# 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

Is methanol even a strong enough reagent for such a reaction?

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.

So, you go through a carbocation intermediate, which undergoes an alkyl shift to generate a more stable carbocation.

Both carbocations are benzylic and secondary, so hyperconjugation isn't the driving force for the rearrangement, as your comment hints at. Instead, it's simply related to the relief of ring strain as you expand a four-membered ring to a five-membered ring. Four-membered rings are pretty unstable to begin with and fusing one to a benzene ring increases the strain even further since you impose more geometrical constraints on the ring.

Interception by methanol and deprotonation gives the desired product.

• Thank you so much!! Your comment was very helpful, as was the diagram. Does it matter which c-c bond in the cyclobutane is broken to form it? And is ring expansion a more important factor than, say, a hydride shift? – cgug123 Nov 13 '16 at 16:20
• 1) Yes, it matters. You can try migrating the other one, like this: i.stack.imgur.com/5BT6m.png You'll find that you end up with a non-benzylic carbocation. While this shift helps to open up the ring size, you lose a lot of electronic stabilisation (delocalisation of the positive charge into the ring). 2) As you can see from the product, yes, the ring expansion is quite important. In general cyclobutanes fused to benzene rings have a very strong tendency to open if they can. There are many more examples (e.g. electrocyclic reactions, retro-aldols). – orthocresol Nov 13 '16 at 16:29