 Important Points Of Woodward-Fieser Rule
A conjugated organic compound’s maximum absorption wavelength (max) can be calculated using the Woodward-Fieser criteria. The chromophores and auxochromes that are present in the molecule determine
- the type
- the position
- and number of the rules.
The following are some significant Woodward-Fieser rules: The Chromophores are conjugated groups in molecules that colour things by absorbing light in the visible spectrum. Few example of chromophore is
- Double bonds.
- carbonyl groups.
- and aromatic rings etc.
Auxochromes: Auxochromes are non-conjugated groups that effect the chromophore’s absorption’s intensity and wavelength. Few examples of auxochromes is given below
- Hydroxyl group.
- Amino group.
- And nitro group etc.
Effect cumulative: The sum of the individual contributions from each chromophore and auxochrome in the molecule determines the absorption wavelength of a conjugated system.
Conjugation length: The maximum shift towards longer wavelengths increases with increasing conjugation length.
Effects of substitutes: Depending on whether they are electron-donating or electron-withdrawing, substitutes connected to a chromophore or auxochrome can shift the max towards shorter or longer wavelengths.
Empirical values: The Woodward-Fieser criteria estimate the absorption maxima of more complicated compounds using empirical values of the absorption maxima of particular chromophores and auxochromes.
Limitations:
 some limitation of woodfieser rule are given below:
- The Woodward-Fieser principles do not apply to all substances.
- They might not be applicable, in instance, to molecules with intricate architectures or many chromophores and auxochromes.
Note: Overall, the Woodward-Fieser criteria can be used to estimate the absorption maxima of conjugated organic compounds based on their structural characteristics.