Daily answer writing · Fri, 30 Oct, 2026
UV spectra of enones: π→π* and n→π* bands, λmax of mesityl oxide and cyclohex-2-enone
An α,β-unsaturated ketone shows two absorption bands in its UV spectrum. Identify the electronic transitions responsible and explain why they differ in intensity. Using the Woodward–Fieser rules, calculate $\lambda_{\max}$ for (a) mesityl oxide, $\ce{(CH3)2C=CH-CO-CH3}$, and (b) cyclohex-2-en-1-one, in ethanol.
Syllabus: Common to CSE Paper II, IFoS Paper II and Geo-Scientist Mains Paper III (spectroscopy): UV–visible spectroscopy, electronic transitions, Woodward–Fieser rules.
Model answer
The two bands
- π→π*: about 215–250 nm, very intense ($\varepsilon \approx 10^4$). Symmetry-allowed, large overlap of π and π*. This is the K-band used in Woodward–Fieser rules.
- n→π*: about 300–330 nm, weak ($\varepsilon < 100$). The O lone pair lies in the nodal plane of π*, so the transition is symmetry-forbidden (R-band). It shifts to shorter λ in polar solvents (blue shift), while π→π* shifts to longer λ.
Woodward–Fieser (enones, ethanol)
Base value, acyclic or six-membered ring enone: 215 nm; each α-alkyl/ring residue +10, each β-alkyl/ring residue +12, exocyclic C=C +5.
(a) Mesityl oxide, $\ce{(CH3)2C=CH-CO-CH3}$: the β-carbon carries two methyl groups, the α-carbon only H.
$$215 + 2(12) = \mathbf{239\ nm}\quad(\text{observed} \approx 237\ \text{nm})$$
(b) Cyclohex-2-en-1-one: C2 (α) carries H; C3 (β) carries one ring residue (C4).
$$215 + 12 = \mathbf{227\ nm}\quad(\text{observed} \approx 225\ \text{nm})$$
Conclusion. Alkyl substitution on the chromophore raises λmax (bathochromic shift), and the additive rules predict it within about 2 nm.
Where the marks are
- Identify π→π* and n→π* with approximate λ and ε (2 marks).
- Reason for weak n→π* (symmetry/overlap forbidden) and solvent shifts (2 marks). An energy-level sketch earns credit.
- Correct base value and increments stated (2 marks).
- Mesityl oxide 239 nm (2 marks); cyclohexenone 227 nm (1 mark), each with substituents shown.
- Comparison with observed values / conclusion (1 mark).