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.
Cholic acid's steroid nucleus (four fused, largely saturated carbocyclic rings, plus the alkyl side chain) is a rigid, entirely non-polar hydrocarbon face. In the body it is conjugated (amide bond) to taurine, converting the terminal –COOH into –CONHCH2CH2SO3– Na+ — a permanently ionic, highly water-soluble sulfonate head group. The three ring hydroxyls (at C3, C7, C12) all sit on one face of the steroid skeleton (the same face, all drawn with the same wedge convention), so the molecule ends up with a polar/hydrophilic face (the OH-bearing convex side plus the ionic sulfonate head) and a non-polar/hydrophobic face (the opposite, all-hydrocarbon convex side) — a facially amphiphilic molecule, geometrically analogous to a soap's separate polar head and non-polar tail, just folded into one rigid steroid body instead of a flexible linear chain.
This is exactly what makes bile salts able to transport lipids through the aqueous intestinal environment: many bile-salt molecules orient their hydrophobic faces inward around a droplet of dietary fat (or cluster their hydrocarbon faces against each other) while their hydrophilic OH/sulfonate faces point outward into the surrounding water, forming a water-soluble micelle. The lipid is solubilised inside the micelle's non-polar interior while the polar outer surface keeps the whole assembly dispersed in the aqueous digestive fluid, exactly as a soap micelle solubilises grease in water.
