INVESTIGATION OF EFFECT OF SUNLIGHT IRRADIATION ON PURE tram-ANETHOLE AND ON SWEET FENNEL ESSENTIAL OIL

摘要

Photochemical behaviour of pure trans-anethole was compared to that of sweet fennel essential oil (85% trans-anethole content). Samples of each were exposed in parallel to sunlight for different time intervals and then were analysed by GC-MS. In both cases, isomerisation of trans -anethole to cis-anethole and dimerisation of trans-anethole were predominat processes. These photochemical processes were more efficient in the essential oil (EO) in comparison to pure trans-anethole and also differences in reactivity were observed. In case of pure trans-anethole the dominant process was dimerisation, and in case of essential oil the dominant process was isomerisation to cis-anethole (evident from the cis-anethole/dimer ratio). Since the dimerisation occurs from the singlet state and the trans/cis isomerisation from the triplet manifold, anisaldehyde initially present in the essential oil (1.44%) may act as triplet sensitiser. In this manner the increased efficiency of the processes and the dominance of cis-anethole in the irradiated EO sample compared to pure trans-anethole can be rationalised.