the production of thymoquinone from thymol and carvacrol
the production of thymoquinone from thymol and carvacrol
the production of thymoquinone from thymol and carvacrol
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Oxidations with H2O2 can involve homolytic pathways via free radical<br />
intermediates <strong>and</strong>/or heterolytic oxygen transfer processes as seen in Figure 5.2<br />
(Sheldon et al. 1998). In Figure 5.2, <strong>carvacrol</strong> is used as reactant which is <strong>the</strong> major<br />
monoterpene component <strong>of</strong> many essential oil.<br />
Heterolytic oxygen transfer process can be divided into two types based on <strong>the</strong><br />
active intermediate: a peroxometal or an oxometal complex. Peroxometal pathways<br />
usually involve early transition elements with d 0 configuration, e.g. Mo(VI), W(VI),<br />
V(V), Ti(IV). Late or first row transition elements, e.g. Cr(VI), V(V), Mn(V), Ru(VI),<br />
Ru(VIII), Os(VIII), generally employ oxometal pathways. Some elements, e.g.<br />
vanadium, can employ oxometal or peroxometal pathways depending on <strong>the</strong> reactant<br />
(Arends et al. 2001).<br />
In <strong>the</strong> presence <strong>of</strong> transition metal complexes oxidation reactions with hydrogen<br />
peroxide are direct activation (unproductive oxidation) <strong>and</strong> productive oxidation<br />
reactions as seen in Figure 5.3 <strong>and</strong> water is only by product. Catalysts (transition metal<br />
complexes) are transferred oxygen <strong>from</strong> hydrogen peroxide to <strong>the</strong> reactant or substrate<br />
by a homolytic cleavage <strong>of</strong> <strong>the</strong> metal oxygen bond (Salem et al. 2000).<br />
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