Question 3 of 13: Penicillin G — Differential Amide Reactivity

Question 3: Penicillin G — Differential Amide Reactivity (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

The β-lactam (four-membered ring) amide is far more reactive than the ordinary, acyclic phenylacetamide side-chain amide. This is the entire basis of penicillin's mechanism of antibacterial action (irreversible acylation of the bacterial transpeptidase enzyme).

1. Ring strain. A normal amide's ideal bond angles are close to 120° (sp² carbonyl carbon, trigonal planar). Forcing that carbonyl carbon into a four-membered ring compresses the internal C–C(=O)–N angle to roughly 90°, a large deviation that stores substantial angle (ring) strain in the β-lactam relative to the unstrained, five/six-atom-chain side amide.
2. Loss of amide resonance. A normal amide's exceptional stability comes from resonance donation of the nitrogen lone pair into the carbonyl (N–C(=O)↔N⁺=C–O⁻), which requires the N lone pair, the C, and the O to be co-planar. In the β-lactam, the rigid fused bicyclic system (fused to the adjacent thiazolidine ring) twists the nitrogen out of that ideal planar geometry, so the lone pair cannot align as well with the carbonyl π* orbital — much of the usual amide resonance stabilisation is lost.
3. Consequence for reactivity. Because the β-lactam carbonyl is both ring-strained and resonance-poor, it behaves kinetically much more like a reactive ketone or ester than like a typical robust amide: nucleophiles (e.g. the serine hydroxyl of the bacterial transpeptidase active site) attack the carbonyl far more readily, and ring-opening relieves the strain irreversibly, permanently acylating (and thereby inactivating) the enzyme.