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.
Every Diels–Alder reaction here is a concerted, suprafacial-suprafacial [4+2] cycloaddition: the diene's terminal carbons form two new σ bonds to the dienophile's alkene carbons, the diene's central single bond becomes the new ring double bond, and — the graded stereochemistry point in all three parts — whatever relative geometry (cis or trans) the dienophile substituents had before the reaction is retained in the product, because both new bonds form on the same face of the dienophile in one concerted step (no intermediate in which the alkene could rotate).
a) Isoprene (2-methyl-1,3-butadiene) + dimethyl maleate. The diene shown is isoprene (a single methyl branch on an internal diene carbon), not plain butadiene — the methyl ends up on the new ring's alkene carbon (it started on an sp2 diene carbon and stays sp2 in the product). Dimethyl maleate is the cis (Z) diester, so its two ester groups stay cis to each other in the ring.
b) 2,3-Dimethyl-1,3-butadiene + dimethyl fumarate. This diene is symmetric (a methyl on each internal carbon), so there is no regiochemistry ambiguity at all — both new ring methyls sit on the ring alkene exactly as drawn. Dimethyl fumarate is the trans (E) diester, so the two esters come out trans to each other in the product.
c) 1,3-Butadiene + 1,4-benzoquinone. 1,4-Benzoquinone offers one of its two equivalent enedione C=C bonds as the dienophile; butadiene adds across it exactly as in parts (a)/(b), leaving the other enedione C=C and both carbonyls untouched. The product is the bicyclic mono-adduct, still bearing a fresh cyclohexene ring (from butadiene's own former single bond, now the new ring alkene) fused to the remaining cyclohexenedione ring.
