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

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 classied 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.
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
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
-C(CH₃)₃> - CH(CH₃)₂ > - CH₂CH₃ > - CH₃

(i) The arrangement of atoms must be identical or almost same in every resonance structure.
(ii) The energy content of all the canomical forms must be almost same.
(iii) Each canocial form must have the same number of unpaired electrons.

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