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04-BS-12 · Undated paper

Question 3 of 13

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

National Exam 04-BS-12, Organic Chemistry — May 2019 sitting (the page-1 header and the running footer, "04-BS-12/May 2019", both give the date). 3 hours, closed-book examination; one Casio/Sharp-approved calculator permitted. NOTES on page 1 state that TEN (10) questions constitute a complete exam paper and only the first 10 as they appear in the answer book are marked, but this sitting prints 13 numbered questions — every question and sub-part below is answered in full.

Reference texts: McMurry, Organic Chemistry, 9th ed. (acid/base theory, functional-group identification, SN1/SN2 mechanisms and stereochemistry, alkyne/acetylide synthesis, electrophilic aromatic substitution, IR/NMR/mass-spectral structure elucidation, named-drug synthesis design, and step-growth polymer chemistry). Every molecular formula, exact mass, and stereochemical (R/S, cis/trans) assignment below.

Question 3 (3/13)

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.

Penicillin G: the strained four-membered β-lactam ring (top) is fused to the five-membered thiazolidine ring; the side-chain amide (left) is an ordinary, unstrained secondary amide.

The β-lactam (four-membered ring) amide is far more reactive than the side-chain (phenylacetamide) amide. An ordinary amide's carbonyl carbon is a poor electrophile because the nitrogen lone pair conjugates efficiently into the C=O (amide resonance), giving the C–N bond significant double-bond character and locking the amide nitrogen roughly planar (sp²-like). In the β-lactam, that same nitrogen is trapped inside a four-membered ring, so ring geometry forces bond angles far from the ideal 120° trigonal-planar arrangement the resonance form wants. Two effects follow directly from this: (1) ring strain destabilises the ground state relative to an unstrained amide, raising its energy and driving reactivity; and (2) the nitrogen lone pair cannot align well with the carbonyl π-system (poor orbital overlap in the strained ring), so amide resonance is disrupted and the carbonyl carbon keeps much more of its intrinsic (ketone/ester-like) electrophilicity. Both effects make the β-lactam carbonyl carbon much more susceptible to nucleophilic attack (by water, or by the active-site serine of bacterial transpeptidase enzymes) — which is precisely the mechanism of action of penicillin: covalent, essentially irreversible acylation of the transpeptidase active site, blocking bacterial cell-wall cross-linking.