Question

Why can't tertiary amines form amides with carboxylic acids?

Answer 1

Because a tertiary amine has no hydrogen, and cannot be eliminated as an amine hydrohalide salt. And I take it you mean amide formation with carbonyl halides.

Explanation:

Typically, we form an amide from an acyl halide, and an amine, in the presence of a base (and typically, this base is a SECOND equiv of amine), e.g.

`RC(=O)Cl +2R'_2NH rarrRC(=O)NR'_2 + R'_2NH*HCl`

The first equiv of `2^@` amine has acted as a nucleophile, and formed a bond with the carbonyl carbon, and the second equiv of amine acts as the base, i.e. it has eliminated the hydrogen halide formed as an amine hydrohalide, where the nitrogen is now quaternized.

We could use a tertiary amine to be the base (and in fact `"triethylamine"`, pungent stuff indeed, is used for this purpose), but `Et_3N` could not be the nucleophile as it has no hydrogens to exchange by means of acid base reaction. `"Triethylamine"` is habitually used as a non-nucleophilic base for this purpose.

If the secondary amine, the one you want to form an amide with, were expensive, or purpose synthesized, you would want to use cheap `Et_3N` for quaternization, and salt elimination.