please explain sn1 and sn2 mechanism. thnks!

Nucleophilic substitution is the reaction of an electron pair donor (the nucleophile, Nu) with an electron pair acceptor (the electrophile). An sp³-hybridized electrophile must have a leaving group (X) in order for the reaction to take place.
Mechanism of Nucleophilic Substitution:-

The term S_N2 means that two molecules are involved in the actual transition state i.e substitution nucleophilic bimolecular.

The departure of the leaving group occurs simultaneously with the backside attack by the nucleophile. The S_N2 reaction thus leads to a predictable configuration of the stereocenter - it proceeds with inversion (reversal of the configuration).Rate depends on both the haloalkane as well as nucleophile.SN2 reactions are fast.Primary carbon seldom formed so SN2 favoured.

In the S_N1 reaction, a planar carbenium ion is formed first, which then reacts further with the nucleophile. Since the nucleophile is free to attack from either side, this reaction is associated with racemization.The rate depends only on the haloalkane.The carbocations formed much slower than it reacts with other molecules,so SN1 reactions are slow. Tertiary carbon most stable so SN1 favoured.

In both reactions, the nucleophile competes with the leaving group.Secondary carbon less stable,either could be favoured.

Thanks & Regards

Aarti Gupta

askiitians Faculty