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### 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, λmax, and the structure of a conjugated π system. The Woodward–Fieser rules predict the value of λmax 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
λmax (calculated) = 240 nm
λmax (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
λmax (calculated) = 249 nm
λmax (observed) = 249 nm

#### Tags

UV-Vis Spectroscopy Woodward-Fieser Rules Conjugated Dienes Absorption Spectra Chromophore Auxochromes Absorption Maxima Structure Substituents Woodward-Fieser Rules Base Wavelength Dienes Trienes Î±,Î²-unsaturated Carbonyl Compounds Base Value Homoannular Diene Heteroannular Diene Double Bond Extended Conjugation Ring Residue Exocyclic Double Bond Alkyl Group -OR Group -Cl -Br Group -NR2 Group

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