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04-BS-12 · December 2019

Question 5 of 13: Reaction-Energy Diagrams

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

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 5: Reaction-Energy Diagrams (equal value)

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.

EnergyReaction coordinateSMProductTSEa (low)
(a) concerted, exothermic, low Ea
EnergyReaction coordinateSMProductTSEa (high)
(b) one-step, endothermic, high Ea
EnergyReaction coordinateABCTS1TS2
(c) two-step, A<C<B, A→B rate-determining
EnergyReaction coordinateSMProductΔH°=-20 kcal/molEa=4 kcal
(d) concerted, ΔH°=-20 kcal/mol, Ea=4 kcal

a) A single smooth hump (one transition state, no intermediate) with the product energy well below the starting material (exothermic) and a short, low hump (low Ea).

b) A single hump again (one step = one TS), but now the product sits above the starting material (endothermic, positive ΔHo) and the hump is tall (high Ea).

c) Two humps with one intermediate (B) between two transition states. The relative energies must satisfy A < C < B: A (starting material) is drawn lowest, C (final product) a little higher than A but still well below the intermediate B, and B sits as a local minimum between the two humps yet is itself the highest-energy stable species among A, B, C. Because A→B is rate determining, the first hump (TS1, from A to B) must be taller than the second hump (TS2, from B to C) — the slow step always has the higher-energy transition state.

d) One smooth hump; the vertical drop from starting material to product is labelled ΔHo = -20 kcal/mol (exothermic), and the height from starting material up to the transition state is labelled Ea = 4 kcal (a small hump relative to the large exothermicity, consistent with a fast, highly exothermic concerted step).