Woodward’s Rule

Sets of empirical rules, often referred to as Woodward s rules or the Woodward-Fieser rules, enable the absorption maxima of dienes (Table 7.11) and enones and dienones (Table 7.12) to be predicted. To the respective base values (absorption wavelength of parent compound) are added the increments for the structural features or substituent groups present. When necessary, a solvent correction is also applied (Table 7.13). 

The correlations presented here are sometimes referred to as Woodward s rules or the Woodward-Fieser rules. 

Empirical rules (Woodward s Rules) of good predictive value are available to estimate the positions of the absorption maxima in conjugated alkenes and conjugated carbonyl compounds. 

Tables 6.3-6.5 record data developed to undertake structural analysis in systems possessing chromophores that are conjugated or otherwise interact with each other. Chromophores within a molecule interact when linked directly to each other or when they are forced into proximity owing to structural constraints. Certain combinations of functional groups comprise chromophoric systems that exhibit characteristic absorption bands. In the era when UV-VIS was one of the principal spectral methods available to the organic chemist, sets of empirical rules were developed to extract as much information as possible from the spectra. The correlations referred to as Woodward s rules or the Woodward-Fieser rules, enable the absorption maxima of dienes (Table 6.3) and enones and dienones (Table 6.4) to be predicted. When this method is applied, wavelength increments correlated to structural features are added to the respective base values (absorption wavelength of parent compound). The data refer to spectra determined in methanol or ethanol. When other solvents are used, a numerical correction must be applied. These corrections are recorded in Table 6.5.

Solvents for Ultraviolet Spectrophotometry Ultraviolet Spectra of Common Liquids Transmittance-Absorbance Conversion Correlation Table of Ultraviolet Active Functionalities Woodward s Rules for Bathochromic Shifts... 

Woodward4 analysed the data on the absorption spectra of dienes and found that alkyl substituents and exocyclic double bonds cause regular bathochromic wavelength displacements. By taking 1 3-butadiene as the reference (Amax at 217 m/x), Woodward found that each alkyl substituent and each exocyclic double bond caused a bathochromic displacement of 5 mp. in substituted symmetrical and unsymmetrical dienes. This empirical rule, often known as Woodward s Rule, has been of value in the elucidation of organic structures. The validity of Woodward s rule is given in Table 4.4. 

Table 4A. Validity of Woodward s Rule Related to the Absorption Spectra of Substituted Dienes ...

XXI is a trialkylated diene. The application of Woodward s rules in the case of a-cyperone has already been mentioned. 

Empirical Parameters for ir — ir Transitions of Conjugated Systems in Ethanol (Woodward s Rules)... 

For each alkyl group substituted on the a, 3- insaturated ketone, a correction of + 10 nm must be made according to Woodward s Rules. Therefore, the calculated X... 

Then, if Smax is over about 1000, we have an allowed transition, usually and the value of A.max gives some idea as to the amount of conjugation involved. The exact position of the peak can provide a lot of information about the conjugated system, using a complex set of rules known as Woodward s rules. 

Empirical rules based on thousands of laboratory observations have been developed over the years relating the wavelengths of the UV absorption maxima to the structures of molecules. R.B. Woodward in 1941 and then L. Fieser and M. Fieser developed rules for predicting the absorption maxima of dienes, polyenes, and conjugated ketones by studying terpenes, steroids, and other natural products. The rules are known as Woodward s Rules or the Woodward-Fieser Rules. 

This tendency to propagate intra-intermolecularly by the unconjugated dienes is greater than can be expected from purely statistical predictions. Butler suggested that this results fh)m interactions between the olefinic bonds. Ultraviolet absorption spectra of several unconjugated diolerins do show bathochromic shifts in the absorption maxima relative to the values calculated from Woodward s rule. This supports Butler s explanation. 

Problem 5.3. Based on the values given in the text for Woodward s rules, calculate the expected absorption maxima for the examples shown above. How do the values you have calculated compare with those provided ... 

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