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Predict the major product of the following reaction:

reaction of cyclobutyral with NaBH4 in MeOH followed by HBr addition

Note: Product A is incorrectly marked as secondary halide, it should have been primary halide

This question appeared in an national level aptitude test.

My reasoning was that for the SN1 rearrangement carbon in the cyclobutane ring (which already has quite a bit of ring strain) undergoes the hybridization change from sp3 to sp2, which puts it at a much higher strain, hence this mechanism should theoretically be disfavoured. On the flip side, the SN2 mechanism in a primary alcohol does not have any such hinderance.

Which effect trumps which? Is the SN1 tertiary stability worth the ring strain?

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    $\begingroup$ Consider the possibility that the ring strain is sufficient that tertiary cation rearranges to relieve that strain $\endgroup$
    – Waylander
    May 23, 2022 at 6:23
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    $\begingroup$ @Waylander Yes that is a possibility, I believe. However the alcohol is primary and hence Sn2 is highly favoured. I am looking for the proportion of products of A is to B. Like how much of A is formed and how much else is B. $\endgroup$
    – Prajval K
    May 23, 2022 at 7:47
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    $\begingroup$ see these questions chemistry.stackexchange.com/questions/128941/… and chemistry.stackexchange.com/questions/112785/… $\endgroup$
    – Waylander
    May 23, 2022 at 8:43
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    $\begingroup$ @Waylander Yes, exactly. Kinetics seems to favour a ring expansion product to compensate increased ring strain. This appears possible in the above reaction. I do understand that Sn1 will be favoured over Sn2 if rearrangement is possible (since it is a polar solvent, MeOH/HBr system). If one takes into account the energy barrier for 1,2-hydride shift, and increased ring strain verses the increased stability of carbocation in polar solvent then how does the Sn1/Sn2 equilibrium look like. It is of my opinion that one will end up with a mixture of products A and B. I might be entirely wrong. $\endgroup$
    – Prajval K
    May 23, 2022 at 9:06
  • $\begingroup$ Also, just an additional curiosity: Will this reasoning hold if the -CHO group is replaced by a ketone ( - CO - CH3 ). What additional factors do I need to consider apart from increased steric hinderance. $\endgroup$
    – Prajval K
    May 23, 2022 at 9:09

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