04-BS-12 · December 2019
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exam 04-BS-12, Organic Chemistry — December 2019. 3 hours, closed-book examination (no calculator; 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. (functional groups, stereochemistry, SN1/SN2 mechanisms and stereochemistry, steroid/bile-acid amphiphilicity, named-drug synthesis design, reaction-energy diagrams, polymer chemistry, arene-oxide metabolism, and mass-spectral/IR/NMR structure elucidation). Every molecular formula, exact mass, and stereochemical (R/S, cis/trans, meso/chiral) 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.
Crack (freebase cocaine) is a neutral covalent molecule held together only by dispersion, dipole, and weak intermolecular forces; it has a well-defined, relatively low boiling/sublimation point and can be vaporised without complete decomposition. Cocaine hydrochloride, by contrast, is an ionic salt — the tertiary amine nitrogen is protonated and paired with a chloride counter-ion, so the solid is held together by strong, long-range electrostatic (ionic lattice) forces. Ionic solids require far more energy to separate into a gas phase than molecular solids of comparable size; in practice cocaine HCl decomposes/chars before it reaches a true boiling point, so in the sense that matters here the freebase has the (much) lower, experimentally accessible boiling point.
Solubility and route of administration. The neutral freebase is nonpolar/lipophilic (no formal charge, only one weak H-bond donor/acceptor site) — it is poorly soluble in water but volatilises cleanly on heating, so it can be smoked: the vapour is inhaled and, being lipophilic, crosses the lipid-rich alveolar/capillary membranes of the lungs almost instantly. The hydrochloride salt is the opposite: the full positive charge on the ammonium nitrogen makes it highly water-soluble (ion-dipole interaction with water) but non-volatile, so it cannot be smoked effectively — instead it is dissolved in water and injected directly into the bloodstream, where its water solubility allows it to mix freely with blood plasma.
