and simplest route is extraction from plants of the Mentha genus and Mentha arvensis (cornmint) in particular. This is achieved by freezing the oil to force the /-menthol to crystallize out. Diethylamine can be added to myrcene (70) in the presence of base and rearrangement of the resultant allyl amine (224) using the optically active catalyst ruthenium (S)-BINAP perchlorate gives the homochiral enamine (225). This can then be hydrolyzed to d-citronellol (209). The chiral center in this molecule ensures that, on acid catalyzed cyclization, the two new stereocentres formed possess the correct stereochemistry for conversion, by hydrogenation, to give /-menthol as the final product. Starting from the petrochemically sourced m-cresol (226), propenylation gives thymol (97), which can be hydrogenated to give a mixture of all eight stereoisomers of menthol (227). Fractional distillation of this mixture gives racemic menthol. Resolution was originally carried out by fractional crystallization, but recent advances include methods for the enzymic resolution of the racemate to give /-menthol.
Estimation of the long-term sustainability of each of these routes is complex and the final outcome is far from certain. In terms of renewability of feedstocks, m-cresol might appear to be at a disadvantage against mint or turpentine. However, as the world's population increases, use of agricultural land will come under pressure for food production, hence increasing pressure on mint cultivation and turpentine, hence, myrcene is a by-product of paper manufacture and is therefore vulnerable to trends in paper recycling and "the paperless office." In terms of energy consumption, and hence current dependence on petrochemicals, the picture is also not as clear as might be imagined. Harvesting and processing of mint requires energy and, if the crop is grown in the same field over time, fertilizer is required and this is produced by the very energy-intensive Haber process. The energy required to turn trees in a forest into pulp at a sawmill is also significant and so turpentine supply will also be affected by energy prices. No doubt, the skills of process chemists will be of increasing importance as we strive to make the best use of natural resources and minimize energy consumption (Baser and Demirci, 2007).
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