04-BS-12 · May 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exam 04-BS-12, Organic Chemistry — May 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. (drug acid–base/salt pharmacokinetics, steroid/bile-acid amphiphilicity, arene-oxide metabolism, cyclopropane stereochemistry and CIP assignment, reaction-energy diagrams, ester equilibria and intramolecular effective molarity, SN2 stereochemistry at a common stereocentre, named-drug synthesis design, epoxide ring-opening stereochemistry, mass-spectral formula discrimination, opioid IR/NMR structure elucidation, keto–enol tautomerism and conjugation/acidity, and condensation-polymer monomer identification). Every molecular formula, mass-balance, exact-mass, and stereochemical (R/S) 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.
a) Oxidation state classification. Benzene (C6H6) → arene oxide (C6H6O) adds one oxygen atom across a ring C=C without removing any hydrogens — this is an oxidation (cytochrome P450 inserts an epoxide oxygen, consuming O2 and NADPH). Arene oxide → phenol is an intramolecular isomerisation (the "NIH shift"): the strained epoxide opens to a benzylic-type cation stabilised by the ring, a 1,2-hydride shift relocates one ring hydrogen, and loss of H+ restores aromaticity as the phenol. No oxygen or hydrogen atom is gained or lost overall in this second step (C6H6O → C6H6O), so it is classified as neither an oxidation nor a reduction.
b) Why phenol is more water-soluble, and why that matters. Phenol's –OH group is a strong hydrogen-bond donor and acceptor, so phenol molecules hydrogen-bond directly with water, whereas benzene has no polar functional group and can only engage water through weak induced-dipole interactions (benzene's water solubility is only ∼0.18 g/100 mL vs. phenol's fully miscible-at-body-temperature behaviour). This matters physiologically because the kidney excretes compounds through an aqueous filtrate (urine); a purely hydrophobic molecule like benzene would simply re-partition back into fatty tissue/cell membranes rather than staying dissolved in urine, so the body cannot eliminate it efficiently until it has been converted to a polar, water-soluble metabolite (phenol, and downstream conjugates such as phenyl sulfate or phenyl glucuronide) that will actually stay dissolved in the aqueous excretory pathway.
