Menthenes

Fig. 5. Alkaline hydrolysis of piperityl chloride to cis- and /n j -2-menthen-l-ol and acid-catalyzed rearrangement to piperitols (78) and (79).

Lsbhe has eaamiued an antheutic sample of thyme oil and found it to contain 30 [>er oeut. of thymol, 17 per ceut. of a terpeue which he could not I deuiify, 15 per cent, of menthene, 21 per cent, nf cymeue, 5 [>er ceut, of liualol, ft per oent. of borueol, and traces of caiwaerol. 

Wallach has synthesised i-menthone by condensing 1, 4-methyl-cyclo-hexanone with bromo-isobutyric ester from the condensation product he prepared i-menthene, which was converted into i-menthenone, by means of its nitrosochloride, whence i-menthone resulted by reduction. 

Figure 3.1 The reactions of ethylene and menthene with bromine. In both molecules, the carbon-carbon doublebond functional group has a similar polarity pattern, so both molecules react with Br2 in the same way. The size and complexity of the remainders of the molecules are not important.

Menthene, a hydrocarbon found in mint plants, has the systematic name 1 - i soprop v -4 - met h y Icyc lohexene. Draw its structure. 

The elimination of HC1 from the isomeric menthyl and neomenthyl chlorides shown in Figure 11.20 gives a good illustration of this trans-diaxial requirement. Neomenthyl chloride undergoes elimination of HC1 on reaction with ethoxide ion 200 times as fast as menthyl chloride. Furthermore, neomenthyl chloride yields 3-menthene as the major alkene product, whereas menthyl chloride yields 2-nienthene. 

The difference in reactivity between the isomeric menthyl chlorides is due to the difference in their conformations. Neomenthyl chloride has the conformation shown in Figure 11.20a, with the methyl ancl isopropyl groups equatorial and the chlorine axial—a perfect geometry for L2 elimination. Loss of the hydrogen atom at C4 occurs easily to yield the more substituted alkene product, 3-menthene, as predicted by Zaitsev s rule. 

A final piece of evidence involves the stereochemistry of elimination. (Jnlike the E2 reaction, where anti periplanar geometry is required, there is no geometric requirement on the El reaction because the halide and the hydrogen are lost in separate steps. We might therefore expect to obtain the more stable (Zaitsev s rule) product from El reaction, which is just what w e find. To return to a familiar example, menthyl chloride loses HC1 under El conditions in a polar solvent to give a mixture of alkenes in w hich the Zaitsev product, 3-menthene, predominates (Figure 11.22). 

Figure 11.22 Elimination reactions of menthyl chloride. E2 conditions (strong base in 100% ethanol) lead to 2-menthene through an anti periplanar elimination, whereas El conditions (dilute base in 80% aqueous ethanol) lead to a mixture of 2-menthene and 3-menthene.

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