04-BS-12 · December 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exam 04-BS-12, Organic Chemistry — December 2016. 3 hours, closed-book examination (no non-communicating calculator restriction beyond the standard aid sheet, 8.5×11", hand-written both sides). Ten questions constitute a complete exam paper (only the first 10 questions as they appear in the answer book are marked, each of equal value) — the source paper in fact prints thirteen questions; all thirteen are answered in full below.
Reference texts: McMurry, Organic Chemistry, 9th ed. (acid/base theory of drugs, SN1/SN2 stereochemistry, carbocation rearrangements, alkyne synthesis via acetylide alkylation, IR/NMR structure elucidation, electrophilic aromatic substitution and synthesis design, amino-acid pKa); Clayden, Organic Chemistry, 2nd ed. (amide resonance and β-lactam reactivity, radical vs. ionic HBr addition mechanisms); a standard biomaterials reference for the poly(ester amide) drug-delivery polymer chemistry of Question 13 (Katsarava-type AABB poly(ester amide)s built from diacids, diols, and protected diamino acids).
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.
The difference is polarity/lipophilicity, which controls where each molecule ends up and how it is cleared.
Ethanol is a small, highly polar molecule (one –OH on a two-carbon chain) that is fully water-miscible. It distributes into the body’s total water compartment (blood, tissue water) and is rapidly and almost completely metabolised by alcohol dehydrogenase / aldehyde dehydrogenase in the liver (to acetaldehyde, then acetate), with a typical elimination rate of roughly one standard drink per hour. Because it is water-soluble, unmetabolised ethanol is also efficiently cleared by the kidneys. Within a day (usually a few hours), essentially none remains to detect.
THC, by contrast, is an extremely lipophilic molecule — a large, fused tricyclic terpenoid with a long pentyl side chain and only one free phenolic –OH, giving it a very high octanol/water partition coefficient. Rather than staying in the blood/water compartment, THC and its metabolites (notably 11-nor-9-carboxy-THC) partition into and accumulate in adipose (fat) tissue. Fat tissue acts as a slow-release reservoir: THC metabolites leach back out of fat into the blood only very slowly over days to weeks, especially with repeated use where the fat depot has built up. Standard urine immunoassays target these long-lived, still-circulating metabolites, so they remain detectable for days to several weeks after last use, long after the parent THC itself has left the bloodstream.
In short: ethanol is water-soluble and metabolised/excreted on an hours timescale, leaving no long-term reservoir, whereas THC is fat-soluble and sequestered in adipose tissue, which slowly re-releases detectable metabolite for weeks.
