How do we prove that alcohols are weaker acids than water?

1 Answer
Feb 20, 2015

The answer will be a little long because I want to provide a clear explanaition for why this happens.

The key to showing why alcohols are weaker acids than water is to consider the fact that an alkyl group is actually an EDG - electron donating group (you'll sometimes see this as ERG - electron releasing group).

I'll explain why this matters from two perspectives - the bond between oxygen and hydrogen in the #"-OH"# group and the stability of the conjugate base.

When comparing the acid strength of alcohols and water, you must look at how "eager" the acidic proton is to jump off. An alcohol's acidity comes from the important difference in electronegativity between the oxygen and the hydrogen in the attached #"-OH"# group.

Oxygen, being the more electronegative of the two atoms in the functional group, will attract the bond electrons towards itself; this in turn will make the proton easier to pull off.

However, in the case of an alcohol, the alkyl group will "push" electrons towards the oxygen; this will increase the electron density on the oxygen atom and reduce the pull the atom has on the two bonding electrons it shares with the acidic proton.

As a direct result, the hydrogen will be a little harder to pull off, since its bond with oxygen is now a little stronger.

So, the acidic proton is less "eager" to jump off #-># weaker acid.

Now for the conjugate bases. The difference in stability is due to the same positive inductive effect of the alkyl group.

Basically, stronger acids have weaker conjugate bases and vice versa. Acid strength is directly related to conjugate base stability. The more stable a conjugate base, the less reactive it is; the less reactive it is, the weaker it will be, which implies that the acid will be stronger.

The conjugate base of a generic alcohol is called an alkoxide ion, or #"R-O"^(-)#. Compare this with water's conjugate base, the hydroxide ion #"HO"^(-)#.

In both cases, oxygen is perfectly capable of accomodating the negative charge; however, in the case of an alkoxide ion, the oxygen must accomodate an extra negative charge because of the aforementioned positive inductive effect of the alkyl group.

This will reduce its stability when compared with the hydroxide ion. A less stable conjugate base is a more reactive conjugate base, and thus a stronger base #-># the acid is weaker.