04-BS-12 · December 2019
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exam 04-BS-12, Organic Chemistry — December 2019. 3 hours, closed-book examination (no calculator; 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. (functional groups, stereochemistry, SN1/SN2 mechanisms and stereochemistry, steroid/bile-acid amphiphilicity, named-drug synthesis design, reaction-energy diagrams, polymer chemistry, arene-oxide metabolism, and mass-spectral/IR/NMR structure elucidation). Every molecular formula, exact mass, and stereochemical (R/S, cis/trans, meso/chiral) 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) Classification. Benzene → arene oxide is an oxidation: cytochrome P450 (a monooxygenase) inserts one oxygen atom across a ring double bond, increasing the oxidation state of the two carbons involved (each gains a bond to the more electronegative oxygen). Arene oxide → phenol is neither an oxidation nor a reduction — it is an intramolecular isomerisation (the NIH shift): the strained epoxide opens to a benzylic cation/arenium-like intermediate, a hydrogen migrates, and re-aromatisation expels a proton to give the phenol. No atoms are added or removed and no electrons are formally transferred to or from an external oxidant/reductant, so the carbon oxidation states are unchanged overall between the oxide and the phenol.
b) Water solubility and its importance. Phenol's –OH group can both donate a hydrogen bond (via its O–H) and accept one (via the oxygen lone pairs) to surrounding water molecules, whereas benzene has no polar functional group and can only engage water through weak dispersion forces. Hydrogen bonding to water is a strong, specific, energetically favourable interaction, so phenol is markedly more water soluble than benzene. Physiologically this matters because the kidneys excrete compounds into urine (an aqueous medium); a lipophilic compound like benzene would simply re-partition into fatty tissue and accumulate, whereas a water-soluble metabolite like phenol (often further conjugated to a sulfate or glucuronide) can be filtered by the kidney and eliminated from the body.
