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.
All four are backside-attack, single-step SN2 substitutions (good leaving group, good nucleophile, no rearrangement possible): the nucleophile displaces the leaving group with inversion of configuration at the carbon bearing the leaving group only — nothing at any other carbon changes.
(a) A deuterium-labelled stereocentre — the reaction carbon itself, so the configuration inverts. The four groups on this carbon are Cl, CH3, D, and H, so it is a genuine (isotopically-differentiated) stereocentre. Methoxide attacks from the face opposite Cl, so the three retained groups (CH3, D, H) are pushed through to the opposite side, exactly like an umbrella flipping inside-out.
(b) A primary, non-stereogenic centre — simple substitution, no stereochemistry to track. 1-Iodopentane is a straight-chain primary iodide; C1 bears two identical H's, so it is not a stereocentre and inversion there has no observable consequence.
(c) The drawn stereocentre is a spectator, three carbons from the reacting carbon — its configuration is untouched. C3 (bearing CH3, H, and the two different chain arms –CH2CH3 and –CH2CH2Cl) is a real stereocentre, but the leaving group (Cl) sits on C1, well away from it. Ethoxide attacks C1 only; C3 never rehybridises and keeps exactly the spatial arrangement it started with.
(d) A ring stereocentre — inversion at C1 flips the ring’s cis/trans descriptor even though C4–CH3 never moves. In the trans starting bromide, Br (C1) and CH3 (C4) sit on opposite faces of the ring. Cyanide attacks C1 from the face opposite Br; since C4 is untouched, the new C–CN bond ends up on the same face as the (unmoved) C4–CH3, converting the relationship from trans to cis.
