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) .