Intensity of the secondarytransition of vanillin a flavouring in agro-food industry

The secondary transition of the benzene chromophore of vanillin is compared to its homologous in paramethoxyacetophenone. It is shown that the longest wavelengths transition in this latter, which could be related to the secondary transition is in fact the overlap of the secondary transition and of the primary one. We show that the intensity of the secondary transition is similar to that of the primary one. The methoxy group in vanillin, at the meta position to the -COH one, quenches a part of the para interaction, so that the two transitions become distinct. The -OCH 3 substituent introducing a pyrocatechol like structure with two ortho -OR substituents should enhance the intensity of the secondary transition as it does when going from phenol to pyrocatechol. But -OCH 3 quenching a part of the para interaction effects on intensity this latter too is quenched.