04-BS-12 · Undated paper
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
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 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.
Approach. Identify the most electronegative/most stabilised site for each acid–base role: for ibuprofen, compare every C–H and O–H bond for conjugate-base stability; for cocaine, compare every lone pair (ester/carbonyl oxygens vs. the aliphatic amine nitrogen) for availability and resulting cation stability.
a) Ibuprofen. Ibuprofen has one O–H (the carboxylic acid) and many C–H bonds (aromatic, benzylic, aliphatic). The carboxylic acid O–H is, by a wide margin, the most acidic proton (pKa ≈ 4.4) because deprotonation gives a carboxylate anion whose negative charge is delocalised symmetrically over two equivalent oxygens by resonance (two equal-length C–O bonds, confirmed crystallographically for carboxylate salts) — no C–H deprotonation (not even the benzylic one alpha to the ring) comes remotely close, since a carbanion conjugate base has nothing analogous to stabilise it.
The conjugate base is the resonance-stabilised carboxylate, —C(=O)O− ↔ −O(O=)C—, drawn as either resonance structure or with the charge shown delocalised.
b) Cocaine. Cocaine carries two ester (carbomethoxy and benzoyloxy) oxygens and one tertiary (tropane) amine nitrogen. The ester carbonyl and ether oxygens are poor bases: their lone pairs are tied up in resonance with the adjacent C=O (conjugation lowers their energy/ availability), and protonating them would disrupt that conjugation and place a formal positive charge next to an already electron-poor carbonyl carbon. The tropane nitrogen's lone pair, by contrast, is a simple, fully localised sp³ lone pair on a trisubstituted aliphatic amine — exactly the functional group class (R₃N:) that is most basic in typical organic structures. It is therefore the most basic site, and protonation gives a stable ammonium ion with no loss of conjugation elsewhere in the molecule.
