Points to remember in General Organic Chemistry

Ortho,Meta and Para Directing Groups

PYQ-2024-Hydrocarbons-Q7

Inductive effect

PYQ-2024-Genral_Organic_Chemistry-Q12 PYQ-2024-Alcohols-Q8,PYQ-2023 Alcohols Q5

The normal $\mathrm{C}-\mathrm{C}$ bond has no polarity as two atoms of same electronegativity (EN) value are connected to each other. Hence the bond is nonpolar.

Consider a carbon chain in 1-Chloro butane, here due to more $\mathrm{EN}$ of $\mathrm{Cl}$ atom $\mathrm{C}-\mathrm{Cl}$ bond pair is slightly displaced towards Cl atom hence creating partial negative $(\delta^{-})$ charge over Cl atom and partial positive $\left(\delta^{+}\right)$ charge over $\mathrm{C}_{1}$ atom.

Now since $C_{1}$ is slightly positive, it will also cause shifting of $C_{1}-C_{2}$ bond pair electrons towards itself causing $C_{2}$ to acquire small positive charge. Similarly $\mathrm{C}_{3}$ acquires slightly positive charge creating an induction of charge in carbon chain. Such an effect is called inductive effect.

Diagram showing I effect

The arrow shows electron withdrawing nature of $-\mathrm{Cl}$ group.

Thus inductive effect may be defined as a permanent displacement of $\sigma$ bond pair electrons due to a dipole. (Polar bond)

Some important points are:

(a) It can also be defined as polarization of one bond caused by polarization of adjacent bond.

(b) It is also called transmission effect.

(c) It causes permanent polarization in molecule, hence it is a permanent effect.

(d) The displacement of electrons takes place due to difference in electronegativity of the two atoms involved in the covalent bond.

(e) The electrons never leave their original atomic orbital.

(f) Its magnitude decreases with distance and it is almost negligible after 3rd carbon atom.

(g) The inductive effect is always operative through $\sigma$ bond, does not involve $\pi$ bond electron.

Types of inductive effects :

(a) - I Effect : The group which withdraws electron cloud is known as - I group and its effect is called - I effect. Various groups are listed in their decreasing - I strength as follows.

$\oplus \quad\quad\quad\quad \oplus \quad\quad\quad\quad \oplus$

$-NR_3 > -SR_2 > -NH_3 > -NO_2 > -SO_{2}R > -CN > -CHO > -COOH > -F > -Cl > -Br > -I > -OR > -OH > -C = CH > -NH_2 > - C_6H_5 > -CH = CH_2 > -H$.

(b) + I effect : The group which release electron cloud is known as + I group and effect is $+\mathrm{I}$ effect.

$-O^{\ominus} > -COO^{\ominus} > -C(CH_3)_3 > -CH(CH_3)_2 > -CH_2-CH_3 > -CH_3 >-D > -H $

The hydrogen atom is reference for $+I$ and $-I$ series. The inductive effect of hydrogen is assumed to be zero.

Ex. Let us consider effect of $COOH$ & $-COO^{-}$ in carbon chain

(i) ${ }^{\ominus} OOC \rightarrow \stackrel{\delta-}CH_2 \rightarrow \stackrel{\delta \delta-}CH_2 \rightarrow CH_3$

(ii) $HOOC \rightarrow \stackrel{\delta+}CH_2 \rightarrow \stackrel{\delta \delta+}CH_3 $

Due to $\mathrm{e}^{-}$ donating nature of $-\mathrm{COO}^{\ominus}$ carbon chain has become partially negative but $-\mathrm{COOH}$ is - I group therefore carbon chain has become partially positive.

Resonance

PYQ-2024-Genral_Organic_Chemistry-Q24, PYQ-2024-Genral_Organic_Chemistry-Q22, PYQ-2024-Genral_Organic_Chemistry-Q16, PYQ-2024-Genral_Organic_Chemistry-Q11, PYQ-2024-Genral_Organic_Chemistry-Q8, PYQ-2023 General Organic Chemistry Q1, PYQ-2023 General Organic Chemistry Q2, PYQ-2023 General Organic Chemistry Q17

Resonance is the phenomenon in which two or more structures involving in identical position of atom, can be written for a particular species, all those possible structures are known as resonating structures or canonical structures. Resonating structures are only hypothetical but they all contribute to a real structure which is called resonance hybrid. The resonance hybrid is more stable than any resonating structure.

Resonance hybrid :

The most stable resonating structure contribute maximum to the resonance hybrid and less stable resonating structure contribute minimum to resonance hybrid.

Conjugation:

A given atom or group 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.

