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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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