Dehydration menthol

On the contrary, the Lewis acid sites present on the snrface of sepiolite make the Cn/sepiolite catalyst extremely active in promoting the ene reaction of citronellal. Thns, citronellal never accnmnlates in the reaction mixtnre bnt it is com erted into isopulegol as soon as it forms. Hydrogenation of isopnlegol is very slow nnder these reaction conditions, bnt this simple catalyst is able to produce menthol in a one-pot-one-step reaction under very mild experimental conditions. Notably dehydration products, which give account of 40% of the reaction mixture obtained over Ni-H-MCM-41 [4], are kept under 20% over both Cu catalysts. 

In order to determine whether trans elimination may occur also in the removal of elements of water from alcohols, the dehydration of 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. 

The dehydration of menthols over alumina, prepared from aluminum isopropoxide and having intrinsic acidic sites, was accompanied by double bond migration of the cycloalkenes produced. The isomerization was, however, suppressed by the preferential neutralization of the strong acid sites with bases. The neutralization of acidic sites thus preventing isomerization was confirmed by von Rudloff (61) who evacuated pyridine-treated alumina for 6 hours, when about 0.8% base was retained. 

The dehydration to 1-butene proceeds most probably via a trans elimination reaction. The formation of 2-butenes, which were the primary products of reaction, can best be explained by a removal of hydrogen from a y-carbon atom, as was indicated in the case of menthols ... 

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. 

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. 

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. 

Boric acid is a mild dehydrating agent suitable for removal of water from some primary, secondary, or tertiary alcohols. Since the acid and the alcohol form first a trimeric metaboric ester, which then regenerates the boric acid when it decomposes to the olefin,36 the reaction is somewhat similar to pyrolysis of carboxylic esters but the boric acid dehydration occurs at appreciably lower temperatures (250-300°). Olefins are readily obtained by heating approximately molar equivalents of boric acid and 1-octanol, 1-heptanol, 1-hexanol, (—)-menthol, cyclohexanol, or 5cyclohexane-methanol, and cyclobutanemethanol.38... 

Professor Rufus T. Firefly of Fredonia State University wanted to stump his ace chemistry student. Sea Water (C-H2O, alias formaldehyde). The professor asked Sea Water to deduce the structure of menthol, the principal flavoring constituent of peppermint, on the basis of the following data the molecular formula of menthol is C10H20O when heated in the presence of sulfuric acid, a dehydration reaction ensues to form compound A ozonolysis of compound A followed by a reductive workup yields 3,7-dimethyl-6-oxo-octanal menthol also reacts with nitric acid and vanadium oxide, when heated, to form 2-isopropyl-5-methyl-hexanedioic acid. 

Since compound A was formed by the dehydration of menthol in sulfuric acid, and the molecular formula of A (CioHib) differs from that of menthol by one H2O molecule,... 

The majority of stereochemical investigations refer to cyclic and rigid substrates. Unlike the corresponding simple pyrolytic eliminations (Section 2.7), the dehydration of menthol and neomenthol enhibits mainly anti rather than syn or syn-clinal stereospecificity, viz. ... 

Decomposition studies of racemic menthol and 3-methylheptan-3-ol gave similar results (Scheme 2.2.). Thus dehydration of racemic menthol over Cu-/-quartz in a flowing N2-air mixture at 430-460°C results in a product with a rotation of -0.16°, which indicated preferential decomposition of the (+)-enantiomer of menthol. The same picture was obtained, but to a somewhat greater extent, for the dehydrogenation of 3-methylheptan-3-ol on Cu-/-quartz in which the (+)-enantiomer reacted and the product exhibited a rotation of -0.12°. 

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. ... 

Thus, the dehydration of menthol and neomenthol exhibits anti rather than syn... 

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