 1-Ethoxy-3-butanone

Show Answer

Answer: (c)

Basic Principles of Organic Chemistry-I

Bond Fission & Reactive Intermediates

Types of bond fission

1.Homolytic bond fission

If a covalent bond breaks in such a way that each atom takes away one electron of the shared pair.

Occurs in non polar bonds

Favoured by high temperature, UV radiations, presence of radical initiators such as peroxides.

$$ \mathrm{RO}-\mathrm{OR} \longrightarrow 2 \mathrm{RO} \quad ; \quad \mathrm{Cl}-\mathrm{Cl} \longrightarrow 2 \mathrm{CI} $$

2.Heterolytic bond fission

When a covalent bond breaks in such a way that both the electrons of covalent bond are taken by one of the bonded atoms.

$$ \mathrm{A} \xrightarrow{\curvearrowright} \mathrm{B} \longrightarrow \mathrm{A}^{+}+\mathrm{B}^{-} \quad ; \quad \mathrm{A}^{-} \mathrm{B} \longrightarrow \mathrm{A}^{-}+\mathrm{B}^{+} $$

$\qquad \qquad \qquad \qquad $ ( $B$ is more electronegative than $A$ ) $ \qquad \qquad$ ( $A$ is more electronegative than $B$ )

It results in the formation of charged species i.e. carbocations and carbanions.

Occurs in polar covalent bonds.

Favoured by polar solvents.

The three important reaction intermediates generated by the bond fission are

I. Carbocations are electron deficient species and have only three shared pairs of electrons (or 6 electrons) around the carbon and thus have positive charge. They are formed by heterolytic cleavage of covalent bonds in which the leaving group takes away with it shared pair of electrons.

$$ \left(\mathrm{CH} _{3}\right) _{3} \mathrm{C}-\stackrel{\sim}{\mathrm{Cl}} \longrightarrow \underset{\substack{\text { carbocation }}}{\left(\mathrm{CH} _{3}\right) _{3} \mathrm{C}^{+}+\mathrm{Cl}^{-}} $$

Carbocations can be classified into the following groups,

1. Alkyl carbocations

When positive charge is present on the alkyl carbon, carbocation is known as alkyl carbocation.

Stability of alkyl carbocations can be explained by

(a) Inductive effect

(b) Hyperconjugation

According to these two effects the stability order is as follows:

If $\alpha$-atom with respect to carbocationic carbon has one or more than one lone pair of electrons then lone pair of electrons strongly stabilizes a carbocation due to delocalization.

2.Vinyl carbocation

When positive charge is present on vinylic carbon then carbocation is known as vinyl carbocation; $\mathrm{H} _{2} \mathrm{C}=\stackrel{\oplus}{\mathrm{C}} \mathrm{H}$.

This carbocation is the least stable because positive charge is present on the electronegative carbon.

3.Allyl carbocation

$\left(\mathrm{CH} _{2}=\mathrm{CH}-\stackrel{\oplus}{\mathrm{C}} \mathrm{H} _{2}\right)$

When positive charge is present on the allylic carbon of the allyl group, the carbocation is known as allyl carbocation.

Allyl carbocations are more stable than alkyl carbocations due to resonance.

Stability of primary, secondary and tertiary allyl carbocations can be compared by (a) Inductive effect (b) Hyperconjugation (c) Resonance.

4. Phenyl methyl carbocations

When positive charge is present on benzyl carbon, carbocation is known as phenyl methyl carbocation.

Phenyl methyl carbocations are of three types:

Stability of phenyl methyl carbocations can be explained by resonance.

Structure

No of resonating structures 10 $\qquad $ 7 $\qquad $ 4

Greater the number of resonating structures, greater is the stability.

5. Cyclopropyl methyl carbocations

(i) These carbocations are very stable carbocations. They are more stable than benzyl carbocations.

(ii) Stability of cyclopropyl methyl carbocations increases with every cyclopropyl group. Thus, additional cyclopropyl group has cumulative additive effect on stability. Thus,

(iii) The special stability is a result of conjugation between the bent orbitals of the cyclopropyl ring and the vacant $p-$ orbital of the cationic carbon.

