SN1 Mechanism :-

SN1 Stands for unimolecular nucleophilic substitution. This mechanism involves two steps consider the hydrolysis of tert-butyl bromide with aqueous NaOH.

Step1 : The alkyl halide ionises to give a planar carbonium ion and it is planar because the central positively charged carbon atom is Sp²hybridized

Step2 : The nucleophile can attack the planar carbonium ion from either side to give the product.

Remember that the primary alkyl halides undergoes substitution by SN2 mechanism. The tertiary alkyl halides undergoes substitution by SN1 mechanism this is because the attack of the nucleophile on the crowd tertiary alkyl halides is quite difficult .

The secondary alkyl halides may undergo nucleophilic substitution by either SN1 or SN2 mechanism depending on the solvent SN1 mechanism will predominate if an ionising solvent is present SN2 mechanism will predominate if ionising solvent is absent.

SN2 Mechanism :-

SN2 stands for bio-molecular nucleophilic substitution. In this mechanism the attack of the nucleophile and the ejection of the halide ion takes place simultaneously. The reactants on their way to products pass through a transition state where the C - O bond is half-formed and the C - X bond is half-broken consider the example of the hydrolysis of methyl bromide by aqueous NaOH. The reaction and the transition states are :