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# Empirical Rules for Absorption Wavelengths of Conjugated Systems

### Woodward-Fieser Rules for Calculating the λmax 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

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.

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

### Woodward-Fieser Rules for Calculating the π → π* λmax 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

λmax (calculated) = Base + Substituent Contributions and Corrections

Examples