Stability of different types of carbocations in decreasing order

Based on their stability carbocations undergo rearrangement as follows,

1. Hydride shift

2.alkyl shift

3. Ring expansion

It releases strain of smaller rings and increases stability. Rearrangement occurs from $\alpha$ positions and produces more stable carbocation.

Carbanion

II.Carbanion

Species bearing a negative charge on carbon and possessing eight electrons in its valence shell. These are produced by heterolytic cleavage of covalent bonds in which shared pair of electrons remain with the carbon atom.

$$ \mathrm{HO}+\stackrel{\ominus}{\mathrm{H}}-\stackrel{\curvearrowright}{\mathrm{C}} \mathrm{H} _{2}-\mathrm{CHO} \longrightarrow \underset{\text { carbanion }}{\mathrm{H} _{2} \mathrm{O}}+\stackrel{\ominus}{\mathrm{C}} \mathrm{H} _{2}-\mathrm{CHO} $$

Stability of carbanions can be explained by

1.Inductive effect

(a) +l effect

Greater the + effect, lower is the stability

(b) -l effect

Greater the -I effect, greater is the stability

2. Electronegativity of carbanionic carbon

Stability of carbanion depends on the % S character of carbanionic carbon. Greater the s character, greater is the stability.

3. Delocalisation or Resonance

Allyl and benzyl carbanions are stabilized by delocalisation of negative charge.

$$ \begin{array}{ccccc} \qquad \mathrm{CH} _{2}=\mathrm{CH}-\stackrel{\ominus}{\mathrm{C}} \mathrm{H} _{2} & <\mathrm{C} _{6} \mathrm{H} _{5}-\stackrel{\ominus}{\mathrm{C}} \mathrm{H} _{2} & <\left(\mathrm{C} _{6} \mathrm{H} _{5}\right) _{2} \stackrel{\ominus}{\mathrm{C}} \mathrm{H} & <\left(\mathrm{C} _{6} \mathrm{H} _{5}\right) _{3}-\stackrel{\ominus}{\mathrm{C}} \end{array} $$

$\qquad $ resonating structures $\qquad \qquad \qquad $ 2 $\qquad \qquad \qquad $ 4 $\qquad \qquad $ 7 $\qquad \qquad \qquad $ 10

Greater the number of resonating structures, greater is the stability.

Stability of different types of carbanions in decreasing order

Benzyl carbanion > Allyl carbanion > $\mathrm{HC} \equiv \overline{\mathrm{C}}>\mathrm{H} _{2} \mathrm{C}=\overline{\mathrm{C}} \mathrm{H}>$ Alkyl carbanion

III. Free radicals

Atom or group of atoms having odd or unpaired electron. They are produced by homolytic cleavage of covalent bond.

$$ {\mathrm{CH} _{3}}- \mathrm{CH} _{3} \xrightarrow{\mathrm{hv}} \dot{\mathrm{CH}} _{3}+\dot{\mathrm{C}} \mathrm{H} _{3} $$

There are seven electrons in the outermost orbit of carbon free radicals.

Owing to the presence of an odd electron, a carbon radical is paramagnetic in nature. Due to this reason free radicals are highly reactive.

Free radicals are neutral electrophiles.

Stability of free radicals can be explained by

1.Inductive effect

Since free radicals are electron deficient species, presence of electron releasing groups will have a stabilizing effect on them. And the stability order will be, Tertiary > secondary > primary > $\dot{\mathrm{C}} _{3}$

2.Hyperconjugation

Stability of alkyl free radicals can be explained by the number of contributing structures formed due to hyperconjugation. Since the delocalization is due to the presence of $\alpha$-hydrogens (H’s attached to $\alpha-\mathrm{C}$ ), more the number of $\alpha$-hydrogens more contributing

structures are formed. Thus the stability order is,

3.Resonance

The stability of allyl and benzyl free radicals can be explained by resonance.

Stability of allyl and benzyl free radicals

Stability of these radicals can be explained by delocalisation or resonance.

