Why does the reaction of the primary alcohol cyclopentylmethanol with hydrogen bromide follow SN1 mechanism?


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> **Example 2**: Identify all the products formed in the following reaction and indicate the major product.
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> [![Example 2](https://i.stack.imgur.com/CVxwl.png)](https://i.stack.imgur.com/CVxwl.png)
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> **Solution**: Three products will be formed in the given reaction as shown below:
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> [![Solution](https://i.stack.imgur.com/51pgG.png)](https://i.stack.imgur.com/51pgG.png)
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In this reaction the primary alcohol undergoes SN1 mechanism by forming a carbocation and then undergoing ring expansion, but primary alcohols prefer SN2 mechanism. So why is this primary alcohol undergoing SN1 mechanism?




**It does not. The proposed mechanism is wrong.**


The conversion of cyclopentylmethanol to 1-bromo-1-methylcyclopentane is not reported *anywhere* in the literature. If you could somehow generate the primary carbocation, then one would expect a rapid 1,2-hydride shift to generate the more stable tertiary carbocation, giving rise to 1-bromo-1-methylcyclopentane.


The ring-expansion product could be rationalized by a concerted 1,2-alkyl shift displacing $\ce{OH2^+}$ without violating first-principles.


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