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
\(R-X+{ E }^{ \oplus }\longrightarrow R-E+{ X }^{ \oplus }\)
\({ R }_{ 2 }NH+N\overset { + }{ O } \longrightarrow { R }_{ 2 }N-NO+{ H }^{ + }\)

Electromeric is a temporary effect which operates in unsaturated compounds (containing >C = C <, > C = O, etc...) in the presence of an attacking reagent.
Let us consider two different compounds
(i) Compounds containing carbonyl group (> C = O) and
(ii) Unsaturated compounds such as alkenes (> C = C <).
When a nucleophile approaches the carbonyl compound, the \(\pi\) electrons between C and O is instantaneously shifted to the more electronegative oxygen. This makes the carbon electron deficient and thus facilitating the formation of a new bond between the incoming nucleophile and the carbonyl carbon atom.

On the other hand When an electrophile such as H⁺ approaches an alkene molecule, the π electrons are instantaneously shified 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.

The electromeric effect, is denoted as E effect. Like the inductive effect, the electromeric effect is also classified as +E and -E based on the direction in which the pair of electron is transfered to form a new bond with the attacking agent.
When the π electron is transfer red towards 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 π electron is transfered away from the attacking reagent, it is called, -E (negativc electromeric) effect.
The attack of CN- on a carbonyl carbon, as shown above, is an example of -E effect.

a) Electrophiles:
Electrophiles are reagents that are attracted towards negative charge or electron rich center. They are either positively charged ions or electron deficient neutral molecules. All Lewis acids act as electrophiles.
Neutral molecules like SnCl₄ can also act as an electrophile, as it has vacant d-orbitals which can accommodate the electrons from others.

b) Nucleophiles:
Nucleophiles are reagents that has high affinity for electro positive centers. They possess an atom has an unshared pair of electrons, and hence it is in search for an electro positive centre where it can have an opportunity to share its elections to form a covalent bond, and gets stabilised.
They are usually negatively charged ions or electron rich neutral molecules (contains one or more lone pair of electrons). AII Lewis bases act as nucleophiles.

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

Inductive effect (I):
(i) Inductive effect is defined as the change in the polarisation of a covalent bond due to the presence of adjacent bonds, atoms or groups in the molecule. This is a permanent phenomenon.
(ii) Let us explain the inductive effect by considering ethylchloride as example. The C-C bond in ethyl chloride is polar.
We know that chlorine is more electronegative than carbon, and hence it attracts the shared pair of electron between C-Cl in ethyl chloride towards itself. is develops a slight negative charge on Chlorine and a slight positive charge on carbon to which chlorine is attached.
To compensate it, the C₁ draws the shared pair of electron between itself and. C₂ . This polarisation effect is called inductive effect.
(iii) The magnitude of the charge separation decreases rapidly, as we move away from C₁ and is observed maximum for 2 carbons and almost insignicant after 4 bonds from the active group.
\(\overset { \delta }{ C } \overset { \delta + }{ { H }_{ 3 } } \longrightarrow \underset { 1 }{ C\overset { \delta + }{ { H }_{ 2 } } } \twoheadrightarrow \overset { \delta - }{ C } { l }_{ 2 }\)
It is important to note that the inductive effect does not transfer electrons from one atom to another but the displacement effect is permanent. The inductive effect represents the ability of a particular atom or a group to either withdraw or donate electron density to the attached carbon. Based on this ability the substituents are classified as +I groups and -I groups. Their ability to release or withdraw the electron through sigma covalent bond is called +I effect and -I effect respectively.
Highly electronegative atoms and atoms of groups which are carry a positive charge are electron withdrawing or -I group
Example : -F, -CI, -COOH, -NO₂, NH₂,
Higher the electronegativity of the substitutent, greater is the -I effect.
The order of the -I effect of some groups are given below :
NH³⁺ > NO₂ > CN > SO₃H > CHO > CO > COOH > COCI > CONH₂ > F > Cl > Br > I > OH > OR, NH₂ > C₆H₅ > H
Highly electropositive atoms and atoms are groups which carry a negative charge are electron donating or +I groups.
Example. Alkali metals, alkyl groups such as methyl, ethyl, negatively charged groups such as CH₃O^-, C₂H₅O^-, COO^- etc.
Lesser the electronegativity of the elements, greater is the +I effect. The relative order of +I effect of some alkyl groups is given below
\(-\mathrm{C}\left(\mathrm{CH}_3\right)_3>-\mathrm{CH}\left(\mathrm{CH}_3\right)_2>-\mathrm{CH}_2 \mathrm{CH}_3>-\mathrm{CH}_3\)
Let us understand the influence of inductive effect on some properties of organic compounds.
Reactivity :
When a highly electronegative atom such as halogen is attached to a carbon then it makes the C-X bond polar. In such cases the -I effect of halogen facilitates the attack of an incoming nucleophile at the polarised carbon, and hence increases the reactivity.

If a - I group is attached nearer to a carbonyl carbon, it decreases the availability of electron density-on the carbonyl carbon, and hence increases the rate of the nucelophilic addition reaction.
Aciditv of carborvlic acids :
When a halogen atom is attached to the carbon which is nearer to the carboxylic acid group, its -I effect withdraws the bonded electrons towards itself and makes the ionisation of H⁺ easy. The acidity of various chloro acetic acid is in the following order. The strength of the acid increases with increase in the -I effect of the group attached to the carboxyl group.
Tiichloro acetic acid > Dichloro acetic acid > Chloro acetic acid > acetic acid

(i) Let us consider the heterolytic fission of the bond C-X present in organic molecule. If the atom X has greater electronegativity than the carbon atom, the former takes away the bonding electron pair and becomes negatively charged while the carbon will lose its electron and thus acquire a positive charge.
(ii) Such a cationic species carrying a positive charge on carbon are known as carbocation or carbonium ion.

There are three types of fission of a covalent bond.
They are,
(i) Homolytic cleavage
(ii) Heterolytic cleavage

(i) Three highly electronegative oxygen atoms are attached to sulphur atom in SO₃. It makes sulphur electron deficient.
(ii) Further, due to resonance, sulphur-acquires a positive charge.
∴ Resonance and electron defficiency make SO₃ an electrophile.

The resonance is a chemical phenomenon which is observed in certain organic compounds possessing double bonds at a suitable position. Certain organic compounds can be represented by more than one structure and they differ only in the position of bonding and lone pair of electrons. Such structures are called resonance structures (canonical structures) and this phenomenon is called resonance. This phenomenon is also called mesomerism or mesomeric effect.

 - COOH. It has -I effect whereas other have +1 effect.

CN^-  It is a negatively charged nucleophile whereas others are neutral nucleophiles.

Both (A) and (R) are correct and (R) is the correct explanation of (A).

Both (A) and (R) are correct and (R) is the correct explanation of (A)