Structure

$$ \left(\mathrm{C} _{6} \mathrm{H} _{5}\right) _{3} \dot{\mathrm{C}}>\left(\mathrm{C} _{6} \mathrm{H} _{5}\right) _{2} \dot{\mathrm{C}} \mathrm{H}>\mathrm{C} _{6} \mathrm{H} _{5} \dot{\mathrm{C}} \mathrm{H} _{2}>\mathrm{H} _{2} \mathrm{C}=\mathrm{CH}-\dot{\mathrm{C}} \mathrm{H} _{2} $$

No. of resonating structures: 10 $\qquad \qquad \qquad 7 \qquad \qquad \qquad 4$ $\qquad \qquad \qquad $ 2

Allyl and benzyl radicals are more stable than alkyl radicals.

Overall Stability order

$\left(\mathrm{C} _{6} \mathrm{H} _{5}\right) _{3} \dot{\mathrm{C}}>\left(\mathrm{C} _{6} \mathrm{H} _{5}\right) _{2} \dot{\mathrm{C}} \mathrm{H}>\mathrm{C} _{6} \mathrm{H} _{5} \dot{\mathrm{C}} \mathrm{H} _{2}>\mathrm{CH} _{2}=\mathrm{CH}-\dot{\mathrm{C}} \mathrm{H} _{2}>\left(\mathrm{CH} _{3}\right) _{3} \dot{\mathrm{C}}>\left(\mathrm{CH} _{3}\right) _{2} \dot{\mathrm{C}} \mathrm{H}>\mathrm{CH} _{3} \dot{\mathrm{C}} \mathrm{H} _{2}>\dot{\mathrm{C}} \mathrm{H} _{3}>\mathrm{CH} _{2}=\dot{\mathrm{C}} \mathrm{H}>$ $\mathrm{CH} \equiv \dot{\mathrm{C}}$ (hyperconjugation and resonance)

Practice Questions

1. The correct stability order for the following species is

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

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

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

Show Answer

Answer: (d)

2. The compound which gives the most stable carbocation by losing $\overline{\mathrm{O}} \mathrm{H}$, is

Show Answer

Answer: (b)

3. Which one of the following carbocations is most stable?

Show Answer

Answer: (a)

4. The order of stability of the following carbocations.

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

(b) I $>$ III $>$ I

(d) II $>$ I $>$ II

Show Answer

Answer: (d)

5. Which of the following pairs is / are correctly matched?

I. Carbocation : electrophile

II. Free radical : paramagnetic

III. Carbene : Incomplete octet

IV. Carbanion: Incomplete octet

(a) Only I

(b) I and II

(d) I, II and III

Show Answer

Answer: (d)

6. Which free radical is most stable?

(a) $\mathrm{C} _{6} \mathrm{H} _{5}-\stackrel{\bullet}{\mathrm{C}} \mathrm{H} _{2}$

(b) $\mathrm{H} _{2} \mathrm{C}=\mathrm{CH}-\stackrel{\bullet}{\mathrm{C}} \mathrm{H} _{2}$

(d) $\begin{aligned} & \mathrm{CH} _{3}-\stackrel{\bullet}{\mathrm{C}}-\mathrm{CH} _{3} \\ &\qquad \dot{\mathrm{C}} \mathrm{H} _{3}\end{aligned}$

Show Answer

Answer: (a)

7. Which allylic carbocation is the most stable?

(a) $\mathrm{CH} _{3}-\mathrm{CH}=\mathrm{CH}-\stackrel{\oplus}{\mathrm{C}} \mathrm{H} _{2}$

(b) $\mathrm{CH} _{3}-\mathrm{CH}=\mathrm{CH}-\stackrel{\oplus}{\mathrm{C}} \mathrm{H}-\mathrm{CH} _{3}$

(d) All have same stability

Show Answer

Answer: (c)

8. Consider the following carbanions :

I. $\mathrm{CH} _{3}-\mathrm{CH} _{2}$

II. $\mathrm{H} _{2} \mathrm{C}=\mathrm{CH}$

III. $\mathrm{HC} \equiv \stackrel{\ominus}{\mathrm{C}}$

Correct order of stability of these carbanions in decreasing order is :

(a) $\mid>$ II $>$ III

(b) $\mid$ II $>$ II $>$ I

(d) $\mid$ II $>$ I $>$ II

Show Answer

Answer: (b)

