2 Woodward-Fieser Rules It is used for finding absorption maxima position (λMax)This differs from observed values by 5-6%.Effect of substituent groups can be reliably quantified by use Woodward –Fieser RuleSeparate values for conjugated dienes and trines and α-β-unsaturated ketnones are available
3 Woodward-Fieser Rules Woodward and the Fiesers performed extensive studies of terpene and steroidal alkenes and noted similar substituents and structural features would predictably lead to an empirical prediction of the wavelength for the lowest energy p p* electronic transitionThis work was distilled by Scott in 1964 into an extensive treatise on the Woodward-Fieser rules in combination with comprehensive tables and examples – (A.I. Scott, Interpretation of the Ultraviolet Spectra of Natural Products, Pergamon, NY, 1964)A more modern interpretation was compiled by Rao in 1975 – (C.N.R. Rao, Ultraviolet and Visible Spectroscopy, 3rd Ed., Butterworths, London, 1975)
4 TerminologyAlkenes are named by dropping the -ane ending of the parent and adding -ene.Conjugated :organic compound; containing two or more double bonds each separated from the other by a single bondDienes: One of a class of organic compounds containing two ethylenic linkages (carbon-to-carbon double bonds) in the molecules. Also known as alkadiene; diolefin.
5 TerminologyEndocyclic double bonds have both carbons in the ring and exocyclic double bonds have only one carbon as part of the ring.Cyclopentene is an example of an endocyclic double bond.Methylenecylopentane is an example of an exocyclic double bond.
31 Fieser RulesIf more than four double bonds are present the Woodward rule is not applicableThe plant pigments contains double bonds up to 11 for this Fieser Rules is used.λMax= (M) +n(48-1.7n)- 16.5Rendo-10 Rexon=no of conjugated double bondsM= no of alkyl or alkyl like SubstituentsRendo= No of endocyclic double bondsRexo = No of exocyclic double bonds
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