Answer to Question #144268 in Organic Chemistry for Gaurav

Question

Discuss the SN1 and SN2 reactions in detail.

Answer 1

The SN1 reaction is a unimolecular reaction, that proceeds stepwise. First, the leaving group leaves, whereupon forms a carbocation that is attacked by the nucleophile. That's why os SN1 speed depends only on substrates concentration. 

SN1 reactions are more likely to proceed with weak nucleophiles (generally neutral substances, such as H2O, CH3OH, etc.) and in polar protic solvents. 

The main limitation of the SN1 reaction is carbocation stability. It increases from primary to tertiary. So the speed of reaction for the SN1 goes from primary (slowest) << secondary < tertiary (fastest).

The SN2 reaction is a bimolecular reaction that takes place in a single step without intermediates when a nucleophile reacts with the substrate from a direction opposite to the leaving group. Because it occurs in one step both the nucleophile and substrate are involved in the rate-determining step. That's why the rate is dependent on both the concentration of the substrate and that of the nucleophile.

SN2 reactions are more likely to proceed with strong nucleophiles(mainly whit negative charges) and in polar aprotic solvents.

The main barrier to the SN2 reaction is a steric hindrance. Because the SN2 is a backside attack, the reaction will proceed only if the empty orbital is accessible. The more groups that are around the leaving group, the slower the reaction will be. That’s why the rate of reaction proceeds from primary (fastest) > secondary >> tertiary (slowest).