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

Types of Conjugation :

(i) Conjugation between $\mathrm{C}=\mathrm{C}$ and

(ii) Conjugation between +ve charge and

(iii) Conjugation between lone pair and

(iv) Conjugation between odd electron and

(v) Conjugation between negative charge and

Mesomeric effect

PYQ-2024-Hydrocarbons-Q3, PYQ-2024-Genral_Organic_Chemistry-Q5

Mesomeric effect is defined as permanent effect of $\pi$ electron shifting from multiple bond to atom or from multiple bond to single bond or from lone pair to single bond. This effect mainly operates in conjugated system of double bond. So that this effect is also known as conjugate effect.

Ex.

Types of Mesomeric effects :

(a) Positive Mesomeric effect ( $+M$ effect) :

When the group donates electron to the conjugated system it shows $+M$ effect.

Relative order of $+\mathrm{M}$ groups (usually followed) :

$-\stackrel{\ominus}O>-NH_{2}>-NHR>-NR_{2}>-OH>-OR>-NHCO>-OCOR>-Ph>-F>-Cl>-Br>-I>-NO$

Ex.

(b) Negative Mesomeric effect ( $-M$ effect) :

When the group withdraws electron from the conjugated system, it shows

$-M$ effect

Relative order of -M groups (usually followed) :

Ex. (i)

(ii)

Hyperconjugation

It is delocalization of sigma electron with p-orbital. Also known as $\sigma ~ \pi$ conjugation or no bond resonance. It may takes place in alkene, alkynes, carbocation, free radical, benzene nucleus.

Necessary Condition : Presence of at least one hydrogen at saturated carbon which is $\alpha$ with respect to alkene, alkynes, carbocation, free radical, benzene nucleus.

(i) Hyperconjugation in alkene

(ii) Hyperconjugation in carbocation

(iii) Hyperconjugation in radical

Electromeric Effect

PYQ-2024-Genral_Organic_Chemistry-Q12

The electromeric effect can be defined as a temporary effect produced when a reagent attacks the multiple bonded compound, causing a complete shift of pi electrons to either of the two atoms of the bond. This total transfer of the shared pair of electrons produces polarity.

Structural Effects on Acidity and Basicity

PYQ-2024-Genral_Organic_Chemistry-Q25

The more stable conjugate base, the weaker the conjugate base is, and the stronger the acid is.

Stability of conjugate base is directly proportional to acidic strength.General Organic Chemistry

Aromatic character [The Huckel $4 n+2$ rule]

PYQ-2024-Genral_Organic_Chemistry-Q30, PYQ-2024-Genral_Organic_Chemistry-Q14, PYQ-2024-Genral_Organic_Chemistry-Q15

The following rules are useful in predicting whether a particular compound is aromatic or non-aromatic. Aromatic compounds are cyclic and planar. Each atom in an aromatic ring is ${sp}^{2}$ hybridised. The cyclic $\pi$ molecular orbital (formed by overlap of p-orbitals) must contain $(4 n+2) \pi$ electrons, i.e.,$ 2,6,10,14 \ldots \ldots . . \pi $ electrons. Where $\mathrm{n}=$ an integer $0,1,2,3$,

Aromatic compounds have characteristic smell, have extra stability and burn with sooty flame.

Comparison between aromatic, anti aromatic and non-aromatic compounds.

Stability of compounds : Aromatic > Non-Aromatic > Anti-Aromatic

PYQ-2023 General Organic Chemistry Q11

(A) Carbocation :

PYQ-2024-Genral_Organic_Chemistry-Q37, PYQ-2024-Genral_Organic_Chemistry-Q39

Definition : A carbon intermediate which contain three bond pair & a positive charge on it is called carbocation.

Hybridization : Carbocation may be $sp^2$ & $sp$ hybridized

Hybridization $\quad \quad \quad \quad \quad $ Example

$sp^2$ $ \quad \quad \quad \quad \quad $ $\stackrel \oplus{C}H_{3}, CH_3 \stackrel \oplus{C}H_{2}, CH_3 \stackrel \oplus{C}HCH_{3}, (CH_3)_3 \stackrel\oplus{C}$

$sp$ $ \quad \quad \quad \quad \quad $ $\mathrm{H}_{2} \mathrm{C}=\stackrel{\oplus}{\mathrm{C}} \mathrm{H}, \mathrm{HC} \equiv \stackrel{\oplus}{\mathrm{C}} $

Carbocations are electron deficient. They have only six electrons in their valence shell, and because of this, carbocations act as Lewis acids. Most of the carbocations are short-lived and highly reactive, they occur as intermediates in some organic reactions. Carbocations react with Lewis bases or ions that can donate the electron pair, that they need to achieve a stable octet of electrons (i.e., the electronic configuration of a noble gas):

Because carbocations are electron seeking reagents, chemists call them electrophiles. All Lewis acids, including protons, are electrophiles. By accepting an electron pair, a proton achieves the valence shell configuration of helium; carbocations achieve the valence shell configuration of neon.

