SpectroscopyStructural Analysis

(Application of Woodward-Fieser Rules)

U.A. Deokate

Copyright © by U. A. Deokate, all rights reserved.

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

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 transition

This 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)

Terminology

Alkenes 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 bond

Dienes: One of a class of organic compounds containing two ethylenic linkages (carbon-to-carbon double bonds) in the molecules. Also known as alkadiene; diolefin.

Terminology

Endocyclic 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.

Woodward Fieser Rule for Dienes & Polyenes

Problem Illustration -1

Problem Illustration -2

Problem Illustration -3

Problem Illustration -4

Woodward Fieser rule for undsaturated carbonyl compound

Problem Illustration - 5

Problem Illustration - 6

Problem Illustration - 1

Problem Illustration - 2

Worked Problem - 1

Worked Problem - 2

Worked Problem - 3

Worked Problem - 4

Worked Problem - 5

Worked Problem - 6

Worked Problem - 7

Worked Problem - 8

EMPIRICAL RULES FOR BENZOYL DERIVATIV

ES

Problem - 1

Problem - 2

Problem - 3

Problem - 4

Problem- 5

Fieser Rules

If more than four double bonds are present the Woodward rule is not applicable

The plant pigments contains double bonds up to 11 for this Fieser Rules is used.

λMax= 114 + 5(M) +n(48-1.7n)- 16.5Rendo-10 Rexo

n=no of conjugated double bonds M= no of alkyl or alkyl like Substituents Rendo= No of endocyclic double bonds

Rexo = No of exocyclic double bonds

Fisher Problem-1

λMax= 114 + 5(M) +n(48-1.7n)- 16.5Rendo-10 Rexo

= 114 + 5 (8) + 11(48-1.7x 11) –0 -0 = 476

Fisher formula

Fisher Problem-2

λMax= 114 + 5(M) +n(48-1.7n)- 16.5Rendo-10 Rexo

= 114 + 5 (8) + 11(48-1.7x 11) –0 -0 = 476

Fisher formula

Fisher Problem-3

λMax= 114 + 5(M) +n(48-1.7n)- 16.5Rendo-10 Rexo

= 114 + 5 (8) + 18(48-1.7x 18) –0 -0 =

Fisher formula