04-BS-12 · December 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exam 04-BS-12, Organic Chemistry — December 2018. 3 hours, closed-book examination (one Casio/Sharp-approved calculator and one hand-written aid sheet 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. (Brønsted acid–base sites in drugs, SN1/SN2 mechanism selection, Williamson ether synthesis, SN2 stereochemistry at a stereocentre, steroid/bile-acid amphiphilicity, named-drug synthesis design, fatty-acid melting-point trends, epoxide/alkene interconversion chemistry, radical stability and antioxidants, Diels–Alder stereochemistry, bicyclic-ketal pheromone synthesis, arene-oxide metabolism, and mass-spectral/IR/NMR structure elucidation). 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.
a) Ibuprofen. Ibuprofen carries one carboxylic acid group (–COOH) and an aromatic ring; every other C–H is an ordinary alkyl/benzylic proton. The carboxylic O–H is by far the most acidic proton in the molecule (pKa ≈ 4.5) because deprotonation gives a carboxylate whose negative charge is delocalised equally over two equivalent oxygens (resonance stabilisation) — no other proton in ibuprofen has any comparable stabilisation available for its conjugate base.
b) Cocaine. Cocaine has three basic-looking oxygens (two ester carbonyls, one ester C–O) and one tertiary amine nitrogen (the ring N–CH3). Protonating an ester oxygen gives a cation with no good resonance/charge stabilisation and destroys the ester's own resonance donation from the alkoxy oxygen into the carbonyl; the tertiary amine's nitrogen lone pair, by contrast, is not delocalised into anything and sits in an sp3 orbital pointed straight at an incoming proton. The tropane nitrogen's lone pair is therefore the most basic electron pair in the molecule (this is the same site that gets protonated to make cocaine hydrochloride, a water-soluble salt, from the free-base "crack" form).
