Q8(d). UV and Woodward–Fieser rules: β-ionone and its ring analogue

Introduction. Both are dienones (C=C–C=C–C=O, extended conjugation); their π→π* λ_(max) is estimated by the Woodward–Fieser rules for enones (ethanol): base value for an acyclic or six-ring α,β-unsaturated ketone 215 nm; extended conjugation +30; each alkyl group or ring residue at the γ or δ carbon +18. The intensity ε tells how well the two C=C lie in one plane.

1. The calculated values (299 and 281 nm) match the observed values 296 nm for X and 281 nm for Y. The extra δ-methyl of X shifts its band to longer wavelength.
2. Intensity: Y has ε = 20800, normal for a planar s-trans dienone. In X the ring methyl on C-2' and the gem-dimethyl group on C-6' crowd the side chain; the ring C=C is twisted out of the plane of the side-chain C=C–C=O. Overlap is reduced, so ε falls to 10700 and λ_(max) is a little below the calculated 299 nm.
3. Both criteria agree: λ_(max) 296 nm, ε 10700 belongs to X; λ_(max) 281 nm, ε 20800 to Y.

Conclusion. Woodward–Fieser gives X: 215+30+3(18) = 299 nm and Y: 215+30+2(18) = 281 nm. Hence 296 nm (ε 10700, lowered by steric twisting) is X (β-ionone) and 281 nm (ε 20800, planar) is Y.

Application. β-Ionone is the perfumery and vitamin A starting material; UV of such dienones is used to follow its purity, and the steric loss of intensity is a general test for twisted conjugated systems.