# Why would this rearrangement happen?

So, I am a first year chemistry student and I saw this chemistry reaction on a previous year's test. I attempted to figure out the mechanism, but I'm rather confused as to how the cyclopentane somehow donates a carbon to the cyclobutane? Why would such a rearrangement occur?

That is how far I get to before I am lost. Is methanol even a strong enough reagent for such a reaction?

• Is this an SN1 reaction where a bromide ion is formed and then a rearrangement occurs to form a tertiary carbocation instead of a secondary carbocation, and then methanol can attack? – cgug123 Nov 13 '16 at 16:09

Methanol is a very poor nucleophile, so the rate of the $\mathrm{S_N2}$ pathway will be extremely slow. This is much likelier to be a $\mathrm{S_N1}$ reaction. The starting material is a benzylic bromide from the point of view of the alkyl halide, both pathways are quite possible, but the lack of a good nucleophile suggests the predominance of the $\mathrm{S_N1}$ route.