UV–Visible absorption spectra of conjugated dienes arise from the lowest energy π → π* transitions. The light-absorbing part of the molecule is called the chromophore, and the substituents directly attached to the chromophore are called auxochromes. A strong correlation exists between the absorption maxima, λ_max, and the structure of a conjugated π system. The Woodward–Fieser rules predict the value of λ_max for a given structure by adding the contributions from various substituents to a base wavelength, which remains fixed for a given class of dienes or trienes. The applicability of Woodward–Fieser rules can be extended to α,β-unsaturated carbonyl compounds.

Woodward–Fieser Rules for Dienes

Base value:
Acyclic diene = 214 nm
Homoannular diene = 214 nm
Heteroannular diene = 253 nm

Substituent contributions:
Double bond extended conjugation = 30 nm
Ring residue = 5 nm
Exocyclic double bond = 5 nm
Substituent groups:
- Alkyl group = 5 nm
- –OR = 6 nm
- –Cl, –Br = 5 nm
- –NR₂ = 60 nm

Figure 1.

Calculation for Figure 1:
Base value (Heteroannular diene) = 214 nm
Three ring residues = 3 (5 nm) = 15 nm
One exocyclic double bond = 5 nm
One –OR group = 6 nm
λ_max (calculated) = 240 nm
λ_max (observed) = 241 nm

Woodward–Fieser Rules for α,β-Unsaturated Carbonyl Compounds

Base value:
Acyclic or six-membered cyclic enones = 215 nm
Five-membered cyclic enones = 202 nm

Substituent contributions:
Double bond extended conjugation = 30 nm
Exocyclic double bond = 5 nm
Ring residue: α = 10 nm; β = 12 nm
Substituent groups:
- Alkyl: α = 10 nm ; β = 12 nm
- –OCH₃: α = 35 nm ; β = 30 nm
- –Cl: α = 15 nm ; β = 12 nm
- –Br: α = 25 nm ; β = 30 nm
- –NR₂: β = 95 nm

Figure 2.

Calculation for Figure 2:
Base value (Acyclic enone) = 215 nm
One α –CH₃ = 10 nm
Two β –CH₃= 2 (12 nm) = 24 nm
λ_max (calculated) = 249 nm
λ_max (observed) = 249 nm