Question

Give an example of an SN2 concerted mechanism with curved arrows. Show the solvent and substrate effect on the reaction, and the stereochemical and regiochemical outcome for the reaction.

Answer 1

• SN2 reaction is a type of nucleophilic substitution reaction. In this mechanism, one bond is formed and one bond is broken simultaneously in a concerted manner in the transition state. The mechanism is shown below-
• SN2 reactions are regioselective. The nucleophile attacks from the backside. Front side attack is less favorable as the negative charge on nucleophile is repelled by the negative charge generated in the leaving group.
• Due to this backside attack, in SN2 reactions the inversion in configuration takes place. The stereochemistry is always inversion in configuration. (R) isomer will become (S) isomer. This is shown below-
• Substrate plays an important role in SN2 reactions. As backside attack takes place in SN2 reactions, the rate will increase if the backside of the molecule is less hindered. The rate will be fastest if the backside group is methyl or primary carbon followed by secondary carbon. The rate is slowest if the group is tertiary carbon. In case of tertiary carbons or hindered molecules SN1 reaction takes place.
• Usually in SN2 reaction polar aprotic solvents are used. Usually the nucleophile is surrounded by solvent molecules. If we take polar protic solvents the solvent molecules will involve in hydrogen bonding with the nucleophile and thus it will reduce its reactivity towards attack to substrate molecule. In other words, polar protic solvents will stabilise the nucleophile. This polar aprotic solvents are used.