Question

For each pair of the following compounds, which would react faster in an Sn1 reaction. Explain.

Answer 1

Substitution reactions are limited by 1) sterics about the reactive carbon (primary reacts more than secondary and tertiary, respectively) that would inhibit the incoming nucleophile 2) the stability of the resulting leaving group and 3) stabilization of the intermediate or transition state. In this case, the intermediate of an Sn1 reaction is a carbocation with sp2 hybridization (planar geometry).

When the chloride leaves from the first sets of compounds, both form secondary carbocations (so no preference either way from that). However, the second compound has an adjacent pi-bond that can stabilize the carbocation through resonance donation of electrons (shown below). Delocalization of the positive charge on different carbons by any means will stabilize the carbocation, resulting in a faster reaction. Thus, the second compound will react faster.

The second pair of compounds is much more simple. Once the bromide leaves, a carbocation exists at the reactive carbon. Carbocations are high-energy, so any factors which stabilize the positive charge will increase the reaction rate. Look at the two carbocations that could form. The first compound would form a primary carbocation and the second compound would form a tertiary carbocation. Each alkyl group attached to the carbon inductively donates electron density to the carbocation, stabilizing it. More alkyl groups lead to more electron donation. Thus, the tertiary carbocation stabilizes the developing carbocation much more than the very unstable primary carbocation. As a result, the second compound would react faster in an Sn1 reaction.