Woodward-Fieser

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. 

Predict the absorbance maximum for unsalurated compounds using the Woodward-Fieser rules... 

As we have already seen, delocalization of electrons by conjugation decreases the energy difference between the HOMO and LUMO energy levels, and this leads to a red shift. Alkyl substitution on a conjugated system also leads to a (smaller) red shift, due to the small interaction between the cr-bonded electrons of the alkyl group with the K-bond system. These effects are additive, and the empirical Woodward-Fieser rules were developed to predict the 2max values for dienes (and trienes). Similar sets of rules can be used to predict the A ax values for a,P-unsaturated aldehydes and ketones (enones) and the Amax values for aromatic carbonyl compounds. These rules are summarized in Table 2.4. 

Using the Woodward-Fieser rules, we can predict the absorption maximum of an unsaturated compound with pleasing accuracy, generally to within 5 nm of the observed value. For example, the calculated absorbance maximum of ergocalciferol (vitamin D2) of 265 nm (Figure 2.8) is only 1 nm different from the observed value at 266 nm (Figure 2.9). 

Using the Woodward-Fieser rules, the of dienes and trienes, a,p-unsaturated aldehydes and ketones (enones), and for aromatic carbonyl compounds can be reliably calculated and predicted. 

Structural analysis from electronic spectra yields little information because of their relative simplicity. In the 1940s, however, before the advent of more powerful identification techniques, UV/VIS visible spectroscopy was used for structural identification. The study of a great number of spectra of various molecules has revealed correlations between structures and the positions of absorption maxima. The most widely known empirical rules, due to Woodward, Fieser and Scott, involve unsaturated carbonyls, dienes and steroids. Using incremental tables based on various factors and structural features, it is possible to predict the position of the n —> n absorption bands in these conjugated systems (Table 11.3). Agreement between the calculated values and the experimentally determined position of absorption bands is usually good, as can been seen by the following four examples ... 

By employing the empirical rules of Woodward-Fieser. predict the position of the maximum absorption of the compounds whose structures are seen below. 

Wave number, 161 WCOT, 29 WDXRF, 238 Weight distribution, 106 Wide bore column, 30 Woodward-Fieser rules, 197 Working electrode, 359... 

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.

Woodward-Fieser rules for ultraviolet-visible spectroscopy 390... 

Conjugated systems show bathochromic shifts in their n->n transition bands. Empirical methods for predicting those shifts were originally formulated by Woodward, Fieser, and Fieser.M This section includes the most important conjugated system rules.15 The reader should consult references 5 and 6 for more details on how to apply the wavelength increment data. 

Table 3.9 Woodward-Fieser rules for enone absorption...

Conjugated Systems (see Section 12.2.3 for Woodward-Fieser Rules)... 

The Woodward-Fieser rules can be used to estimate the UV/VIS absorption as follows ... 

The following triene on partial hydrogenation gives three products. Which are separated by glc. How UV spectroscopy and application of Woodward-Fieser rules will help to identify the products. 

The Woodward-Fieser rules for predicting the wavelength of the lowest-energy IT - 7T absorption band of an ajS-unsaturated ketone have been subjected to a critical re-evaluation, with particular attention to the torsion angle between the C=0 and C=C components. The remarkable reliability of the original rules , whether applied to s-cis- or s-trans-cnoncs, seems to be due to a fortuitous cancellation of effects associated with conformation, provided that the system is not excessively twisted from planarity. Modified rules are now presented in two forms. The simpler scheme, applicable (in ethanol) to planar or moderately twisted systems whether cyclic or acyclic, uses parent values of 209 nm for s-trans and 215 nm for s-cis, with alkyl shifts of -I-10 nm for a-substitution or... 

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. 

---