(a) What contributing structure(s) would account for the double-bond character of the carbon-nitrogen bond in amides? (b) What does this resonance mean in terms of orbitals?

Amides contain a carbon-nitrogen bond that has a double-bond character due to the resonance between the amide's lone pair of electrons on nitrogen and the carbonyl group's pi bond. The contributing structures that account for this double-bond character are: (i) The nitrogen atom has a lone pair of electrons, and the carbon atom is double-bonded to the oxygen atom. (ii) The nitrogen atom is double-bonded to the carbon atom, and the oxygen atom has a negative charge and a lone pair of electrons. We can represent these contributing structures using resonance arrows as follows: \[ \mathrm{ R-C(=O)-NH_2 \rightleftharpoons R-C(-O^-)-N(=)H_2 } \] where R represents the rest of the molecule.

Resonance involves the delocalization of electrons within certain molecules, and in terms of orbitals, this could be explained as the overlap between the atomic orbitals of the atoms involved. In the case of amides: 1. A lone pair (non-bonding) orbital on the nitrogen atom overlaps with the pi* (antibonding) orbital of the carbon-oxygen double bond, which is in a higher energy level. 2. This overlap results in the delocalization of the lone pair of electrons on the nitrogen, creating a partial pi bond between the carbon and nitrogen atoms. 3. Simultaneously, the pi bond between carbon and oxygen weakens due to the presence of the new pi bond between carbon and nitrogen, making it more like a single bond. 4. This combination of electron delocalization and weakened pi bonds gives the carbon-nitrogen bond its double-bond character, making it shorter and stronger than a typical single bond between carbon and nitrogen. In summary, the resonance in amides results from the overlap of atomic orbitals on the nitrogen, carbon, and oxygen atoms, leading to the delocalization of electrons and the double-bond character observed in the carbon-nitrogen bond.