State and illustrate with two examples Woodward-Fieser rules?

Answer 1

The Woodward-Fieser rules are designed to predict the absorbance maximum of conjugated systems in the ultraviolet/visible region of the spectrum based on the connectivity of atoms withing the system. More conjugation and more substitution tends to give shifts to longer wavelength (lower energy) light. Alkyl groups tend to make the wavelength longer by 5 nm, other conjugated systems increase the wavelength by 30 nm, with the base wavelength set to that of buta-1,3-diene (lambdamax=217). 2,4-dimethylpenta-1,3-diene has 3 alkyl groups. This give 217 nm + 15 nm = 232 nm predicted wavelength, which matches the actual wavelength. 2,4-dimethylhexa-2,4-diene has 4 alkyl groups. This give 217 nm + 20 nm = 237 nm predicted wavelength. The actual wavelength is 241 nm. As can be seen from the examples, this is really only an approximation, and one the tends to deviate more from reality the further one gets from the base structure.

Answer 2

The Woodward-Fieser rules are used to estimate the absorption wavelength of conjugated dienes or enones in UV-Visible spectroscopy. These rules are based on the positions of substituents on the diene or enone molecule. For example, electron-withdrawing groups such as -CHO and -COOH at the 2-position of a diene will lead to a bathochromic (red) shift in absorption wavelength. Conversely, electron-donating groups like -OH at the 4-position will cause a hypsochromic (blue) shift in the absorption wavelength.