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Empirical Rules for Absorption Wavelengths of Conjugated Systems

  • Page ID
    1245
  • 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

    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.

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

    Examples

    diendata.gif

    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

    enonedat.gif

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