Question
What do you understand by:
(i) Substitution reactions
(ii) Addition reactions?
Solution
(i) Substitution reactions. In a substitution reaction, a group or radical attached to a carbon atom is removed and another one enters in its place; no change in the degree of unsaturation or structure of the substance occurs.
The product is called substitution product. For example.
The substitution reactions are further classified into three types depending upon the nature of the attacking reagent.
(a) Electrophilic substitution reaction (SE reactions):
In these reactions, the attacking reagent is an electrophile. For example,
(b) Nucleophilic substitution reactions (SN reactions):
Substitution reactions which are brought about by nucleophiles are called nucleophilic substitution reactions.
(c) Free radical substitution reactions. Substitution reactions brought about by free radicals are called free radical substitution reactions. Such reactions are carried either at high temperature or in the presence of ultraviolet light. For example, chlorination of methane to form chloromethane consists of the following three fundamental steps:
(a) Initiation:
(ii) Addition reactions. Reactions which involve combination between two reacting molecules to yield a single molecule of the product are called addition reactions. This addition occurs at the carbon atoms joined by a double or triple bond, leading to change in the degree of unsaturation of the molecule and the molecule becomes nearly saturated.
Addition reactions are of three types depending upon the nature of attacking reagent.
(a) Electrophilic addition reactions. Addition reactions brought about by electrophiles are called electrophilic addition reactions. For example the addition of bromine or HBr to ethylene.
(b) Nucleophilic addition reactions. Addition reactions brought about by nucleophiles are called nucleophilic addition reactions. For example, the addition of HCN to aldehydes.
(c) Free radical addition reactions. Addition reactions brought about by free radicals are called free radical addition reactions. For example, the addition of HBr to alkenes in the presence of peroxides.
This reaction takes place as follows:
Initiation:
The product is called substitution product. For example.
The substitution reactions are further classified into three types depending upon the nature of the attacking reagent.
(a) Electrophilic substitution reaction (SE reactions):
In these reactions, the attacking reagent is an electrophile. For example,
(b) Nucleophilic substitution reactions (SN reactions):
Substitution reactions which are brought about by nucleophiles are called nucleophilic substitution reactions.
(c) Free radical substitution reactions. Substitution reactions brought about by free radicals are called free radical substitution reactions. Such reactions are carried either at high temperature or in the presence of ultraviolet light. For example, chlorination of methane to form chloromethane consists of the following three fundamental steps:
(a) Initiation:
(ii) Addition reactions. Reactions which involve combination between two reacting molecules to yield a single molecule of the product are called addition reactions. This addition occurs at the carbon atoms joined by a double or triple bond, leading to change in the degree of unsaturation of the molecule and the molecule becomes nearly saturated.
Addition reactions are of three types depending upon the nature of attacking reagent.
(a) Electrophilic addition reactions. Addition reactions brought about by electrophiles are called electrophilic addition reactions. For example the addition of bromine or HBr to ethylene.
(b) Nucleophilic addition reactions. Addition reactions brought about by nucleophiles are called nucleophilic addition reactions. For example, the addition of HCN to aldehydes.
(c) Free radical addition reactions. Addition reactions brought about by free radicals are called free radical addition reactions. For example, the addition of HBr to alkenes in the presence of peroxides.
This reaction takes place as follows:
Initiation:
