# 1911 Encyclopædia Britannica/Terpenes

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​TERPENES, in organic chemistry, the generic name of a group of hydrocarbons of the general formula (C$_5$H$_8$)$_n$, and the more important oxygen derivatives, mainly alcohols, aldehydes and ketones, derived from them. They may be classified into several distinct groups: hemiterpenes, C$_5$H$_8$; terpenes proper, C$_{10}$H$_{16}$; sesquiterpenes, C$_{15}$H$_{24}$; and polyterpenes (C$_5$H$_8$)$_n$. In addition to these, a series of open-chain olefine terpenes is known.
The chief sources of the terpenes and their derivatives are the essential oils obtained by the distillation or extraction by pressure of various plants, chiefly of the Coniferae and different species of Citrus. Certain of these oils consist very largely of hydrocarbons; for example, those of turpentine, citron, thyme, orange, pine-needle, goldenrod (from Solidago canadensis) and cypress, while others contain as their chief constituents various alcoholic and ketonic substances. With the exception of camphene, all the terpenes are liquids, boiling approximately between 160° and 190° C., so that it is almost impossible to separate them from the various essential oils by fractional distillation. In order to prepare the individual members pure, advantage is taken of the different physical properties of their derivatives. The terpenes all possess a characteristic odour and are fairly stable to alkalis, but are easily decomposed by acids or by heating to a sufficiently high temperature. Many polymerize readily, or are transformed into isomers by boiling with dilute alcoholic sulphuric acid. Some oxidize rapidly on exposure to air, passing into resinous substances. The formation of addition compounds with the halogens, halogen hydrides, and with nitrosyl chloride, is characteristic of many, whilst others unite readily with nitrogen peroxide. According to A. v. Baeyer (Ber., 1895, 28, p. 648; 1896, 29, p. 10) the nitrosochlorides are not simple addition products, but bi-molecular compounds or bisnitrosochlorides.
Hemiterpenes
The best known is Isoprene, C$_5$H$_8$, which is obtained on distilling caoutchouc or gutta-percha. It was synthesized by W. Euler (Ber., 1897, 30, p. 1989) by distilling the addition compound of methyl iodide and 2·3·5-trimethylpyrollidine with caustic potash. It is an unstable liquid which boils at 33.5° C., and on heating rapidly polymerizes to dipentene, the same change being effected by hydrochloric acid. In ethereal solution it combines with bromine to form an unstable liquid dibromide; it also unites with one molecule of hydrobromic acid to form the same tertiary bromide as dimethylallylene; this points to its being β-methyldivinyl, CH$_2$:C(CH$_3$)·CH:CH$_2$ (V. A. Mokiewsky, Jour. Soc. Phys. Chim. Russ., 1900, 32, p. 207).
Terpenes Proper
The terpenes proper may be subdivided into the simple monocyclic terpenes and the more complex (usually bicyclic) terpenes. The monocyclic terpenes are hydro derivatives of paracymene. A. v. Baeyer proposed the following nomenclature: the dihydroparacymenes are called terpadienes, the tetrahydrocymenes becoming terpenes and the hexahydrocymene terpan, the carbon atoms being numbered as shown in the inset formula:
In the more complex terpenes the name camphene is retained, and camphane is used for the dihydrocamphene. G. Wagner (Ber., 1894, 27, p. 1636 Anm.) designates the hexahydrocymenes menthans, the tetrahydrocymenes menthenes, and the dihydrocymenes menthadienes. The position ​of the double linking in the molecule is shown by the use of the symbol Δ followed by the number of the carbon atom immediately preceding it.
Monocyclic Terpene Group
Limonene, Δ1:8(9) terpadiene, C$_{10}$H$_{16}$, 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 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 pleasant-smelling liquids, which boil at 175–176° C. They differ from each other only in rotatory power. Dry hydrochloric acid gas converts them into optically active limonene hydrochloride, while in the moist condition it gives dipentene dihydrochloride. When heated to a sufficiently high temperature they are converted 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 (O. 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, C$_{10}$H$_{16}$·2HCl, best 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 50° C. and boil at 118–120° C. (10 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, Δ1:4(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, Δ1:4(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 Δ1:5 terpadiene and Δ2:1(7) terpadiene (pseudo-phellandrene) (F. W. Semmler, Ber., 1903, 36, p. 1749). 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 vacuo. The hydrocarbons obtained from elemi oil and eucalyptus oil correspond to Δ1·5 terpadiene. A similar hydrocarbon was obtained by C. Harries and M. Johnson (Ber., 1905, 38, p. 1832) by converting carvone hydrobromide into Δ6 terpenone-2, then, by phosphorus pentachloride, into chlor-2-phellandrene, which is finally reduced.
Sylvestrene, Δ1:8(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 anhydride. 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. Villiger, 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. Chem. Soc., 1907, 22, p. 268) by the application of the Grignard reaction to the ethyl ester of γ-ketohexahydrobenzoic acid (1). By the action of magnesium methyl iodide this ester yields the lactone of γ-hydroxy-hexahydro-meta-toluic acid, which is transformed by hydrobromic acid into the corresponding γ-bromo-hexahydro-meta-toluic acid. This latter substance by the action of pyridine yields tetrahydro-meta-toluic acid, the ester of which by magnesium methyl iodide is converted into Δ·1-meta menthenol-8 (2). The meta-menthenol on dehydration by potassium bisulphate yields carvestrene (3) of boiling-point 179–180° C.
A synthetical monocyclic terpene, viz. 1-methyl-4-isopropyl dihydrocymene was prepared by A. v. Baeyer (Ber., 1893, 26, p. 232). Succino-succinic ester is converted into the methyl isopropyl derivative, which on hydrolysis and elimination of carbon dioxide yields 1-methyl-4-isopropyldiketohexamethylene. This ketone is then reduced to the secondary alcohol, the hydroxyl groups replaced by bromine, and hydrobromic acid is then removed from 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.
Alcohol and Ketone Derivatives
Menthol (terpan-ol-3), C$_{10}$H$_{20}$O. The laevo variety is the chief portion of oil of peppermint; it may be prepared by reducing the menthone obtained by E. Beckmann and M. Pleissner (Ann., 1891, 262, p. 21) from pulegone hydrobromide with sodium and alcohol. It crystallizes in prisms which melt at 43° C. and boil at 212° C. It is readily oxidized by chromic acid to the corresponding ketone menthone. By the action of phosphorus pentoxide, or zinc chloride, it is converted into menthene, C$_{10}$H$_{18}$, 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 C$_{10}$H$_{19}$Cl, which, according to I. L. Kondakow (Jour. prakt. Chem., 1899 [2], 60, 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. (20 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 peppermint odour. W. H. Perkin, junr. (Proc. Chem. Soc., 1905, 21, p. 255) synthesized it from 1·4 methylcyclohexanone: sodium carbonate converts α-bromhexahydro-para-toluic acid (1) into Δ·1-tetrahydro-para-toluic acid and α-oxyhexahydro-para-toluic acid, and the latter on treatment with dilute sulphuric acid yields 1·4-methylcyclohexanone (2), which by the action of magnesium isopropyl iodide and subsequent hydrolysis is converted into tertiary menthol (3).
Terpin (terpan-diol 1·8), C$_{10}$H$_{18}$(OH)$_2$, is known in two stereoisomeric forms, cis-terpin and trans-terpin. The trans- form is obtained by adding silver acetate to a glacial acetic acid solution of dipentene dihydrochloride, filtering and neutralizing the filtrate by caustic soda. It is then extracted with ether, and the acetyl derivative so obtained is hydrolysed by alcoholic potash. It crystallizes in prisms, which melt at 156–158° C., and boil at 263–265° C. It is converted into terpineol by dilute sulphuric acid. The cis-compound melts at 104–105° C. and may be prepared by heating its hydrate. Terpin hydrate, C$_{10}$H$_{18}$(OH)$_2$·H$_2$O, crystallizes in prisms which melt at 116° C. It is prepared by acting with dilute mineral acids on limonene or dipentene. When boiled with glacial acetic acid it is converted into terpineol, while concentrated hydriodic acid at 210° C. reduces it to hexahydrocymene. When heated with dilute sulphuric acid it gives a number of compounds, which may be considered as arising from the loss of one or two molecules of water from one molecule of terpin.
Cineol, C$_{10}$H$_{18}$O, is an inner oxide of terpin. It is found in the oils of wormseed, cajaput, eucalyptus, laurel, galanga, camphor and of lavender. It may be prepared by passing a current of dry hydrochloric acid gas into wormseed oil, the precipitated hydrochloride being then distilled in a current of steam (O. Wallach and W. Brass, Ann., 1884, 225, p. 297). It is an inactive liquid, which boils at 176° C. The oxygen atom in the molecule does not appear to possess either an alcoholic, ketonic, aldehyde or acid function.
