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04-BS-12 · May 2017

Question 11 of 13: Vitamin C — Ascorbate Radical and Why That H Is So Easily Removed

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

Notes on this paper

National Exam 04-BS-12, Organic Chemistry — May 2017. 3 hours, closed-book examination (no textbook aid beyond one double-sided aid sheet); a Casio or Sharp approved calculator is permitted. The paper prints thirteen questions using plain "Question N:" numbering; all thirteen are answered in full below.

Reference texts: McMurry, Organic Chemistry, 9th ed. (functional-group reactivity, amide/β-lactam resonance, stereochemistry and specific rotation, SN1/SN2 and epoxide-opening regiochemistry, cyclohexane conformational analysis, IR/NMR spectroscopy, electrophilic aromatic substitution and multi-step synthesis design, polymer/step-growth chemistry); Atkins, Physical Chemistry, 11th ed. (entropy of intramolecular vs. intermolecular reactions).

Question 11: Vitamin C — Ascorbate Radical and Why That H Is So Easily Removed

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.

vitamin C (ascorbic acid)
X, the ascorbate monoanion (deprotonated at C3–OH)

X is ascorbate: the more acidic of the ring's two enediol –OH groups (C3–OH, conjugated directly with the ring C=O through the C2=C3 double bond) is already deprotonated. The indicated hydrogen the question asks about is the remaining enediol O–H (at C2), which sits directly conjugated with that same enolate/carbonyl system.

  1. Homolysis of the C2–O–H bond. An oxidising radical abstracts this hydrogen atom, leaving one unpaired electron on the C2 oxygen.
  2. The resulting radical is delocalised, not localised. Because that oxygen sits on a carbon (C2) that is doubly bonded to C3, which itself bears the ring carbonyl and the already-anionic C3-oxygen system, the unpaired electron can be drawn shifted — by resonance — from the C2 oxygen onto C3 and on to the ring carbonyl oxygen. The radical (and the pre-existing negative charge) are shared over an extended O–C=C–C(=O)–O conjugated system, not confined to a single atom.
  3. Why this makes the H unusually easy to remove. Homolytic bond dissociation energy is lowered whenever the resulting radical is resonance-stabilised; spreading the odd electron over three atoms (two oxygens and the ring carbon) instead of leaving it isolated on one oxygen lowers the energy of the radical dramatically, in exactly the same way resonance lowers the energy of a delocalised carbanion. A lower-energy radical means a lower bond dissociation energy for that particular O–H, so it is abstracted in preference to an ordinary, non-conjugated O–H (such as the two side-chain –OH groups on the exocyclic CH(OH)CH2OH tail, which have no comparable conjugated system to stabilise their radicals).

This resonance-stabilised, relatively unreactive radical is the whole basis of ascorbate's antioxidant action: by sacrificially donating this one, easily-removed and well-stabilised hydrogen atom, ascorbate converts a reactive, damaging radical into a much less reactive one, halting a radical chain oxidation before it can propagate further.