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.
All eight transformations radiate out from three hub compounds — cyclopentene, cyclopentene oxide, and cyclopentanol — so the fastest way through the question is to recognise each hub's own standard reaction toolkit.
| Step | Transformation | Reagents | Why |
|---|---|---|---|
| (a) | cyclopentene → cyclopentane | H2, Pd/C | catalytic hydrogenation of the alkene |
| (b) | cyclopentene → cyclopentene oxide | mCPBA (a peroxyacid) | direct epoxidation |
| (c) | cyclopentene → cis-diol | OsO4, then NaHSO3/NMO (or cold, dilute KMnO4/OH–) | syn dihydroxylation — both new OH's add to the same face |
| (d) | bromocyclopentane → cyclopentane | Bu3SnH, AIBN, Δ (or: Mg, Et2O then H3O+) | radical (or Grignard/protonation) reductive removal of the halide |
| (e) | epoxide → cyclopentanol | LiAlH4, then H3O+ | hydride opens the strained epoxide, adding just one H and one OH (net epoxide reduction) |
| (f) | epoxide → trans-diol (+ enantiomer) | H3O+, H2O | acid-catalysed epoxide hydrolysis, backside (SN2-like) water attack ⇒ anti opening ⇒ trans-diol, racemic |
| (g) | cyclopentanol → bromocyclopentane | PBr3 (or HBr) | converts –OH directly to –Br |
| (h) | cyclopentanol → cyclopentanone | PCC, CH2Cl2 (or Na2Cr2O7/H2SO4) | oxidation of a 2° alcohol to a ketone |
Steps (c) and (f) are the pedagogical core of this web: both start from the same C=C (or its epoxide) but give opposite diol stereochemistry because OsO4 delivers both new oxygens from the same face of the alkene in one concerted step (syn addition ⇒ cis-diol), whereas acid-catalysed epoxide hydrolysis first forms the epoxide with both oxygens already syn, then water attacks the protonated epoxide from the face opposite the oxygen (backside, SN2-like), flipping one of the two stereocentres and netting an overall anti addition (⇒ trans-diol) relative to the original alkene.
