The question Why is methanol more acidic than water? deals with the reasoning of why methanol is more acidic than water. However, as mentioned in the comments of that question, the acidity constant of water is $14.0$, as confirmed by two sources$^{[1][2]}$, with one of them offering a very convincing explanation. The Wikipedia page$^{[3]}$ for methanol quotes its acidity constant to be $15.5$.

So it would seem, water is more acidic than methanol. But an answer to the question mentioned above gives a good reasoning for why methanol is more acidic than water (it doesn't deal with the pKa of water, though). Also, the answer mentions that water would be much worse an acid in DMSO. Another question implies that methanol and other alcohols are more acidic than water.

On the other hand, it is also true that we regard alkoxide ions as being more strongly basic than the hydroxide ion (thus we use them primarily for elimination reactions).

This leaves us in a fix. So what is the ultimate truth?

[1]: What is pKa of water? - Chemistry ChemLibreTexts
[2]: Water (Data Page)
[3]: Methanol

  • 2
    $\begingroup$ It hardly makes sense to compare the pKa of methanol in water with that of water in water. I've heard this question before, and to me it is like splitting hair. $\endgroup$
    – Karl
    Aug 25 '16 at 19:54
  • 2
    $\begingroup$ So the super uber ultimate answer is: it depends $\endgroup$
    – Mithoron
    Aug 26 '16 at 0:21
  • $\begingroup$ @Karl But the question I have linked to, has compared the acidities of alcohols in aqueous solutions with water itself! And the same answer has indeed used pKa values to compare their acidities. My concern is they might be using the wrong pKa for water. $\endgroup$ Aug 26 '16 at 6:23
  • $\begingroup$ @Mithoron I already realised that when I read that question I have mentioned. But right now, I just want to compare acidities of their aqueous solutions! $\endgroup$ Aug 26 '16 at 6:25
  • 5
    $\begingroup$ If you want a clear-cut answer, get rid of all those complicated intermolecular interactions and study the compounds' acidity in the gas phase. The hydroxide anion releases more energy when protonated, compared to a methoxide anion. Thus, excluding all but the most fundamental factors, methanol is more acidic than water. $\endgroup$ Aug 26 '16 at 23:46

First, water's $\ce{pK_a}$ in water is $\ce{14}$ as explained here. This means that $\ce{H2O}$ is slightly dissociated in liquid form, such that $\ce{[H+] = [HO-] = 10^{-7}}$.

Second, I would say that caring too much about which compound is the best acid is a bit like arguing by definition. Acidity is a fuzzy concept used as a shorthand for deeper meaning. This means there is little point arguing about the shorthand when we have the actual properties it synthesizes.

In this case (sou rce):

$$ \begin{array} {|c|c|c|l|} \hline \ce{solvent} & \ce{pK_a\ of\ water} & \ce{pK_a\ of\ methanol} & \ce{Interpretation} \\ \hline \ce{water} & 14.0 & 15.5 & \ce{water\ is\ more\ dissociated\ than\ methanol} \\ \hline \ce{DMSO} & 31.4 & 29.0 & \ce{methanol\ is\ more\ dissociated\ than\ water}\\ \hline \end{array} $$ So you can argue (as in the answer you linked) that methanol is more acidic than water in the abstract sense, because $\ce{DMSO}$ doesn't stabilize the anions by hydrogen bonding and so is closer to the "ideal" case (whatever that may be).

But what's the point ? You already have the $\ce{pK_a}$ values, any practical question you might have about the concentrations of theses solutes in $\ce{H2O}$ and $\ce{DMSO}$ is already answerable.

  • $\begingroup$ Thank you for your answer! Just one more thing, can I then assume the methoxide ion to be a stronger base than hydroxide ion, (in DMSO)? I was intending to use this answer for that purpose also, but I realise that maybe that would be too simplistic! $\endgroup$ Aug 27 '16 at 6:05
  • 2
    $\begingroup$ No, it's the other way around. Methanol being more dissociated means that the methoxide is a weaker base than the hydroxide (in DMSO). In water the trend is reversed because of better hydrogen bonding with the smaller hydroxide ion. $\endgroup$
    – SCH
    Aug 27 '16 at 10:38

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