3- Menthene Menthol

When the dehydration of menthol is carried out on an acidic alumina or at a long contact time the 2-menthene can isomerize to the more stable 1- and 3-menthenes. In order to avoid the consecutive reactions which proceed by acid catalysis, the alumina can be modified either by adding pyridine to the menthol or by passing ammonia over the catalyst during dehydration. 

Evidence was presented to show participation of the hydrogen on the y-carbon atom relative to the hydroxyl group. This leads to the formation of l-p-menthene from menthol, optically active cam phene from bornanols, allylbenzene from 2-phenylpropanol, etc. 

Chemical Properties. Hydrogenation of menthols yields / -menthane oxidation with chromic acid or catalytic dehydrogenation yields menthones. Dehydration under mild conditions yields 3-/ -menthene as the main product. Reaction with carboxylic acids or their derivatives yields menthyl esters, which are used mainly as aroma substances and in pharmaceutical preparations and formulations. The esterification of menthols with benzoic acid is used on an industrial scale in the resolution of racemic menthol. 

Menthol from (+)-3-Carene. An Indian manufacturing process for (—)-menthol starts from 3-carene, the major component of Indian turpentine oil (55-65%). (+)-3-Carene isomerizes to (+)-2-carene, which can be pyrolyzed to (+)-tra s-2,8-/ -menthadiene. Isomerization of the latter yields (+)-isoterpi-nolene, which is hydrogenated to give >50% (+)-3-/ -menthene. Epoxidation and subsequent rearrangement lead to a menthone isomenthone mixture,... 

Exercise 30-6 Reduction of the ketone group of (—)-menthone, which has its alkyl groups trans to one another, gives two products, known as (—(-menthol and (+)-neomenthol. These two substances differ considerably in their reactions. (+)-Neomenthol undergoes dehydration either in methanoic acid or when treated with phosphorus pentachloride, whereas (—(-menthol gives a methanoate ester with methanoic acid and a chloride with phosphorus pentachloride. What is the relationship between neomenthol and menthol, and why do they behave differently with methanoic acid and phosphorus pentachloride What is the likely structure of the menthene from dehydration of neomenthol (Review Sections 8-8D, 12-3D, and 12-5.)... 

Of the two isomeric alcohols menthol and neomenthol, the former dehydrates preferably into menthene-2, while the latter gives menthene-3 preferably. 

Lithium-ethylamine reduction at one or both double bonds of carvone, and of carvenone (58) to carvomenthone only, is reported.The effect of solvent on the lithium or potassium amide-reduction of p-cymene to menthenes and menthadienes has been examined.Hydrogenation of carvone (Vol. 4, p. 32), using palladium-polysaccharide exchange resin, favours endocyclic over exocyclic double-bond reduction, more so than with Pd-C or Pd-BaS04, whereas platinum or rhodium on exchange resins exhibit no special selectivity.Optimum conditions for the catalytic hydrogenation of thymol, and the catalytic dehydrogenation of menthol, to menthone have been determined.Cathodic reduction of carvomenthone (to... 

Although metals have generally been considered to exert only a dehydrogenating action in the reactions involved in the pyrolysis of the alcohols, it is possible for them to act toward dehydration in certain instances. By passing isopropanol over reduced copper at 320° C. it is possible to obtain acetone and propene, and by using ethanol, to obtain acetaldehyde and ethylene. By treating menthol in the same way menthene is formed. A determination of the water formed in each of these cases suggests that the catalytic action of the metal is one of direct dehydration.44... 

Important information about the stereochemistry of dehydration has been obtained by studying the transformation of cyclic alcohols many of these reactions have proved suitable for the selective synthesis of alkenes [2]. The dehydration of menthol (5) and neomenthol (6) illustrates the usefulness of such processes (Scheme 3 axial hydrogens participating in water loss to form the major menthene isomers are shown) [55]. The regioselectivity observed points to an anti elimination mechanism. Isomer 6, with a trans OH/H configuration in the most stable conformation, reacts faster than compound 5. 

Manganese(II) iodide, 312-313 Maimich reaction, 194, 231, 339 Marschallc cyclization, 456 Massoialactone, 308 Maytansenoids, 318 Maytansine, 325 Meerwein-Ponndorf reaction, 16 Meldrum s acid, 143, 313-314, 484 p-Menthene-1, 141 Menthol, 535... 

Dehydration of racemic menthol on Ni-/-quartz at 560°C gave products containing 62.4% menthenes and a rotation of -0.15°. This catalyst was strongly carbonized during reaction and lost its activity, but alter oxidation and reduction with hydrogen it gave a product with a rotation of -0.08. ... 

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