9. Arrange the given carbanions in decreasing order of their stability :

I. $\left(\mathrm{CH} _{3}\right) _{3}-\stackrel{\ominus}{\mathrm{C}}$

II. $\stackrel{\ominus}{\mathrm{C}} \mathrm{Cl} _{3}$

III. $\left(\mathrm{CH} _{3}\right) _{2} \stackrel{\ominus}{\mathrm{C}} \mathrm{H}$

IV. $\mathrm{C} _{6} \mathrm{H} _{5}-\stackrel{\ominus}{\mathrm{C}} \mathrm{H} _{2}$

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

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

(d) $\mid$ I $>$ III $>$ I $>$ IV

Show Answer

Answer: (c)

10. Which of the following contains three pairs of electrons in the valence shell?

(a) Carbocations

(b) Carbanions

(d) All of these

Show Answer

Answer: (a)

11. Arrange the given carbocations in decreasing order of stability

(a) $\quad$ I $>$ II $>$ III $>$ IV

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

(d) $\quad$ II $>$ III $>$ I $>$ IV

Show Answer

Answer: (b)

12. Consider the following carbocations & give decreasing order of stability

I. $\mathrm{C} _{6} \mathrm{H} _{5}-\stackrel{\oplus}{\mathrm{C}} \mathrm{H} _{2}$

II. $\mathrm{C} _{6} \mathrm{H} _{5}-\mathrm{CH} _{2}-\stackrel{\oplus}{\mathrm{C}} \mathrm{H} _{2}$

III. $\mathrm{C} _{6} \mathrm{H} _{5}-\stackrel{\oplus}{\mathrm{C}} \mathrm{H}-\mathrm{CH} _{3}$

IV. $\mathrm{C} _{6} \mathrm{H} _{5}-\stackrel{\oplus}{\mathrm{C}}\left(\mathrm{CH} _{3}\right) _{2}$

(a) $\quad$ I $>$ II $>$ III $>$ IV

(b) $\mid$ I $>$ I $>$ III $>$ IV

(d) $\mid$ V $>$ III $>$ I $>$ II

Show Answer

Answer: (d)

13. Which carbocation is the most stable?

Show Answer

Answer: (b)

14. Which one of the carbanions is most stable?

Show Answer

Answer: (a)

15. Which one of the following carbocations is most stable due to resonance?

(a)

(b) $\quad \mathrm{C} _{6} \mathrm{H} _{5}-\stackrel{\oplus}{\mathrm{C}} \mathrm{H} _{2}$

(d)

Show Answer

Answer: (d)

NUCLEOPHILES AND ELECTROPHILES

Reagents in organic chemistry can be broadly classified as electrophilic or nucleophilic reagents. Some of their features are,

Electrophiles

Electron loving species

May be positively charged or neutral

Positive electrophiles

$\mathrm{H}^{+}, \mathrm{Cl}^{+}, \mathrm{NO} _{2}, \mathrm{R}^{+},+\mathrm{NO}$

Neutral electrophiles

$\mathrm{SO} _{3}, \mathrm{BF} _{3}, \mathrm{AlCl} _{3}, \mathrm{R},: \mathrm{CR} _{2}$ (carbenes), : $: \mathrm{NR}$ (nitrenes), $\mathrm{FeCl} _{3}, \mathrm{SnCl} _{4}$.

Being electron deficient, they act as Lewis acids and accept a pair of electrons.

Attack the substrate molecule at the site of highest electron density, to form a covalent bond.

Nucleophiles

Nucleus loving species

May be negatively charged or neutral

Negative nucleophiles

$\mathrm{H}^{-}, \mathrm{Cl}^{-}, \mathrm{I}^{-}, \mathrm{CN}^{-}, \mathrm{NH} _{2}^{-}, \mathrm{RCOO}^{-}, \mathrm{R}^{-}$

Neutral necleophiles

$\mathrm{H} _{2} \underset{0}{0}, \mathrm{NH} _{3}, \mathrm{RNH} _{2}, \mathrm{R}-\ddot{0}-\mathrm{H}, \mathrm{R}-\ddot{0}-\mathrm{R}, \mathrm{R}-\mathrm{MgX}$

Being electron rich, they actas Lewis bases and attack on electrophilic carbon.

Attack the substrate molecule at the site of lowest electron density.

Ambident nucleophiles

Species having two nucleophilic centres, for example $\mathrm{C} \equiv \mathrm{N}$..

