# Which mechanism should a reaction between the cyclopentylmethanol and the HBr should follow?

I just thought that the reaction should follow $$\mathrm{S_{N}2}$$ mechanism this is because the alkyl halide is primary but the book says that the reaction will follow $$\mathrm{S_{N}1}$$ mechanism resulting in the formation of tertiary carbonation and a ring expansion product. Can anyone please explain the reason behind this ?

• Mar 11 '20 at 23:56
• @jezzo is but the alcohol mentionedis also a primary alcohol that's why I am confused because I also thought that the reaction should follow SN2 mechanism. Mar 12 '20 at 0:29

It's a tricky example, but the $$S_N1$$ process is favored because the solvent is protic and the resulting 6-member ring is more stable than a 5-member ring.
If ring expansion weren't possible, I believe your intuition is correct that the $$S_N2$$ mechanism would dominate. See this discussion. You're not alone!