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.
a) Epoxide. Treat cyclohexene with a peroxyacid (e.g. mCPBA) in one step: concerted syn addition of one oxygen across the double bond gives the epoxide directly.
b) Nitrile. Two steps: (i) Markovnikov hydrohalogenation with HBr installs a secondary bromide (both alkene carbons of cyclohexene are equivalent, so this is unambiguous); (ii) treat the bromocyclohexane with NaCN — SN2 displacement of bromide by cyanide installs the nitrile directly on the ring carbon.
c) Propyl ether. One pot, two reagents: oxymercuration (Hg(OAc)2, 1-propanol as the nucleophile/solvent) adds propanol across the double bond Markovnikov-fashion without carbocation rearrangement, then NaBH4 reductively removes the mercury, giving cyclohexyl propyl ether cleanly.
d) trans-2-Mercaptocyclohexanol. Two steps, reusing target (a)'s epoxide directly: epoxidise cyclohexene (mCPBA) to give the same 1,2-epoxycyclohexane as in part (a), then open the epoxide with a sulfur nucleophile (NaSH). Epoxide-opening by a strong nucleophile is an SN2 backside attack, which necessarily delivers the incoming group anti to the oxygen that remains as the alcohol — giving the trans product; since the starting alkene and epoxidation are non-stereospecific with respect to which face reacts, both enantiomers form equally (racemic, "+ enantiomer").
e) trans-2-Ethynylcyclohexanol. Same two-step epoxide-opening strategy as (d), reusing the identical epoxide intermediate again, but with lithium (or sodium) acetylide, HC≡C-, as the ring-opening nucleophile in place of HS-. The same anti-periplanar SN2 opening logic applies, giving the racemic trans product.
