Empirical Rules for Absorption Wavelengths of Conjugated Systems

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Woodward-Fieser Rules for Calculating the λ_max of Conjugated Dienes and Polyenes

Core Chromophore	Substituent and Influence
Transoid Diene 215 nm	R- (Alkyl Group) .... +5 nm RO- (Alkoxy Group) .. +6 X- (Cl- or Br-) ......... +10 RCO₂- (Acyl Group) .... 0 RS- (Sulfide Group) .. +30 R₂N- (Amino Group) .. +60 Further π -Conjugation C=C (Double Bond) ... +30 C₆H₅ (Phenyl Group) ... +60
Cyclohexadiene* 260 nm
(i) Each exocyclic double bond adds 5 nm. In the example on the right, there are two exo-double bond components: one to ring A and the other to ring B. (ii) Solvent effects are minor. * When a homoannular (same ring) cyclohexadiene chromophore is present, a base value of 260 nm should be choosen. This includes the ring substituents. Rings of other size have a lesser influence.

λ_max (calculated) = Base (215 or 260) + Substituent Contributions

Examples

Woodward-Fieser Rules for Calculating the π → π* λ_max of Conjugated Carbonyl Compounds

Core Chromophore	Substituent and Influence
R = Alkyl 215 nm R = H 210 nm R = OR' 195 nm	α- Substituent R- (Alkyl Group) +10 nm Cl- (Chloro Group) +15 Br- (Chloro Group) +25 HO- (Hydroxyl Group) +35 RO- (Alkoxyl Group) +35 RCO₂- (Acyl Group) +6 β- Substituent R- (Alkyl Group) +12 nm Cl- (Chloro Group) +12 Br- (Chloro Group) +30 HO- (Hydroxyl Group) +30 RO- (Alkoxyl Group) +30 RCO₂- (Acyl Group) +6 RS- (Sulfide Group) +85 R₂N- (Amino Group) +95 γ & δ- Substituents R- (Alkyl Group) +18 nm (both γ & δ) HO- (Hydroxyl Group) +50 nm (γ) RO- (Alkoxyl Group) +30 nm (γ) Further π -Conjugation C=C (Double Bond) ... +30 C₆H₅ (Phenyl Group) ... +60
Cyclopentenone 202 nm

(i) Each exocyclic double bond adds 5 nm. In the example on the right, there are two exo-double bond components: one to ring A and the other to ring B. (ii) Homoannular cyclohexadiene component adds +35 nm (ring atoms must be counted separately as substituents) (iii) Solvent Correction: water = –8; methanol/ethanol = 0; ether = +7; hexane/cyclohexane = +11

λ_max (calculated) = Base + Substituent Contributions and Corrections

Examples

Contributors

William Reusch, Professor Emeritus (Michigan State U.), Virtual Textbook of Organic Chemistry

Woodward-Fieser Rules for Calculating the λmax of Conjugated Dienes and Polyenes

Examples

Woodward-Fieser Rules for Calculating the π → π* λmax of Conjugated Carbonyl Compounds

Contributors

Woodward-Fieser Rules for Calculating the λ_max of Conjugated Dienes and Polyenes

Woodward-Fieser Rules for Calculating the π → π* λ_max of Conjugated Carbonyl Compounds