Stability : Carbocations are stabilized by

(i) + I effect

(ii) $+M$ effect

(iii) Hyperconjugation

(iv) Delocalization of charge

General stability order :

(B) Carbanion :

Definition: A carbon intermediate which contain three bond pair and a negative charge on it, is called carbanion.

Hybridization : Hybridization of carbanion may be $sp^3, sp^2$ & $sp$

Hybridization $ \quad \quad \quad \quad \quad $ Example

Stability of carbanion: Carbanions are stabilized by electron withdrawing effect as

(i) - I effect

(ii) $-\mathrm{m}$ effect

(iii) Delocalization of charge

(C) Free radicals :

Homolysis of covalent bond results into free radical intermediates possess the unpaired electrons.

It is generated in presence of Sun light, Peroxides or High temperature

Free radical : An uncharged intermediate which has three bond pair and an unpaired electron on carbon.

(i) It is Neutral species with odd $\mathrm{e}^{-}$

(ii) It is paramagnetic in nature due to odd $\mathrm{e}^{-}$

(iii) No rearrangement is observed generally.

(iv) Carbon atom having odd electron is in $\mathrm{sp}^{2}$ hybridized state

(v) Any reaction if it is carried out in the presence of sunlight, peroxide or high temperature it generally proceeds via free radical intermediate.

Stability of free radical : It is stabilized by resonance, hyperconjugation and + I groups.

Ex. $(H_3C)_{3}C^{\bullet}>H_3 C-\dot{C} H-CH_3>H_3 C-\dot{C} H_2>\dot{C} H_3$

(Stability order)

(D) Carbenes (Divalent Carbon intermediates) :

Definition : There is a group of intermediates in which carbon forms only two bonds. These neutral divalent carbon species are called carbenes. Most carbenes are highly unstable that are capable of only fleeting existence. Soon after carbenes are formed, they usually react with another molecules. Methods of preparation of carbene :

(E) Nitrenes:

The nitrogen analog of carbenes are nitrenes. They are very much reactive since in them octet of $\mathrm{N}$ is incomplete. In nitrenes only one valencies of $\mathrm{N}$ are satisfied.

$$ \mathrm{R}-\ddot{\mathrm{N}} \quad \quad \quad \mathrm{H}-\ddot{\mathrm{N}} $$

(F) Benzyne :

The benzene ring has one extra $\mathrm{C}-\mathrm{C} \pi$ bond in benzyne

Clearly, we can see that the newly formed $\pi$ bond cannot enter in resonance with other $\pi$ orbitals of ring. since it is in perpendicular plane.

It is also important to note that hybridization of each carbon involved in ‘Benzyne bond’ is $\mathrm{sp}^{2}$ since the overlap between these $\mathrm{sp}^{2}$ hybrid orbitals is not so much effective.

Retardation factor

PYQ-2023 General Organic Chemistry Q9

$\mathrm{R}_{\mathrm{f}}=\frac{\text { Distance moved by the substance from base line }}{\text { Distance moved by the solvent from base line }}$

Conformation

PYQ-2024-Genral_Organic_Chemistry-Q20 , PYQ-2023 General Organic Chemistry Q3

Sawhorse projections

Newman projections

Relative stability of conformations:

• In staggered form of ethane, the electron clouds of carbon-hydrogen bonds are as far apart as possible. Thus, there are minimum repulsive forces, minimum energy and maximum stability of the molecule. On the other hand, when the staggered form changes into the eclipsed form, the electron clouds of the carbon – hydrogen bonds come closer to each other resulting in increase in electron cloud repulsions. To check the increased repulsive forces, molecule will have to possess more energy and thus has lesser stability.

• The repulsive interaction between the electron clouds, which affects stability of a conformation, is called torsional strain. Magnitude of torsional strain depends upon the angle of rotation about C–C bond. This angle is also called dihedral angle or torsional angle. Of all the conformations of ethane, the staggered form has the least torsional strain and the eclipsed form, the maximum torsional strain. Therefore, staggered conformation is more stable than the eclipsed conformation.

Bond Cleavage

PYQ-2024-Genral_Organic_Chemistry-Q2

Homolytic fission is the symmetrical breakdown of a covalent connection in which each leaving atom removes one electron from the bonded pair.

Heterolytic fission is the unsymmetrical breakdown of a covalent bond in which one of the leaving atoms retains the bonded pair.