Type of Mechanism

Type of Mechanism:- Reaction mechanisms are classified mainly into two classes

(i)Free Radical mechanism

(ii)Ionic or polar mechanism

(i)Free radical mechanism:- this type of mechanism explains those reactions in watch hemolytic bond fission occurs. The hemolytic bond fission is favoured by the following factors

a) Gaseous state of reactant

b) Non-polar solvents

c) Presence of energy in the form of UV, sunlight, heat, etc

d) Presence of substances like peroxides which form free radicals readily on decomposition.

The mechanism of reactions involving attach of free radicals on the substrate molecule is referred to free radical mechanism. The mechanism of free radical reactions involves three steps initiation, propagation and termination. Each of these steps, in turn, may involve two or more steps.

(ii)Ionic or polar mechanism:- This type of mechanism applies to those reactions in which heterolytic bond fission occurs. The heterolytic bond fission is favoured by the following factors.

a)Liquidstate of reactants

b) Presence of olar solvents

c) Inductive, mesomeric or electromeric effects whichdisplace electrons in the substrate

d) absence of peroxides.

The heterolytic bond fission prodices positive or negative centres in the substrate, i.e., carbonium ions or carbanions come into existence. These centres are attacked by nucleophiles or electrophiles. The mechanism of reactions involving the attack of nucleophilic or electrophilic reagents on polar substrate molecules is referred to as polar or ionic mechanism.