Question

1) How does increasing the degree of unsaturation affect the rate of the Sn1 and the Sn2 reaction?

2) How does shifting from a cycloalkyl bromide to an aromatic bromide affect the rate of the Sn1 and the Sn2 reaction?

Answer 1

1. SN1 and SN2 reactions are shown by alkyl halides and/or alcohols. The reactions occur easily for unsaturated alkyl halides and alcohols. When the staring alkyl halide or the alcohol is unsaturated, then chances of elimination reaction increases at the expense of SN1 and SN2 reactions. However, with a proper choice of the staring alkyl halide or alcohol, unsaturated organic compounds can be made to undergo SN1 and SN2 reactions. In general, the rate of SN1 reactions are supposed to increase with the degree of unsaturation. SN1 reactions proceed via a carbocation mechanism. An increased degree of unsaturation helps to stabilize the carbocation via hyperconjugation or resonance. SN2 reactions occur in a single step mechanism and the degree of unsaturation cannot play a significant role to enhance the SN2 reaction rate.

2. The rate of SN1 reactions tend to increase when the alkyl group in an alkyl bromide is shifted from cycloalkyl to an aromatic group. This is due to the fact that SN1 reactions proceed via carbocation mechanism and aromatic groups help to stabilize carbocation intermediates by extended conjugation or resonance effects.