Molar absorptivity phenolic compounds

Aromatic compounds have very high molar absorptivities that usually lie in the vacuum ultraviolet region and are not useful for routine analysis. Modest absorption peaks are found between 200 and 300 nm. Substituted benxene compounds show dramatic effects from electron-withdrawing substituents. These substituents are known as auxo-chromes since they do not absorb electromagnetic radiation but they have a significant effect on the main chromophore. For example, phenol and aniline have molar absorptivities that are six times the molar absorptivity of benzene or toluene at similar wavelengths. 

Phenol and phenolic derivatives, cresol and related compounds were analyzed by Folin-Ciocalteau reagent at 700 m/i, standardizing each compound separately by its Folin response. Their slopes (molar absorption coefficients) were 8000-10,000 M-1cm-1. Interestingly, methoxyl aromatics with no free hydroxyl groups were not reactive. 

The solvent often exerts a profound influence on the quality and shape of the spectrum. For example, many aromatic chromophores display vibrational fine structure in non-polar solvents, whereas in more polar solvents this fine structure is absent due to solute-solvent interaction effects. A classic case is phenol and related compounds which have different spectra in cyclohexane and in neutral aqueous solution. In aqueous solutions, the pH exerts a profound effect on ionisable chromophores due to the differing extent of conjugation in the ionised and the non-ionised chromophore. In phenolic compounds, for example, addition of alkali to two pH units above the pKa leads to the classical red or bathochromic shift to longer wavelength, a loss of any fine structure, and an increase in molar absorptivity (hyper chromic... 

Eleven phenolic acids present in olive oils (e.g., tyrosol, hydroxytyrosol, vanillic acid, syringic acid, caffeic acid, o- and j-coumaric acid, -hydroxybenzoic acid) were studied using a C g column and a 70-min 6/94 -y 100/0 methanol/water (3% acetic acid) gradient [283]. Acetonitrile adversely affected chromatographic performance and required lengthy re-equilibration times between injections. Molar absorptivities are tabulated for all compounds and detection limits ranged from I to 6 ng injected (analyte dependent). 

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