Question

How can CH3OH be a lewis acid?

Answer 1

I remember the definition by sticking to how ammonia, a base, is a lewis base because it can donate electrons. Therefore, a lewis acid accepts electrons.

Methanol is a lewis acid due to it also being a Bronsted-Lowry acid. That's because:

• Methanol can donate a proton, i.e. it is a Bronsted-Lowry acid. (Obviously, with a proton available, it can donate it.)
• It has to accept electrons (i.e. it is a Lewis acid) in order to donate a proton. It's a "trade".

We can see that here:

Note that this is not thermodynamically favored in this case. This is just an easy illustration of Lewis acid behavior. If I were to pick a weak base such that the conjugate acid is WEAKER than methanol (higher `"pKa"` than `15.9`), then the example would have thermodynamic favorability.

So, tert-butoxide, let's say, would be a better choice of Lewis base instead of ammonia, since the pKa of tert-butyl alcohol is about `18`.