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#1 2011-03-24 00:30:13

orgopete
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Why is methanol more acidic than water, but less than ethanol?

This is an excellent question, however I think water is the outlier. The alcohols seem to me to follow a predictable pattern while water (or hydrogen) does not follow that pattern.

If you compare the acidity of protonated dimethyl ether (-3.8 )  with protonated THF (-2.1), this seems consistent with methanol (15.5), ethanol (16), isopropanol (16.5), and t-butanol (17). If methyl groups are replaced with hydrogen, the protonated methanol (-2.2) and hydronium (-1.7) seem to have a trend counter to the electron donation of substituted carbons. In virtually all other circumstances, carbon is a better electron donor than hydrogen.

I agree with another suggestion to consider the gas phase acidities in which the acidities do follow the predicted trend. I am wary of gas phase acidities (probably because I do not know whether they are measuring the same ionic reaction). However, I do think that we can learn from the gas phase acidities and how they differ from solution pKa values. In the gas phase,  solvent effects are removed. How might hydrogen bonding affect pKa values? Could hydrogen bonding reverse the predicted pKa values? In solution, pKa measurements combine solvent effects. Hydrogen bonding could stretch O-H bonds and make the non-bonded electrons of oxygen more available. Rather than hydrogen making the electron less available as is the case for the alcohol acidities, hydrogen bonding could pull the protons away from the oxygen atoms making the oxygen electrons more basic. This would be an effect opposite of a proton being an electron withdrawing group, but consistent with hydrogen bonding.

     http://www.curvedarrow.com/chem/ZZ49C00A82.jpg

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