Let us consider the heterolytic fission of the bond C-X present in an organic molecule. If the carbon atom has greater electronegativity than the atom X, the former takes away the bonding electron pair and acquires a negative charge. The resulting carbon anion is known as carbanion.

(i) The benzylic free radical is resonance stabilised. It possess more resonance structures.
(ii) Allylic free radical has only two resonating structures, so it has less delocalisation.

When an electrophile such as H⁺ approaches an alkene molecule, the π electrons are instantaneously shifted to the electrophile and a new bond is formed between carbon and hydrogen. is makes the other carbon electron decient and· hence it acquires a positive charge.

When the π electron is transferred toward the attacking reagent, it is called +E (positive electromeric) effect.

The addition of H⁺ to alkene as shown above is an example of +E effect.
When the electron is transfered away from the attacking reagent, it is called, -E (negative electromeric) effect.

The attack of CN^- on a carbonyl carbon, as shown above, is an example of -E effect

(i) C₂H₄ is CH₂=CH₂ is a simple alkene. A is ethylene.
(ii) Ethylene (A) reacts with dil H₂SO4 to give ethanol (B)
\(\underset { Ethylene\\ \quad \quad \quad (A) }{ { CH }_{ 2 }{ CH }_{ 2 } } \overset { dil{ H }_{ 2 }{ SO }_{ 4 } }{ \longrightarrow } \underset { ethanol\\ \quad \quad (B) }{ { CH }_{ 3 }-{ CH }_{ 2 }-OH } \) 
(iii) Ethylene (A) reacts with Cl₂ to give 1,2 dichloro ethane (C)
\(\underset { (Ethylene)\\ \quad \quad \quad (A) }{ { CH }_{ 2 }={ CH }_{ 2 } } \overset { { cl }_{ 2 } }{ \longrightarrow } \underset { (1,2-dicholoro\quad ethene) }{ \underset { \overset { | }{ Cl } }{ { CH }_{ 2 } } -\underset { \overset { | }{ Cl } }{ { CH }_{ 2 } } } \) 

Substitution reactions are classified into three types. They are,
(i) Nucleophilic substitution reaction.
(ii) Electrophilic substitution reaction.
(iii) Free radical substitution reaction.
(i) Nucleophilic substitution reaction: A nucleophilic substitution reaction in organic chemistry is a type of reaction where a nucleophilic gets attached to the positive charged atoms or molecules of the other substance. A good example of a nucleophilic substitution reaction is the hydrolysis of alkyl bromide, under the basic conditions where in the nucleophile is nothing but the base \(\overset { \ominus }{ O } H\), whereas the leaving group is the Br^Ө. The reaction for the following is as give below.
\(\underset { (mentyl\quad bromide) }{ { CH }_{ 3 }Br } \overset { aq.\overset { \ominus }{ O } H }{ \longrightarrow } \underset { (methanol) }{ { CH }_{ 3 }OH } +{ Br }^{ \ominus }\) 

(ii) Electrophilic substitution reaction: The electrophilic substitution reaction involves the electrophiles. The electrophilic reactions occur mostly with the aromatic compounds. This types of substitution reaction are basically defined as those chemical reactions where the electrophile replaces the functional group in a compound but not the hydrogen atom, sometimes hydrogen atom can be also replaced by electrophiles. For example:

(iii) Free radical substitution reaction: Free radical substitution reaction involving free radicals are a reactive intermediate
\(A-X+\overset { \bullet }{ Y } \longrightarrow A-Y+\overset { \bullet }{ X } \) 
\({ CH }_{ 4 }+\overset { \bullet }{ C } l\longrightarrow \overset { \bullet }{ C } { H }_{ 3 }+HCl\) 
\(\overset { \bullet }{ C } { H }_{ 3 }+\overset { \bullet }{ C } l\longrightarrow { CH }_{ 3 }Cl\)

Addition reactions are classified into three types. They are,
(i) Electrophilic addition reaction
(ii) Nucleophilic addition reaction
(iii) Free radical addition reaction
(i) Electrophilic addition reaction: An electrophilic addition reaction can be described as an addition reaction in which a reactant with multiple bonds as in a double or triple bond undergoes has its n bond broken and two new a bond are formed.

(il) Nucleophilic addition reaction: A nucleophilic addition reaction is an addition reaction where a chemical compound with an electron deficient or electrophilic double or triple bond, a n bond, reacts with a nucleophilic which is an electron rich reactant with the disappearance of the double bond and creation of two new single or a bonds.

