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.
The mechanism call in each part comes from the same two-factor test: substrate class (1°/2°/3° carbon bearing the leaving group) and nucleophile strength (a charged nucleophile is strong/favours SN2; a neutral solvent molecule such as CH3OH is weak and favours SN1 by solvolysis whenever the substrate can support a carbocation).
| Part | Substrate | Nucleophile | Mechanism | Product |
|---|---|---|---|---|
| a | 3° (2-chloro-2-methylpentane) | CH3OH, weak/neutral | SN1 | 2-methoxy-2-methylpentane |
| b | 1° (exocyclic CH2Br) | –SH, strong | SN2 | cyclohexylmethanethiol |
| c | 2° (iodocyclohexane) | CH3CH2O–, strong | SN2 | ethoxycyclohexane |
| d | 2° (2-bromobutane) | CH3OH, weak/neutral | SN1 | 2-methoxybutane |
a) The substrate is a fully substituted tertiary carbon (two methyls + a propyl chain + Cl); a tertiary carbocation is easily stable enough to form, and CH3OH is a weak, un-charged nucleophile/solvent — classic SN1 conditions (ionisation is rate-limiting; SN2 backside attack on a 3° centre is blocked sterically in any case). Note the two methyl branches on the reacting carbon make it not a stereocentre, so no R/S label is needed for the product.
b) The leaving group sits on a primary, unhindered exocyclic CH2, and –SH is a strong, negatively-charged, highly polarisable nucleophile — ideal SN2 conditions (backside attack is fast and unhindered; a primary carbocation is far too unstable for SN1 to compete).
c) Iodocyclohexane is a secondary substrate, but ethoxide is a strong, small, negatively-charged nucleophile — strong nucleophiles push secondary substrates toward SN2 (the question asks only for SN1 vs. SN2, so E2 competition from this moderately basic nucleophile is not scored here).
d) 2-Bromobutane is also secondary, but here the nucleophile is neutral, weak CH3OH (solvolysis conditions) — a secondary carbocation is stable enough to form under solvolysis, so this pair reacts by SN1 (with racemisation at the planar cationic centre, since the starting bromide's stereochemistry is not specified here).