Compounds which behave as electrophile as well as nucleophile

Compound in which carbon is bonded with electronegative atom $(0, \mathrm{~N}, \mathrm{~S})$ by multiple bond

Electron Displacements in a covalent bond

Electron displacements may take place in an organic compound, either due to the presence of an atom or group attached to it or in the presence of an attacking reagent. Such electron displacements are.

1.Inductive effect

Displacement of $\sigma$ electrons along a saturated carbon chain whenever an electron withdrawing or electron donating group is present at the end of the carbon chain.

permanent effect

weakens with increasing distance from the substituent

+l effect

if the substituent attached at the end of carbon chain is electron donating, effect is + leffect.

$ \left(\mathrm{CH} _{3}\right) _{3} \mathrm{C}>\left(\mathrm{CH} _{3}\right) _{2} \mathrm{CH}>\mathrm{CH} _{3} \mathrm{CH} _{2}>\mathrm{CH} _{3}>\mathrm{D}>\mathrm{H}$

t-butyl $\qquad $ isopropyl $\qquad $ ethyl $\qquad $ methyl

I effect

if the substituent attached at the end of carbon chain is electron withdrawing, effect is -1 effect.

$\mathrm{NO} _{2}>\mathrm{CN}>\mathrm{SO} _{3} \mathrm{H}>\mathrm{CHO}>\mathrm{CO}>\mathrm{COOH}>\mathrm{COCl}>\mathrm{COOR}>\mathrm{CONH} _{2}>\mathrm{F}>\mathrm{Cl}>\mathrm{Br}>\mathrm{I}>\mathrm{OH}>\mathrm{OR}>$ $\mathrm{NH} _{2}>\mathrm{C} _{6} \mathrm{H} _{5}>\mathrm{H}$

Applications

Electron withdrawing group increases acidic strength $\Rightarrow$ Greater the -I effect, greater is the acidity.

$$ \mathrm{FCH} _{2} \mathrm{COOH}>\mathrm{ClCH} _{2} \mathrm{COOH}>\mathrm{BrCH} _{2} \mathrm{COOH}>\mathrm{ICH} _{2} \mathrm{COOH} $$

As the distance between electron withdrawing group and $\mathrm{COOH}$ group increases, acidity decreases.

Electron donating group decreases acidic strength $\Rightarrow$ Greater the $+I$ effect, lesser is the acidity.

2.Resonance effect

Delocalisation of $\pi$ electrons or non-bonded pair of electrons in a conjugated system.

Conjugated system

A given atom or group of atoms is said to be in conjugation with an unsaturated system if, i. It is directly linked to one of the atoms of the multiple bond through a single bond, and

ii. It has a $\pi$ bond, positive charge, negative charge, odd electron or lone pair.

Type of conjugation

1. $\pi, \pi$ conjugation - All conjugate positions have $\pi$ bonds,

2. Positive charge, $\pi$ conjugation - All conjugate positions have $\pi$ bonds and only one conjugate position has positive charge

3. Negative charge, $\pi$ conjugation - All conjugate positions have $\pi$ bonds and only one conjugate position has negative charge

4. Odd electron, $\pi$ conjugation - All conjugate positions have $\pi$ bonds and one conjugate position has odd electron

5. Lone pair, $\pi$ conjugation - All conjugate positions have $\pi$ bonds and one conjugate position has lone pair

Resonance effect is a permanent effect

Resonating structures are not real, rather real structure is a hybrid of all resonating structures. It is difficult to represent the actual resonance structure by a single structure.

Due to delocalization, the carbon - carbon bond lengths in a resonating structure become equal. And the value of a carbon - carbon bond length is in between that of a carbon carbon single bond and a carbon - carbon double bond.

Delocalization lowers the resonance energy thereby stabilizing the conjugated system.

Stability of resonating structures

1. Species having complete octet is more stable than species having incomplete octet.

2. Species in which positive charge resides on most electropositive atom and negative charge on most electronegative atom is more stable.

3. Increase in charge separation decreases the stability of a resonating structure.

Stability order - I $>$ II = IV $>$ III

Steric inhibition of resonance

Most important condition for resonance to occur is that the involved atoms in resonating structures must be coplanar for maximum delocalisation. If bulky groups are present on adjacent atoms planarity of orbitals is inhibited. This is known as steric inhibition of resonance.

