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Originally appearing in Volume V26, Page 648 of the 1911 Encyclopedia Britannica.
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MONOCYCLIC TERPENE GROUP Limonene, At :8(9) terpadiene, C10H16, is known in three forms, namely d-limonene, l-limonene, and i-limonene or dipentene. d-Limonene is the chief constituent of oil of orange-rind, and is ALCOHOL AND KETONE DERIVATIVES also found in oil of lemon and oil of bergamot. l-Limonene is found in oil of fir-cones and in Russian peppermint oil. Both are Menthol (terpan-ol-3), C1oH20O. The laevo variety is the chief pleasant-smelling liquids, which boil at 175-176° C. They differ portion of oil of peppermint; it may be prepared by redwing the from each other only in rotatory power. Dry hydrochloric acid menthone obtained by E. Beckmann and M. Pleissner :nn., 1891, gas converts them into optically active limonene hydrochloride, 262, p. 21) from pulegone hydrobromide with sodium and alcohol. while in the moist condition it gives dipentene dihydrochloride. It crystallizes in prisms which melt at 43° C. and boil at 212° C. When heated to a sufficiently high temperature they are con- verted into dipentene. Four optically active nitrosochlorides are known, two corresponding to each of the active limonenes, and these on heating with alcoholic potash are converted into d- and l-carvoxime. Dipentene (i-limonene) is found widely distributed in many essential oils, e.g. of camphor, Russian turpentine, cubebs, bergamot, cardamom, &c., and is also a product of the dry distillation of many vegetable resins. It may be produced by heating many terpenes (pinene, camphene, sylvestrene, limonene) for several hours at 250-270° C.; or by the polymerization of isoprene at 300° C. To obtain pure dipentene it is best to heat dipentene hydrochloride with anhydrous sodium acetate and glacial acetic acid (0. Wallach, Ann. Chem. Pharm., 1887, 239, p. 3). It is a pleasant-smelling liquid, which boils at 175-176° C., and polymerizes on heating to high temperatures. When warmed with alcoholic sulphuric acid it yields terpinene, whilst concentrated sulphuric acid or phosphorus pentasulphide convert it into paracymene. Dipentene dihydrochloride, CioH16-2HC1, beet prepared by passing a current of hydrochloric acid gas over the surface of a glacial acetic acid solution of dipentene, crystallizes in rhombic tables which melt at 5o° C. and boil at 118-12o° C. (to mm.). It is apparently a trans-compound. for A. v. Baeyer (Ber., 1893, 26, p. 2863) has obtained a cis-dihydrochloride of melting-point 25° (circa), by the action of hydrochloric acid on cineol. Terpinolene, 01:4(8) terpadiene, has not as yet been observed in essential oils. It is formed by the action of hot dilute sulphuric acid on terpineol, terpin hydrate and cineol. It is an inactive liquid boiling at 183-185° C., and is readily converted into terpinene by acids. Terpinene, DI : 4(8) terpadiene (?), is found in cardamom oil and in oil of marjoram. It is formed by the action of alcoholic sulphuric acid on dipentene, terpin hydrate, cineol phellandrene or terpineol; or by the action of formic acid on linalool. Phellandrene is . a mixture of Al : 5 terpadiene and 02: 1(7) terpadiene (pseudo-phellandrene) (F. W. Semmler, Ber., 1903, 36, p. 17491. It is found as d-phellandrene in oil of water-fennel and oil of elemi, and as l-phellandrene in Australian eucalyptus oil and oil of bay. It is an exceedingly unstable compound, and must be extracted from the oils by distillation in vacua. The hydrocarbons obtained from elemi oil and eucalyptus oil correspond to A1.5 terpadiene. A similar hydrocarbon was obtained by C. Harries and M. Johnson (Bee., 1905, 38, p. 1832) by converting carvone hydro-bromide into A6 terpenone-2, then, by phosphorus pentachlcride, into chlor-2-phellandrene, which is finally reduced. Svlvestrene, Al :8(9) meta-terpadiene, is found in Swedish and Russian oil of turpentine and in various pine oils. It boils at 175-176° C. and is dextro-rotatory. It is one of the most stable of the terpenes and gives a characteristic deep blue colour on the addition of a drop of sulphuric acid to its solution in acetic an-hydride. On treating the hydrobromide with bromine in the presence of iodine, a product is obtained which on reduction yields meta-cymene (A. v. Baeyer and V. Villigcr, Ber., 1898, 31, p. 2067). Carvestrene is obtained by the distillation of carylamine or vestrylamine hydrochloride (A. v Baeyer, Ber., 1894, 27, pp. 3485 seq.). It is regarded by Baeyer as i-sylvestrene. It was synthesized by W. H. Perkin and G. Tattersall (Proc. Chein. Soc., 1907, 22, p. 268) by the application of the Grignard reaction to the ethyl ester of y-ketohexahydrobenzoic acid M. By the action of magnesium methyl iodide this ester yields the lactone of y-hydroxy-hexahydrometa-toluic acid, which is transformed by hydrobromic acid into the corresponding y-bromo-hexahydro-meta-toluic acid. This latter substance by the action of pyridine yields tetrahydro-metatoluic acid, the ester of which by magnesium methyl iodide is converted into 0•I-meta menthenol-8 (2). The meta-menthenol on dehydration by potassium bisulphate yields carvestrene (3) of boiling-point 179-180° C. of the double linking in the molecule is shown by the use of the symbol A followed by the number of the carbon atom immediately preceding it. dioxide yields I-methyl-4-isopropyldiketohexamethylene. This ketone is then reduced to the secondary alcohol, the hydroxyl groups replaced by bromine, and hydrobromic acid is then removed froth the bromo-compound by boiling it with quinoline, leaving the terpene. It is a liquid which boils at 174° C. and shows a complete terpene character. It is readily oxidized by chronic acid to the corresponding ketone menthone. By the action of phosphorus pentoxide, or zinc chloride, it is converted into menthene, CioHis, and when heated with anhydrous copper sulphate to 250° C. it yields para-cymene. It is reduced by hydriodic acid and phosphorus to hexahydrocymene. The phosphorus haloids yield haloid esters of composition C10H19C1, which, according to I. L. Kondakow (Jour. prakt. Chem., 1899 [2]. 6o, p. 257) are to be regarded as tertiary esters; a similar type of reaction is found in the case of carvomenthol. A d-menthol has been prepared from the i-mixture obtained by reducing menthone with sodium. The mixture is benzoylated, and the liquid d-menthol benzoate separated and hydrolysed. Tertiary menthol (terpan-ol-4), a liquid boiling at 97-101 ° C. (2o mm.), has been obtained by the hydrolysis of the ester prepared by heating menthene with trichloracetic acid (A. Reychler and L. Masson, Ber., 1896, 29, p. 1844). It possesses a faint pepper-mint odour. W. H. Perkin, junr. (Proc. Chem. Soc., 1905, 21, p. 255) synthesized it from 1.4 methylcyclohexanone: sodium carbonate converts a-bromhexahydro-para-toluic acid (I) into i .1-tetrahydro-para-toluic acid and a-oxyhexahydro-para-toluic acid, and the latter on treatment with dilute sulphuric acid yields I.4-methylcyclohexanone (2), which by the action of magnesium isopropyl iodide and subsequent hydrolysis is converted into tertiary menthol (3). CHa • CIIC Br • CO3H -sCHa • CHC /OH
ADOLPHE MONOD (1802–1856)

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