Thymol hydrogenation

The remaining isomeric menthols, neomenthol, isomenthol, and a trace of neoisomenthol, can be epimerized, under the conditions used for the thymol hydrogenation, to give ca. 6 3 1 equilibrium mixture of ( )-menthol, ( )-neomenthol, and ( )-isomenthol, respectively. ( )-Menthol can, again, be distilled from the equilibrium mixture. 

Mcnlhol can be raccmizcd under thymol hydrogenation conditions (also with Raney-Ni). 

Addition of propylene to m-cresol produces thymol. Hydrogenation of thymol gives a mixture of menthol isomers. Treatment of any one of the eight isomers with the same copper chromite catalyst that is used for thymol hydrogenation causes racemization to the same equilibrium mixture of isomers. This fact is used to good effect in the process. The hydrogenation product is optically inactive, being composed of equal amounts of d- and L-isomers of each of the four conformational isomers. The balance between these is 62-64% menthol, 18-20% neomenthol, 10-12% isomenthol and 1-2% neoisomenthol. Since these are pairs of diastereomers, their physical properties differ. Thus,... 

Figure 10.6. Liquid-phase hydrogenation of thymol (2-isopropyl-5-methylphenol) over 50%Ni/Cr2O3 at 160°C in cyclohexane at different hydrogen pressures 0.4, 1.4 and 2.4 MPa (A.I. Allakhverdiev, N.V.Kul kova, D.Yu.Murzin, Kinetics of thymol hydrogenation over a Ni-Cr203 catalyst, Ind. Eng. Chem. Res. 34 (1995) 1539).

Stereoselective thymol hydrogenation comparative study of charcoal-supported, platinum, rhodium and iridium catalysts... 

We have investigated the changes in the product distribution and in the stereoisomeric composition of the menthol isomers, using well characterized Pt, Pd and Ir catalysts with the same high dispersion (1-2 nm large particles) and on the same active charcoal support. Moreover the hydrogenation of pure menthone was studied on the same catalysts and under the reaction conditions used for thymol hydrogenation. 

Figure 1. Product distribution vs. hydrogen consumption during thymol hydrogenation over 4,18% Pt/C.

The selectivity to ML increases with temperature (333 to 373 K), the ML/(ML-t-NML) ratio varying between 0.18 and 0.22 for thymol hydrogenation and between 0.23 and 0.31 in the case of pure menthone hydrogenation. However, the selectivity to NIML is always highly favored (ca. 60%). 

Piperitone is of considerable technical im portance. It is a colourless oil of a pleasant peppermint-like smell. (-)-Piperilone has b.p. 109-5-110-5 C/I5mm. Piperitone yields thymol on oxidation with FeCl. On reduction with hydrogen in presence of a nickel catalyst it yields menthone. On reduction with sodium in alcoholic solution all forms of piperitone yield racemic menthols and womenthols together with some racemic a-phel)andrene. 

To prepare the standard pH buffer solutions recommended by the National Bureau of Standards (U.S.), the indicated weights of the pure materials in Table 8.15 should be dissolved in water of specific conductivity not greater than 5 micromhos. The tartrate, phthalate, and phosphates can be dried for 2 h at 100°C before use. Potassium tetroxalate and calcium hydroxide need not be dried. Fresh-looking crystals of borax should be used. Before use, excess solid potassium hydrogen tartrate and calcium hydroxide must be removed. Buffer solutions pH 6 or above should be stored in plastic containers and should be protected from carbon doxide with soda-lime traps. The solutions should be replaced within 2 to 3 weeks, or sooner if formation of mold is noticed. A crystal of thymol may be added as a preservative. 

A)-Menthol can also be made synthetically by hydrogenation of thymol [89-83-8], which can be produced by isopropylation of y -cresol with propylene (143,144). 

The synthetic process starts with the isopropylation of m-cresol to yield thymol. After catalytic hydrogenation a mixture of stereoisomers is obtained from which (+)-menthol is isolated. The process requires much separation and recycling work. In contrast, the semi-synthetic process of Takasago (Scheme 5.5) leads essentially to stereopure (—)-menthol [19]. 

Optical resolution of enantiomeric mixtures which have been obtained by short chemical syntheses continues to be the method of choice for a wide variety of compounds. For instance, the industrial synthesis of (-)-menthol starts from thymol which is catalytically hydrogenated to furnish all four diastereomers in racemic form. 

At higher concentrations, the antioxidant activities of thymol and carvacrol were close to that of a-tocopherol and were in fact responsible for the antioxidant activity of many EOs which contain them [12, 17, 139, 153, 163, 164, 168, 170-174]. The high potential of phenolic components to scavenge radicals might be explained by their ability to donate a hydrogen atom from their phenolic hydroxyl groups [175]. 

Alkylation of m-cresol with propene in the presence of an aluminium catalyst results in the formation of thymol, which upon hydrogenation gives a rnkture of all eight isomers of menthol, D-menthol, L-menthol, neomenthol, isomenthol and neoisomenthol (Scheme 13.3). The preferred isomer is L-menthol, because of its ability to induce physiologically the sense of cold which is desired in many products such as chewing gum and toothpaste L-menthol is about... 

In aromatherapy the family of phenols are called substituted phenols, as one or more of the five remaining available hydrogen atoms of the ring is replaced by another group of atoms. Examples are carvacrol, thymol and eugenol (Fig. 3.6). 

Konuspaev et al. studied the hydrogenation of thymol over Ni-based catalysts.139 The effects of the catalyst support and the solvent on the cis-trans isomerization of the intermediate ketones, which governed the stereoselectivity, have been examined. Allakhverdiev et al. hydrogenated thymol with the catalysts modified by inorganic com-... 

Pulegone.—Pulegone is present in pennyroyal, Mentha pulegium. It yields menthone by addition of two hydrogens, which, by reduction, yields menthob and this, by loss of hydrogen, is converted into thymol. The position of the ketone group is thus proven. 

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