Question

Explain why it is impossible to form an amide by the condensation of a tertiary amine with a carboxylic acid.

Short answer

It is impossible to form an amide by the condensation of a tertiary amine with a carboxylic acid because the tertiary amine has no hydrogen to form water molecule as a byproduct.

Step-by-step solution

Condensation of amines with carboxylic acids

The reaction of amines with carboxylic acid gives amides. It is not a direct reaction. When amines react with carboxylic acids, it forms ammonium carboxylate salt, which on heating produces amide.

Step 2: Conditions for amide formation

Amides can be formed when there is a hydrogen atom in the amine group. This hydrogen and the hydroxyl group in carboxylic acid combine to form water molecule as a byproduct. So, only primary $\left({\text{- HNR}}_2\right)$ and secondary amines $\left(-{\text{H}}_2\text{NR}\right)$ give positive result for the amide formation.

Since tertiary amines${\text{- NR}}_3$have no hydrogen atom on the nitrogen, it cannot contribute hydrogen to form water molecule as a byproduct.