Question

According to several sources, the SNi

reactions follow a mechanism allowing carbocation formation while others disagree. Precisely, for example, according to A Guidebook to Mechanism in Organic Chemistry - Peter Sykes, it states that

Whether the breaking of the C-O and the S-Cl bond occurs simultaneously, or whether the former occurs first, is still a matter of debate.

Thus, it's unknown if a carbocation formation occurs or not. Now my question is if carbocation formation occurs, wouldn't that imply hydride and methyl shifts are possible within the cation? However, in the case when the reaction is concerted, such rearrangements are not possible. Take the case of the following reaction for example,
3-Methylbutan-2-ol+SOCl2 →2-Chloro-3-methylbutane (Major)
Is the above more likely or the following after carbocation rearrangements:
3-Methylbutan-2-ol+SOCl2 →2-Chloro-2-methylbutane (Major)
Now shouldn't the latter imply the carbocation formation and the former reject it? Which reaction occurs in reality and why is it the reaction mechanism undetermined exactly?

Answer 1

The SN1 reaction mechanism involves carbocation formation with possible hydride and methyl shifts, leading to rearranged products, while the SN2 reaction is a concerted process without rearrangements. The reaction of 3-Methylbutan-2-ol with SOCl2 could proceed via either mechanism, resulting in different major products, and factors such as solvent and leaving group influence the actual reaction pathway.

Step-by-step explanation:

Carbocation formation is a key aspect of unimolecular nucleophilic substitution (SN1) reactions. In the particular case of 3-Methylbutan-2-ol reacting with SOCl2, whether the reaction mechanism involves carbocation formation or is a concerted process like SN2, which doesn't involve such an intermediate, is a matter of debate.

Carbocation intermediates allow for the possibility of hydride and methyl shifts, potentially leading to rearranged products. However, if the reaction proceeds through a concerted SN2 mechanism, such rearrangements are not possible.

For tertiary substrates, SN1 reactions are common due to the stability provided by inductive effects from adjacent carbons. The possibility of forming rearranged products implies the formation of a carbocation. In contrast, for primary substrates, SN2 mechanism is more common, involving a single concerted step without carbocation intermediates.

Considering the reaction of 3-Methylbutan-2-ol and SOCl2, if carbocation formation occurs, rearrangement to form 2-Chloro-2-methylbutane might be expected if it leads to a more stable carbocation. However, if the reaction mechanism is concerted (SN2), the major product would likely be 2-Chloro-3-methylbutane.

The exact predominance of one mechanism over the other can be influenced by factors such as the solvent or the leaving group's ability.