Q5(d). λ_(max) by Woodward–Fieser and Scott rules
Predict λ_(max) for the following molecules:
(i) [structure] (ii) [structure]
Answer in brief
(i) 6-Methoxy-1-tetralone: 246 + 3 + 25 = 274 nm (Scott). (ii) The cross-ring dienone: 215 + 30 + 12 + 18 + 5 = 280 nm (Woodward–Fieser). Both are π→π* bands; the weak n→π* bands lie above 300 nm.
Key points
- Introduction. The π→π* band of a conjugated carbonyl compound is predicted by adding increments to a base value: Scott's rules for aryl ketones ArCOR and Woodward–Fieser rules for α,β-unsaturated ketones (ethanol solution).
- The chromophore is Ar-CO-R. The ring carries the alkyl ring residue (C-4) ortho to the C=O and OCH₃ para to it.
The full model answer
Written out in full, as it should go in the answer booklet. The solution page with its diagrams follows it.
Introduction. The π→π* band of a conjugated carbonyl compound is predicted by adding increments to a base value: Scott's rules for aryl ketones ArCOR and Woodward–Fieser rules for α,β-unsaturated ketones (ethanol solution).
(i) 6-Methoxy-1-tetralone (aryl ketone)
The chromophore is Ar-CO-R. The ring carries the alkyl ring residue (C-4) ortho to the C=O and OCH₃ para to it.
| Base value, ArCOR (alkyl ketone) | 246 nm |
| o-Ring residue | + 3 |
| p-OCH₃ | + 25 |
| Calculated λ_(max) | 274 nm |
λ_(max)≈ 274 nm
(ii) 1-(Cyclohex-2-en-1-ylidene)propan-2-one (dienone)
The C=O is conjugated with the exocyclic Cα=Cβ and, through it, with the ring Cγ=Cδ: an acyclic dienone. Cβ carries one ring residue (C-6); Cδ carries one ring residue (C-4); Cα=Cβ is exocyclic to the six-membered ring. The diene part is not homoannular (only one of its C=C is in the ring).
Figure: C=O, Cα=Cβ (exocyclic) and Cγ=Cδ (in the ring).
| Base value, acyclic α,β-unsaturated ketone | 215 nm |
| Double bond extending conjugation | + 30 |
| β-Ring residue | + 12 |
| δ-Ring residue | + 18 |
| Exocyclic C=C (Cα=Cβ) | + 5 |
| Calculated λ_(max) | 280 nm |
λ_(max)≈ 280 nm
Conclusion. (i) 6-Methoxy-1-tetralone: 246 + 3 + 25 = 274 nm (Scott). (ii) The cross-ring dienone: 215 + 30 + 12 + 18 + 5 = 280 nm (Woodward–Fieser). Both are π→π* bands; the weak n→π* bands lie above 300 nm.
Application. Such calculations distinguish isomeric enones and dienones (for example the position of the C=C in steroid ketones) and confirm the extent of conjugation in natural products.
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All parts of Question 5
- 5(a) DNA bases and Watson–Crick pairing10 marks
- 5(b) Baeyer–Villiger oxidation of an electron-rich aldehyde10 marks
- 5(c) Photochemistry of norbornenone: 1,3-acyl shift and ketene loss10 marks
- 5(d) λ_(max) by Woodward–Fieser and Scott rules10 marks
- 5(e) IR spectra of two isoxazolone tautomers10 marks