Rearrangement vs. Ring Strain in Cyclobutanecarboxaldehyde Nucleophilic Reaction


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> Predict the major product of the following reaction:
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> [![reaction of cyclobutyral with NaBH4 in MeOH followed by HBr addition](https://i.stack.imgur.com/98dP9.jpg)](https://i.stack.imgur.com/98dP9.jpg)
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> *Note*: Product **A** is incorrectly marked as secondary halide, it should have been primary halide
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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?