In dimethylaniline, the orbital having lone pair of electrons present on nitrogen atom is in the plane of benzene ring hence lone pair takes part in delocalization.

In $\mathrm{N}, \mathrm{N}$-dimethyl-2, 6-dinitroaniline, the $\mathrm{N}\left(\mathrm{CH} _{3}\right) _{2}$ group is out of the plane of benzene ring owing to the presence of two bulky nitro groups so lone pair of electrons on the nitrogen atoms of $\dot{\mathrm{N}}\left(\mathrm{CH} _{3}\right) _{2}$ group cannot get delocalized through lone pair, $\pi$ conjugation.

Steric inhibition of resonance has profound effect on

1. Physical properties

2. Acidity and basicity

3. Reactivity of organic compounds

Of the two compounds given above (b) is a stronger base than (a) because the steric inhibition of resonance in (b) prevents the lone pair to take part in delocalization and thus increases the availability of lone pair on nitrogen.

Another example is of 2, 4, 6-trinitro-N, N-dimethylaniline which is a much stronger base than 2, 4, 6-trinitroaniline. In 2, 4, 6-trinitroaniline, $\mathrm{H}$-bonding between the oxygen atoms of $\mathrm{NO} _{2}$ groups and the $\mathrm{H}$-atoms of $\mathrm{NH} _{2}$ group holds the entire molecule in the planar orientation. Delocalization of the lone pair of nitrogen with the ring carbon atom takes place and the three $\mathrm{NO} _{2}$ groups are able to exert their powerful electron withdrawing effect thus decreasing the basicity.

In nitrobenzene (I), bond length between carbon-nitrogen is in between single and double bond due to resonance.

In compound (II) bond length between carbon-nitrogen is only of single bond due to inhibition of resonance.

Groups in conjugation with benzene ring can be classified as :

+R group

Groups having - ve charge or at least one lone pair of electrons donate electrons to the benzene ring or any other conjugated system by resonance effect eg.

-R group

Groups having +ve charge on key atom when bonded with electronegative atom by multiple bond, withdraw electrons from the benzene ring or a conjugated system by resonance effect

Resonance effect operates only at ortho and para positions.

3.Hyperconjugation effect

Hyperconjugation effect is the phenomenon when $\sigma$ electrons involve themselves in conjugation with $\pi$ - electrons or p-orbitals. Delocalization of $\sigma$ electrons of a $\mathrm{C}-\mathrm{H}$ bond of an alkyl group takes place when it is attached to an atom of unsaturated system or to an atom with an unshared p-orbital. It is a permanent effect.

Structural requirement for hyperconjugation

Compound should have atleast one $\mathrm{sp}^{2}$ carbon of either alkene, alkyl carbocation or alkyl free radical.

Hybridisation of $\alpha$-carbon should be $\mathrm{sp}^{3}$ and this $\mathrm{sp}^{3}$ carbon should have atleast one hydrogen.

Types of hyperconjugation

1. $\sigma(\mathrm{C}-\mathrm{H}), \pi$ conjugation-occurs in alkenes and alkyl arenes.

2. $\sigma(\mathrm{C}-\mathrm{H})$, +ve charge conjugation-occurs in alkyl carbocation.

3. $\quad \sigma(\mathrm{C}-\mathrm{H})$, odd electron conjugation-occurs in alkyl free radicals

Hyperconjugative structures

Since there is no bond between $\mathrm{C}$ and $\mathrm{H}$ atoms in these structures, hyperconjugation is known as no bond resonance. Although a free proton has been shown but it is still bound quite firmly to the $\pi$ cloud and hence it is not free to move.

Applications of hyperconjugation

This effect has been able to explain a number of otherwise unexplained facts,

1.Stability of alkenes

Greater the number of alkyl groups attached to the doubly bonded carbon atoms, greater is the stability of alkene. In other words, more substituted alkenes are more stable

Greater the number of $\alpha \mathrm{H}$ ’s, greater the number of hyperconjugative structures, more is the stability of alkene.

2. Directive influence of alkyl groups

Due to hyperconjugation, electron density at 0 and $p$ positions increases hence, electrophilic substitution in toluene occurs at ortho and para positions.

