Woodward-Fisher rule and applications
The Woodward-Fieser Rules, developed by Robert B. Woodward and William S. Fieser, are a set of empirical rules used in organic chemistry to estimate the maximum absorption wavelength (λmax) of a compound's UV-visible absorption spectrum. These rules are particularly valuable for predicting the λmax of conjugated organic compounds, especially those containing chromophores like double bonds and aromatic rings. Understanding the λmax of a compound can be useful in various applications, including the identification, characterization, and analysis of organic molecules.
The Woodward-Fieser Rules are expressed as a series of additive and subtractive contributions to estimate λmax based on the compound's structural features. The rules take into account the type and number of conjugated double bonds, the presence of certain functional groups, and the degree of extension of the conjugated system.
Here are some common contributions to the λmax predictions using the Woodward-Fieser Rules:
Conjugated Double Bonds (CD): Each conjugated double bond typically contributes around 30-40 nm to the λmax. For example, a compound with three conjugated double bonds might have an estimated λmax of 300-400 nm.
Carbonyl Groups (K): Ketones and aldehydes with a conjugated system contribute about 5 nm to the λmax per carbonyl group.
Aromatic Rings (AR): Each aromatic ring contributes around 15-20 nm to the λmax.
Substituents: Certain substituents can modify the λmax. Electron-donating groups (e.g., alkyl groups) generally shift λmax to longer wavelengths (redshift), while electron-withdrawing groups (e.g., halogens) shift it to shorter wavelengths (blueshift).
Applications of the Woodward-Fieser Rules:
ble Spectroscopy: The primary application is in UV-visible spectroscopy, where these rules help predict the absorption maxima of compounds before experimental measurements are conducted. This is especially useful in identifying and characterizing organic compounds.
Organic Synthesis: Chemists can use the rules to guide the design of new molecules with specific absorption properties, such as dyes and pigments. By strategically incorporating conjugated systems and functional groups, they can control the λmax of the resulting compounds.
Quantitative Analysis: The rules can be used in quantitative analysis techniques such as spectrophotometry, where knowing the λmax allows for the determination of the concentration of a compound in a solution based on its absorbance.
Drug Discovery: In medicinal chemistry, the Woodward-Fieser Rules can be used to estimate the λmax of drug candidates. This can be valuable in drug design, formulation, and studying the pharmacokinetics and pharmacodynamics of compounds.
Polymer Chemistry: Conjugated polymers, used in various applications such as organic photovoltaics and organic electronics, often have well-defined absorption spectra. The rules can aid in designing and tailoring the optical properties of these materials.
It's important to note that the Woodward-Fieser Rules provide estimates and approximations and may not always accurately predict the exact λmax for every compound, especially when complex interactions occur between different chromophores or functional groups. Nevertheless, they serve as valuable guidelines for quickly assessing the UV-visible absorption properties of organic molecules.
Anjalee Mishra
