What is considered an uncommon bond acceptor in molecular interactions?

In molecular interactions, a tertiary amine is considered an uncommon bond acceptor primarily due to its lack of lone pairs of electrons that can participate in hydrogen bonding. Tertiary amines have three alkyl groups attached to the nitrogen atom, which can create steric hindrance and reduce their ability to engage as hydrogen bond acceptors compared to primary and secondary amines.

Primary and secondary amines, along with alcohols and carboxylic acids, typically have lone pairs of electrons that make them effective hydrogen bond acceptors. Carboxylic acids can donate and accept hydrogen bonds due to the presence of both a carbonyl and a hydroxyl group. Thus, while tertiary amines can form hydrogen bonds in certain contexts, their effectiveness as bond acceptors is less common compared to the other functional groups listed, keeping them in a secondary position in terms of hydrogen bonding capability.