04-BS-12 · December 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exam 04-BS-12, Organic Chemistry — December 2016. 3 hours, closed-book examination (no non-communicating calculator restriction beyond the standard aid sheet, 8.5×11", hand-written both sides). Ten questions constitute a complete exam paper (only the first 10 questions as they appear in the answer book are marked, each of equal value) — the source paper in fact prints thirteen questions; all thirteen are answered in full below.
Reference texts: McMurry, Organic Chemistry, 9th ed. (acid/base theory of drugs, SN1/SN2 stereochemistry, carbocation rearrangements, alkyne synthesis via acetylide alkylation, IR/NMR structure elucidation, electrophilic aromatic substitution and synthesis design, amino-acid pKa); Clayden, Organic Chemistry, 2nd ed. (amide resonance and β-lactam reactivity, radical vs. ionic HBr addition mechanisms); a standard biomaterials reference for the poly(ester amide) drug-delivery polymer chemistry of Question 13 (Katsarava-type AABB poly(ester amide)s built from diacids, diols, and protected diamino acids).
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.
(a) Ibuprofen — the carboxylic acid O–H is the most acidic proton. Ibuprofen has only one acidic functional group, the arylpropanoic –COOH; its O–H is far more acidic (pKa ≈ 4.4) than any aromatic or alkyl C–H because deprotonation gives a carboxylate whose negative charge is delocalised symmetrically over both oxygens by resonance — no other proton in the molecule has any comparable stabilisation available to its conjugate base.
(b) Cocaine — the tertiary amine nitrogen lone pair is the most basic site. Cocaine carries three types of lone pairs: the tropane-ring tertiary amine nitrogen, and two ester carbonyl oxygens (the benzoate and the methyl ester). Amine nitrogen lone pairs are essentially localised and freely available to accept a proton; ester carbonyl-oxygen lone pairs are strongly delocalised into the C=O π system by resonance (the classic amide/ester resonance structure with a C–O+ double bond), which stabilises the neutral ester and makes that oxygen a far weaker base. The bridgehead tropane nitrogen is therefore the most basic site by a wide margin, and protonation there gives the ammonium conjugate acid.
