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.
Cholic acid's steroid nucleus is a rigid, fused four-ring hydrocarbon skeleton that is almost entirely nonpolar (only bearing three hydroxyl groups, all clustered on one face of the ring system, with the opposite face presented by axial methyl groups and C–H bonds). Conjugation with taurine (or glycine) converts the terminal carboxylic acid into an amide bearing a strongly polar, fully ionised sulfonate (or carboxylate) group. The resulting bile salt is therefore a genuine amphiphile: one face of the steroid ring system is hydrophobic (like a soap's long alkyl tail) while the hydroxyl-bearing face plus the ionic taurine/glycine head group are hydrophilic (like a soap's carboxylate head).
In water, bile salts spontaneously orient with their hydrophobic faces turned inward, toward a dietary lipid droplet, and their hydrophilic faces turned outward, toward the surrounding water — forming small aggregates (micelles) with the lipid sequestered in the nonpolar interior. This is exactly how a soap emulsifies grease: the hydrophobic tail dissolves in the oil, the hydrophilic head keeps the whole particle suspended in water. In the gut this micellar packaging is what allows fat-soluble lipids, cholesterol, and fat-soluble vitamins to be carried through the aqueous intestinal contents to the absorptive surface of the intestinal wall.
