Question 12 of 13: Curcumin Tautomerism, Acidity, Colour, and Antioxidant Activity

Question 12: Curcumin Tautomerism, Acidity, Colour, and Antioxidant Activity (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.

a) Keto form and enol stability. The drawn structure is curcumin's enol tautomer: a 1,3-diketone-derived enol in which the central –C(OH)=CH– unit is stabilised by a strong intramolecular hydrogen bond to the neighbouring carbonyl oxygen (shown explicitly as the dashed H···O contact), forming a pseudo-six-membered chelate ring. Converting this enol to its keto tautomer simply moves that one proton from the enol oxygen onto the adjacent carbon, restoring a normal (non-hydrogen-bonded) 1,3-diketone flanked on both sides by the same conjugated aryl-vinyl "cinnamoyl" arms:

Most simple enols are minor, fleeting tautomers (<1% at equilibrium) because a plain C=C–OH is not intrinsically more stable than its keto form. Curcumin's enol is unusual because it is stabilised both by the strong internal O–H···O=C hydrogen bond and by extended conjugation: the enol form places the C=C–OH in full conjugation with the remaining carbonyl and with both aryl "cinnamoyl" arms simultaneously, creating one long, fully conjugated push-pull system across the whole molecule — an electronic stabilisation the keto form (with an sp³-like interrupting CH₂ at the centre) cannot match. Together these effects make the enol tautomer of curcumin the major species at equilibrium, unlike almost any simple ketone.

b) Why the enol O–H is more acidic than an ordinary alcohol O–H. Deprotonating the enol –OH gives an anion whose negative charge is not localised on that one oxygen: it is delocalised by resonance onto the adjacent carbonyl oxygen (through the conjugated C=C–C=O system), giving a symmetric, enolate/carboxylate-like resonance-stabilised anion. Deprotonating an ordinary alcohol gives a simple alkoxide with the negative charge localised entirely on one oxygen — no comparable resonance delocalisation is available. Because the enolate's conjugate base is much better stabilised, the enol proton is removed far more easily (lower pK_a, more acidic) than a typical alcohol proton.

c) Why curcumin is coloured. Curcumin has an unusually long, fully conjugated π-system: two aromatic rings connected through alkene and carbonyl/enol units in one continuous conjugation pathway spanning the entire molecule. Extending conjugation lowers the energy gap between the highest-occupied and lowest-unoccupied molecular orbitals (HOMO–LUMO gap) until it falls low enough to absorb visible light (curcumin absorbs strongly around 420–430 nm, in the blue-violet region); the light that is not absorbed (predominantly yellow/orange wavelengths) is transmitted or reflected, which is the colour we perceive.

d) Why curcumin is an antioxidant. Curcumin carries phenolic –OH groups on both aryl rings. A phenolic O–H can donate its hydrogen atom to a reactive free radical (hydrogen-atom transfer), quenching the radical chain before it can propagate (e.g. before it can abstract a hydrogen from a lipid and continue a peroxidation chain reaction). This works because the resulting phenoxyl radical is itself resonance-stabilised, delocalised into the aromatic ring and, thanks to curcumin's extended conjugation, even further into the central diketone/enol system — making the radical intermediate low enough in energy that the H-donation step is thermodynamically favourable, exactly the same mechanism that makes vitamin E and BHT effective antioxidants.