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Example 2: Identify all the products formed in the following reaction and indicate the major product.

Example 2

Solution: Three products will be formed in the given reaction as shown below:

Solution

In this reaction primary alcohol undergoes $\mathrm{S_N1}$ mechanism by forming a carbocation and then undergoing ring expansion, but primary alcohols prefer $\mathrm{S_N2}$ mechanism. So why is this primary alcohol undergoing $\mathrm{S_N1}$ mechanism?

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    $\begingroup$ The Sn1 mechanism should be considered relatively unreasonable here. You might consider a Sn1-like mechanism instead where ring expansion is the thing that displaces water, i.e., the C-C bond is the nucleophile. $\endgroup$ – Zhe Apr 16 at 0:49
  • $\begingroup$ But the carbocation is formed in Sn1 mechanism and ring expansion can only take place if carbocation is formed,also in this reaction the product formed by Sn1 mechamism is major product $\endgroup$ – saurya yadav Apr 16 at 6:11
  • $\begingroup$ "ring expansion can only take place if carbocation is formed" That is incorrect. The ring expansion could displace the leaving group. $\endgroup$ – Zhe Apr 16 at 13:24
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    $\begingroup$ The bottom line here is the the provide solution is wrong. $\endgroup$ – Zhe Apr 16 at 13:25
  • $\begingroup$ Is the given answer to the question regarding major product 1-bromo cyclohexane? $\endgroup$ – J_B892 Sep 13 at 8:05
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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 OH2+ without violating first-principles.

What is the source for this mechanism?

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  • $\begingroup$ 1-bromo -1-methylcyclopentane is not the major product here so I'm not concerned about it,maybe they wanted to show other possibilities...what I'm not getting is why primary alcohol would follow Sn1 reaction and form major product. .....this mechanism was in my textbook. $\endgroup$ – saurya yadav Apr 16 at 8:04

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