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04-BS-12 · Undated paper

Question 6 of 13

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Notes on this paper

National Exam 04-BS-12, Organic Chemistry — May 2019 sitting (the page-1 header and the running footer, "04-BS-12/May 2019", both give the date). 3 hours, closed-book examination; one Casio/Sharp-approved calculator 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. (acid/base theory, functional-group identification, SN1/SN2 mechanisms and stereochemistry, alkyne/acetylide synthesis, electrophilic aromatic substitution, IR/NMR/mass-spectral structure elucidation, named-drug synthesis design, and step-growth polymer chemistry). Every molecular formula, exact mass, and stereochemical (R/S, cis/trans) assignment below.

Question 6 (6/13)

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.

Approach. Every internal alkyne can, in principle, be disconnected at the C(sp)–C(sp3) bond on either side of the triple bond. The rule that picks the right disconnection every time: acetylide ion is a strong, unhindered base as well as a nucleophile, so the alkyl halide partner must always be primary (or methyl) — pairing it with a secondary or tertiary halide invites E2 elimination instead of the desired SN2 alkylation. When a target alkyne has one side primary and the other side secondary/ tertiary/aryl, only the disconnection that leaves a primary halide is synthetically viable.

6(a) target: HC≡C–CH2CH2CH(CH3)2 (5-methylhex-1-yne).

a) This alkyne is terminal, so there is only one possible disconnection: the acetylide is simply the anion of acetylene itself (HC≡C−, from HC≡CH + NaNH2), alkylated with 1-halo-3-methylbutane (isoamyl halide, X–CH2CH2CH(CH3)2, a primary halide — clean SN2).

6(b) target: CH3–C≡C–CH(CH3)–CH2CH3 (4-methylhex-2-yne).

b) Disconnecting at the methyl side gives acetylide = the anion of 3-methylpent-1-yne, HC≡C–CH(CH3)CH2CH3, alkylated with CH3I (methyl iodide, primary/methyl — ideal). The alternative disconnection (propyne's acetylide + a secondary 2-halobutane) is avoided: a secondary halide with a strong, bulky base like an acetylide anion favours E2 elimination over substitution, so it is not the intended answer.

6(c) target: cyclohexyl–C≡C–CH2CH2CH3.

c) Disconnecting at the ring gives acetylide = the anion of cyclohexylacetylene (cyclohexyl–C≡CH, from cyclohexylacetylene + NaNH2), alkylated with 1-halopropane (n-propyl halide, primary). The reverse disconnection would require a secondary cyclohexyl halide as the electrophile — again the wrong choice for the same E2-avoidance reason.