04-BS-12 · Undated paper
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
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 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. All four targets share one strategic idea: build the carbon skeleton by alkylating an acetylide anion (from acetylene) with a ≤2-carbon electrophile, set the alkene geometry by a stereospecific partial reduction of the resulting internal alkyne (Lindlar's catalyst → cis; Na/NH3(l), a dissolving-metal reduction → trans), and finish with a simple functional-group interconversion where the target is a bromide or aldehyde rather than an alcohol. Parts (b), (c) and (d) deliberately reuse the same trans-alkenyl skeleton.
a) cis-2-buten-1-ol (4 carbons, cis).
Sodium acetylide (from acetylene + NaNH2) attacks formaldehyde (1 carbon) to install the CH2OH terminus (propargyl alcohol). The remaining terminal alkyne C–H is then deprotonated and alkylated with methyl iodide (1 carbon) to extend the other end, giving 2-butyn-1-ol. Hydrogenation over Lindlar's catalyst (Pd/CaCO3, poisoned with lead and quinoline) delivers syn addition of H2 across the triple bond and stops at the alkene stage, giving the cis alkene directly.
b) trans-hex-3-en-1-ol (6 carbons, trans).
Sodium acetylide opens ethylene oxide (2 carbons) to install a two-carbon –CH2CH2OH tail (3-butyn-1-ol). The remaining terminal alkyne is deprotonated and alkylated with ethyl bromide (2 carbons) to extend the far end, giving hex-3-yn-1-ol. A dissolving-metal reduction (Na in liquid NH3) proceeds through a trans-vinyl radical/anion intermediate and delivers the trans alkene selectively — the geometric complement of Lindlar hydrogenation used in part (a).
c) trans-hex-3-enyl bromide.
Treating the alcohol from part (b) with PBr3 converts –OH to –Br by an SN2-type mechanism at the unhindered primary carbon, with no reagent present that could touch the internal alkene — the trans geometry set in part (b) survives unchanged.
d) trans-hex-3-enal.
PCC (pyridinium chlorochromate) is a mild, anhydrous oxidant that stops cleanly at the aldehyde (no over-oxidation to the carboxylic acid, unlike aqueous Cr(VI) reagents) and, being neither acidic/aqueous nor a source of Br2/H+, again leaves the internal alkene untouched.