(iii) Free radical addition reaction: It is an addition reaction in organic chemistry involving free radicals. The addition may occur between a radical and a non radical or between two radicals.
\({ CH }_{ 2 }={ CH }_{ 2 }+H-Br\overset { Benzoyl\quad peroxide }{ \longrightarrow } { CH }_{ 3 }-{ CH }_{ 2 }-Br\)

All organic reactions can be understood by following the electron movements.
(i) Lone pair becomes a bonding pair.
(ii) Bonding pair becomes a lone pair.
(iii) A bond breaks and becomes another bond.
The electron movement depends on the nature of the substrate, reagent and the prevailing conditions.
Type 1.A lone pair to a bonding pair

Type 2. A bonding pair to a lone pair

Type 3. A bonding pair to an another bonding pair

There are three types of electron movement viz.,
1. lone pair becomes a bonding pair.
2. bonding pair becomes a lone pair
3. a bond breaks and becomes another bond
Type 1: A lone pair to a bonding pair

Type 2: A bonding pair to a lone pair

Type 3: A bonding pair to an another bonding pair

(i) In triphenylmethyl cation, due to resonance the positive charge can move at both the o- and p-positions of each benzene ring.

[Six more structures are possible due to resonance in other two benzene rings]
(ii) Totally nine resonance structures are possible for 3 benzene rings
(iii) ∴ Triphenylmethyl cation is highly stable due to these nine resonance structure.

(i) 2-methyl butane has four different sets of equivalent H-atoms.
(ii) Removal of H-atom from any of C-atom gives four different carbocations.
(i) \(C{ H }_{ 3 }\rightarrow \overset { \beta }{ \underset { \overset { | }{ C{ H }_{ 3 } } }{ CH } } \rightarrow \overset { \alpha }{ C{ H }_{ 2 } } \rightarrow \overset { + }{ C{ H }_{ 2 } } \)
(ii) \(C{ H }_{ 3 }\rightarrow \underset { \overset { | }{ C{ H }_{ 3 } } }{ CH } -\overset { + }{ C{ H } } \leftarrow C{ H }_{ 3 }\)
(iii) \(C{ H }_{ 3 }\rightarrow \overset { + }{ \underset { \overset { | }{ C{ H }_{ 3 } } }{ C } } \leftarrow C{ H }_{ 2 }-C{ H }_{ 3 }\)
(iv) \(\overset { + }{ C{ H }_{ 2 } } \leftarrow \underset { \overset { | }{ C{ H }_{ 3 } } }{ CH } \leftarrow C{ H }_{ 2 }-C{ H }_{ 3 }\)
(iii) Stability order of carbocation is 3⁰ > 2⁰ > 1⁰
(iv) Though I & IV are primary carbocations, I has (α-CH₃ group at β-carbon and while IV has -CH₃ at α-carbon.
(v) +I effect decreases with distance, hence IV is more stable than I
(vi) ∴ The overall stability is I < IV < II < III

(i) CH₃ - OH : Does not exist as resonance hybrid due to absence of π-electrons.
(ii) R-CO NH₂ : Can exist as resonance hybrid due to the presence of non-bonding electrons on N and n-electrons on C = O bond.

(iii) CH₃-CH = CH-CH₂NH₂: Does not exist as resonance hybrid, because the lone pair on N-atom is not conjugated with n-electrons of the double bond

In this reaction an atom or a group of atoms attached to a carbon atom is replaced by a new atom or a group of atoms. Based on the nature of the attacking reagent, this reactions can be classied as
i) Nucleophilic substitution
ii) Electrophilic substitution
iii) Free radical substitution
(i) Nucleophilic substitution:
This reaction can be represented as

Here Y- is the incoming nucleophile or and attacking species and x- is the leaving group.
Example: Hydrolysis of alkyl halides
\(C{ H }_{ 3 }\overset { aqueous\quad OH }{ \longrightarrow } C{ H }_{ 3 }OH+Br\)
Aliphatic nucleophilic substitution reactions take places either by S_N1 or S_N2 mechanism.
(ii) Electrophilic substitution:

Here Y+ is an electrophile
Example: Nitration of Benzene

(iii) Free radical substitution:
\(A-X+{ Y }^{ . }\longrightarrow A-Y+\overset { . }{ X } \)
\(C{ H }_{ 4 }+\overset { . }{ C } l\longrightarrow \overset { . }{ C } { H }_{ 3 }+HCl\)
Aliphatic electrophilic substitution
A general aliphatic electrophilic substitution is represented as
\(R-X+{ E }^{ \oplus }\longrightarrow R-E+{ X }^{ \oplus }\)
\({ R }_{ 2 }NH+N\overset { + }{ O } \longrightarrow { R }_{ 2 }N-NO+{ H }^{ + }\)