Answer to Question #61058 in Organic Chemistry for Amit Sharma

Question

a) Explain the following:
i) Primary halides which have an unsaturated group attached to the carbon react much faster
than bromomethane in SN2 reactions.
ii) SN1 reactions do not exhibit complete racemisation and inversion always exceeds retention.

Answer 1

1. No free carbocation is generated, therefore SN2 displacement afford unrearranged products. However, sometimes SN2 reaction, leads to allylic rearrangement. The attack of nucleophile take place at the end of the Pi-system (C3-atom). As result steric hindrance is less than in the case of attack by C1-atom, where gorup Br is very big (as in the case of MeBr) and it is difficult for nucleofile to came close to the reactive center. As result allyl bromide is more reactive in SN2 reaction than methylbromide.

2. Although it’s often said that the SN1 proceeds with “racemization” of stereocenters, in practice a 50/50 split of stereocenters may not be obtained due to “ion pairing” effects. In other words, the leaving group could leave, but not fully dissociate from the vicinity of the carbocation, which could block a nucleophile from attacking the nucleophile from that face. For that reason it’s a little bit more correct to say that it proceeds with a “mixture of retention and inversion” rather than “racemization”.