Question

Why methanol is stronger acid than water?

Answer 1

Answer:

Methanol is more acidic than water because its conjugate base (methoxide) is weaker than water's conjugate base (hydroxide). The methyl group slightly takes away electron density from the oxygen atom, unlike the hydrogen in water, which actually contributes more electron distribution.

Answer 2

Methanol is slightly more acidic than water. Their pKa values, in water, are 15.5 and 15.7, respectively. All other aliphatic alcohols, however, are less acidic than water.

Is the following reasoning correct? This is my best rationalization; is there anything better or anything that can be added?

In both hydroxide and methoxide, we have an O bearing negative charge. In the former, we have a hydrogen attached to the O. In the latter, we have a carbon attached to the O. Carbon is more electronegative than hydrogen, therefore, carbon should be able to better withdraw electron density via induction from the O. This is a stabilizing interaction.

We often think of methyl groups as inductively donating, but that's molecular profiling. They can inductively donate to a carbocation, partly because a carbocation is highly electronegative, so the carbocation pulls the electrons toward itself pretty well (so there's both pushing and pulling of electrons going on). When you have a methyl group attached to an O, however, the methyl group is often inductively withdrawing. NMR data supports this.

Now the question is why aren't other aliphatic alcohols more acidic than water? In longer-chain aliphatic alkoxides, you don't just have a methyl group attached to the O bearing the negative charge - you have a bunch of methyl groups strung together. These lessen the amount of inductive withdrawal that the alpha carbon can do. Each−CH2− unit attached to the alpha −CH2− is somewhat inductive donating to the alpha −CH2−.

Thank u