3. Carbon-Carbon double bond length in alkenes

Because of hyperconjugation, $\mathrm{C} _{2}-\mathrm{C} _{3}$ single bond in propene acquires partial double bond character hence is little shorter (1.49Å) than normal C-C single bond (1.54Å) in propene.

4. Stability of alkyl carbocations and alkyl free radicals

Stability $\propto$ number of resonating structures $\propto$ number of $\alpha$ H’s.

5. Reversal of inductive effect order of alkyl groups

When an alkyl group is attached to an unsaturated system such as a double bond or benzene ring, order of inductive effect is reversed

This is because greater the $\alpha-H$ ’s, more hyperconjugative structures are formed and more is the electron releasing tendency of alkyl group

Applications of conjugation and resonance

1.Aromatic character of compounds

According to Huckel’s rule, a compound is aromatic if it is

Cyclic

Planar

Conjugated

Has $(4 n+2) \pi$ electrons, $n=0,1,2,3, \ldots \ldots$

2. Antiaromatic compounds

Planar conjugated cyclic systems containing $4 n \pi$ electrons ( $n=1,2,3, \ldots .$.

3. Non aromatic systems

Some antiaromatic compounds adopt non planar geometries to become stable. They have $4 \pi$ electrons butare non planar.

cyclooctatetraene (tub shaped)

Types of Organic Reactions

Organic reactions are broadly divided into the following four categories

I) Substitution Reactions

When an atom or group of atoms from a molecule is replaced by another atom or group of atoms, it is known as a substitution reaction.

ii) Addition Reactions

When an atom or group of atoms are added to a substrate molecule, which is unsaturated, it is known as addition reaction. In this reaction there is a net gain of atoms in the product molecule.

iii) Elimination Reactions

When atoms or group of atoms are removed from a molecule resulting in the formation of multiple linkage in that molecule, it is known as elimination reaction. The loss of atoms or group of atoms may occur from the same atom or different atoms in the molecule.

iv) Rearrangement Reactions

When atoms or group of atoms within the molecule simply change their position to give the product, it is known as a rearrangement reaction. It may appear as an internal substitution reaction has occured.

Practice Questions

1. Which among the following compounds behaves both as an electrophile as well as nucleophile?

I. $\mathrm{H} _{2} \mathrm{C}=\mathrm{CH} _{2}$

II. $\mathrm{H} _{2} \mathrm{C}=\mathrm{CH}-\mathrm{CH} _{2}$

III.

IV.

(a) only 1

(b) 1 and 2

(d) 2,3 and 4

Show Answer

Answer: (c)

2. Consider the following alkenes:

The correct sequence of these alkenes in increasing order of their stability is :

(a) III > II > I

(b) I > II > III

(d) II > III > I

Show Answer

Answer: (c)

3. Arrange the following resonating structures of formic acid in decreasing order of stability :

(a) I > II > IV > II

(b) I > II > III > IV

(d) I > IV > II > III

Show Answer

Answer: (b)

4. The correct stability order of the following resonance structures is

I. $\mathrm{H} _{2} \mathrm{C}=\mathrm{N}^{+}=\mathrm{N}^{-}$

II. $\mathrm{H} _{2} \mathrm{C}^{+}=\mathrm{N}=\mathrm{N}^{-}$

III. $\mathrm{H} _{2} \mathrm{C}^{-}=\stackrel{+}{\mathrm{N}} \equiv \mathrm{N}$

IV. $\mathrm{H} _{2} \overline{\mathrm{C}}-\mathrm{N}=\stackrel{+}{\mathrm{N}}$

(a) $\quad$ (I) $>$ (II) $>$ (IV) $>$ (III)

(b) $\quad$ (I) $>$ (III) $>$ (II) $>$ (IV)

(d) $\quad$ (III) $>$ (I) $>$ (IV) $>$ (II)

Show Answer

Answer: (a)

5. Arrange the following groups in order of decreasing $-\mathrm{R}$ (or $-\mathrm{M})$ power :

$\mathrm{NO} _{2}$
$\mathrm{SO} _{3} \mathrm{H}$
$\mathrm{CF} _{3}$
$\mathrm{CHO}$

(a) $1>3>2>4$

(b) $1>2>4>3$

(d) $4>3>2>1$

Show Answer

Answer: (a)

