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Organic Chemistry: What makes methanol (pKa15.54) a stronger acid than water (pKa 15.74)?

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Written 28 Nov, 2012. Asked to answer byBrian Bi.

The pKa's that you cite are in water, where solvation and H-bonds can make alarge difference so it's hard to gather any information that is of predictive valuefrom analyzing this alone.

If you take a look at DMSO acidities instead (of course there is still solvation inDMSO, so this relies on the assumption that it's much less determining afactor, which is true), it's apparent that methanol is actually quite a bit moreacidic (2.5 pKa units roughly). What's suggested is that the conjugate base ofmethanol is stabilized by a hyperconjugative donation of p-type lone pairs onoxygen into the C-H antibonding orbitals on the methyl. This hyperconjugationis stronger on methoxide than methanol.

Returning to the problem of solvation, note that changing the solvent to waterincreases the acidity of both species by a huge factor. The acidities are closer toeach other because water solvates hydroxide (relative to water) better than itsolvates methoxide (relative to methanol). .

Richard Liu8 upvotes byTristan Tan,Sean Ridout,David Pechersky, (more)

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Downvote Comment 1

Written 26 May, 2011.

That's a very small difference so the effect can't be large. I would think that themethyl group might stabilize the negative charge on the oxygen afterdeprotonation. Maybe through hyperconjugation, but that seems a little hardto conceive.

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