16.10 : UV–Vis Spectroscopy: Woodward–Fieser Rules

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