6. Which one of the following statements is not correct for electrophile?

(a) Electron deficient species are electrophile

(b) Electrophiles are Lewis acids

(d) $\mathrm{AICl} _{3}, \mathrm{SF} _{6}, \mathrm{IF} _{7}$ and $\mathrm{SO} _{3}$ are electrophiles

Show Answer

Answer: (c)

7. $+R$ power of the given groups in decreasing order is :

$-0^{-}$
$-\mathrm{NH} _{2}$
$-\mathrm{OH}$
$-NHCOCH_3$

(a) $1>2>3>4$

(b) $4>3>2>1$

(d) $1>4>3>2$

Show Answer

Answer: (a)

8. In pyridine

Number of conjugated electrons is

(a) 6

(b) 8

(d) 5

Show Answer

Answer: (a)

9. Decreasing - I power of given groups is :

$\mathrm{CN}$
$\mathrm{NO} _{2}$
$\mathrm{NH} _{3}$
$\mathrm{F}$

(a) $2>1>4>3$

(b) $2>3>4>1$

(d) $3>2>1>4$

Show Answer

Answer: (d)

10. In which of the following pairs, none of the species is an electrophile?

(a) $\quad \mathbf{N H} _{3}$ and $\mathbf{N F} _{3}$

$\qquad $ (I) $\qquad \quad $ (II)

(b) $\mathrm{PH} _{3}$ and $\ddot{\mathrm{P} C l} _{3}$

$\qquad $ (I) $\qquad \quad $ (II)

(d) $\mathrm{H} _{2} \stackrel{\text { Ọa }}{ }$ and $\ddot{\mathrm{CCCl}}$

$\qquad $ (I) $\qquad \quad $ (II)

Show Answer

Answer: (c)

11. Which of the following reactions would generate an electrophile?

I. $\left(\mathrm{CH} _{3}\right) _{3} \mathrm{CBr}+$ anhy $\mathrm{AlCl} _{3}$

II. $\mathrm{CH} _{3}-\mathrm{CH}=\mathrm{CH} _{2}+\stackrel{\oplus}{\mathrm{H}}$

III. $\mathrm{C} _{6} \mathrm{H} _{5} \mathrm{COOH}+\mathrm{H} _{3}{ }^{\oplus}$

IV. $\mathrm{HNO} _{3}+\mathrm{H} _{2} \mathrm{SO} _{4}$

(a) I, II and IV

(b) I, II and III

(d) II, III and IV

Show Answer

Answer: (a)

12. In which of the following compounds delocalisation is not possible?

(a) 1,4-pentadiene

(b) 1,3-butadiene

(d) benzene

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Answer: (a)

13. Which among the following species is an ambident nucleophile?

(a) $\mathrm{CH} _{3}-\mathrm{CH} _{2}$

(b) $\mathrm{H} _{2} \mathrm{C}=\mathrm{CH} _{2}$

(d) $\mathbf{\mathrm { N }} _{3}$

Show Answer

Answer: (c)

14. Which among the following is an electrophile?

(a) $\mathrm{CO} _{2}$

(b) $\mathrm{SO} _{3}$

(d) All of these

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Answer: (d)

15. Which one of the following species is a nucleophile:

(a) $ \mathrm{NF} _{3}$

(b) $\mathrm{PCl} _{3}$

(d) $\mathrm{OF} _{2}$

Show Answer

Answer: (c)

16. Which one of the following compounds on gentle heating will undergo facile homolytic bond cleavage?

(a)

(b) $\left(\mathrm{CH} _{3}\right) _{3} \mathrm{C}-\mathrm{O}-\mathrm{C}\left(\mathrm{CH} _{3}\right) _{3}$

(d) $ (CH _{3}) _{3}C-O-O-C(CH _{3}) _{3}$

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Answer: (d)

17. Resonance is due to:

(a) delocalisation of sigma electrons

(b) migration of $\mathrm{H}$ atoms

(d) delocalisation of pi electrons

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Answer: (d)

18. Hyperconjugation effect is possible in which of the following species.

(a) $\stackrel{\oplus}{\mathrm{C}} \mathrm{H} _{3}$

(b) $\mathrm{C} _{6} \mathrm{H} _{5}-\mathrm{CH} _{3}$

(d)

Show Answer

Answer: (b)