Can mesylates be reversed?

[BACK TO THE PAST]

I'm trying to clarify something that Kaplan didn't explain too well. I know mesylates turn alcohols into better leaving groups, but is it possible to convert a mesylate back into a hydroxyl group? Like how acetals can be reversed under acidic conditions. I saw a question implying it might be possible, but that seems kinda rare. Is this just one of those exceptions MCAT wants us to know?

 

[george.mx]

I understand this can be a point of confusion but no, you can't directly reverse a mesylation reaction like you can hydrolyze an acetal. The mesylate group is extremely stable, and the anion it forms is a very poor nucleophile, which is why it's such a good leaving group.

Instead of a reversal, think of it as a substitution. The question you saw was likely about using the mesylate as an excellent leaving group in a substitution reaction. You could use a strong nucleophile, like hydroxide, to displace the mesylate and form a hydroxyl group.

So while you can't reverse it, you can use its properties to replace it with a hydroxyl group. This is a fundamental concept in substitution reactions, not a rare exception! Hope that helps, and good luck with your studying!

 

[Mike et al]

Kaplan's great at drilling facts, not chemistry sense, reversing a mesylate back to an -OH isn't standard, it's synthetic overkill. MCAT loves exceptions, but don't chase unicorns...know the forward reaction cold, ignore the noise