There’s two main ways that acids can help with reactions of carbonyl compounds.
Acid catalyzes addition reactions.
We already talked about acids as the “carbonyl aphrodisiac”. The first way is that acid makes carbonyls more electrophilic. This means that once protonated, a carbonyl will be much more likely to react with poor nucleophiles (such as water and alcohols) than normal.
Acid catalyzes elimination reactions
The second way that acid helps carbonyl reactions is through making groups such as OH, OR, and NHR into better leaving groups. How so? Because protonation of each of these groups will generate -OH2(+), -OHR(+), or -NH2R(+), which are weaker bases – and thus, better leaving groups.
But… there are limits
Acid catalysis only works if it’s compatible with the nucleophile. For instance, it wouldn’t make sense to add acid to a Grignard reaction, right? Since Grignard reagents are strong bases, addition of acid will only end up protonating (i.e. destroying) the Grignard, and this stopping the reaction dead in its tracks. Similarly, acid won’t help reactions of NaBH4 or LiAlH4 with aldehydes or ketones; instead, it will react with the nucleophile.
So acid catalysis only works when protonation is reversible.
For our purposes, that means when the nucleophile is only weakly basic, such as with water, alcohols, amines, and similar species.
Tomorrow: a really cool reaction that turns aldehydes and ketones into alkenes.
Thanks for reading! James
P.S. Relevant post: The power of acid catalysis