Terpineol (Δ1-terpen-ol-8), C$_{10}$H$_{17}$(OH). The term “terpineol” has been used to denote what is now known to be a mixture of various isomeric alcohols. Liquid terpineols have been isolated from the oils of Erigeron canadense, of marjoram and of camphor. Liquid terpineol is generally prepared by the action of dilute sulphuric acid on terpin hydrate. It consists of a mixture of various isomers, from which a solid terpineol melting at 35° C. and an isomeric Δ·8(9) terpen-ol-1, melting at 32° C., have been isolated (K. Stephan and J. Halle, Ber., 1902, 35, p. 2147. See also G. Bouchardat, Comptes rendus, 1887, 104, p. 996; 1895, 121, p. 141; Schimmel & Co., Semi-annual Reports, Oct. 1897, p. 11; J. Godlewsky, Chem. Centralblatt, 1899 (I.), p. 1241). Solid terpineol exists in active and racemic forms. The active form was obtained by F. W. Semmler (Ber., 1895, 28, p. 2190) by replacing the halogen atoms in the active monohydrobromide of limonene by the hydroxyl group; it has also been obtained by the action of acetic acid on linalool. The racemic variety has been prepared by the action of formic acid on geraniol, and was synthesized by the following method (W. H. Perkin, junr., Jour. Chem. Soc., 1904, 85, p. 654). γ-Cyanpentane tricarboxylic ester (1) (prepared by the ​action of cyanacetic ester on β-iodopropionic ester) is hydrolysed to pentane-αγε-tricarboxylic acid (2), which when boiled with acetic anhydride and distilled gives δ-ketohexahydrobenzoic acid (3). The ester of this acid, when treated with the Grignard reagent, yields δ-oxyhexahydrotoluic acid (4), which is converted into the corresponding brom-compound by fuming hydrobromic acid. This latter compound on treatment with dilute alkali or pyridine yields Δ·3-tetrahydro-para-toluic acid (5), the ester of which with magnesium and methyl iodide furnishes terpineol (6):—
This synthesis determines the constitution of terpin (7) and of dipentene (8), since the former is produced by the action of 5 per cent. sulphuric acid on terpineol, and the latter by heating terpineol with acid sodium sulphate.
Terpineol adds on nitrosyl chloride to form a nitrosochloride, which on elimination of hydrochloric acid yields the oxime of an unsaturated oxyketone; this on boiling with acids is converted into inactive carvone. When reduced by the method of Sabatier and Senderens it forms hexahydrocymene (A. Haller, Comptes rendus, 1905, 140, p. 1303); when oxidized with Caro’s reagent it yields trioxyhexahydrocymene (A. v. Baeyer and V. Villiger, Ber., 1899, 32, p. 3625). For an isomeric terpineol (Δ·8(9) terpenol-1) see A. v. Baeyer, Ber., 1894, 27, pp. 443, 815.
Menthone (terpan-one-3), C$_{10}$H$_{18}$O, occurs with menthol in oil of peppermint. It was first obtained by M. Moriya (Jour. Chem. Soc., 1881, 39, p. 77) by oxidizing menthol with chromic acid mixture at 120° C., and was described as an inactive compound; but R. W. Atkinson (ibid., 1882, 41, p. 50) showed that when menthol was oxidized at 135° C. a strongly dextro-rotatory menthone was produced. For the preparation of l-menthone and d-isomenthone (Beckmann’s d-menthone) see E. Beckmann, Ann., 1889, 250, p. 325; 1891, 262, pp. 21 seq. The menthone obtained by Beckmann by the reduction of pulegone hydrobromide was shown by C. Martine (Ann. chim. phys., 1904 (8), 3, p. 49) to be not completely identical with l-menthone; it is consequently designated P-menthone. An inactive menthone has been synthesized as follows. β-Methyl pimelic ester is converted by sodium ethylate into methyl-1-cyclohexanon-3-carboxylic ester-4, into which the isopropyl group is introduced (also in position 4) by the action of isopropyl iodide and sodium ethylate. The ester is then hydrolysed, and carbon dioxide eliminated from the carboxyl group, when inactive menthone is obtained (A. Einhorn and L. Klages, Ber., 1901, 34, p. 3793). It boils at 204–206° C., whereas Beckmann’s menthones boil at 208° C. A. Haller and C. Martine (Comptes rendus, 1905, 140, p. 130) synthesized natural menthone from isopropyl iodide and the sodium derivative of methyl-1-cyclohexanone-3. It has also been prepared by condensing methylhexanone with ethyl acetate, the resulting methyl-1-acetyl-4-cyclohexanone-3 being converted into the isopropyl derivative, yielding acetylmenthone, which is then hydrolysed to menthone (G. Leser, Comptes rendus, 1902, 134, p. 1115). A. Koltz and L. Hesse (Ann., 1905, 342, p. 306) convert methylhexanone (1) by means of ethyl oxalate and subsequent hydrolysis into methylhexanone oxalic acid (2), the isopropyl ester of which on treatment with a methyl alcohol solution of caustic potash yields d-menthone (3).
O. Wallach (Ann., 1900, 312, p. 171) showed that the oximes of cyclic ketones are converted by phosphorus pentoxide into iso-oximes, which are readily decomposed by concentrated hydrochloric acid to yield aliphatic amino-acids; in this way menthone may be converted into ε-amido-decylic acid,
(CH$_3$)$_2$CH·CH(NH$_2$)·(CH$_2$)$_2$·CH(CH$_3$)·CH$_2$·CO$_2$H.
Diosphenol, C$_{10}$H$_{16}$O$_2$, which occurs in the essential oil of bucco leaves (Borosma betulina) may be synthesized by oxidizing oxymethylene menthone. Sodium in alcoholic solution reduces it to para-terpane-di-ol (2·3).
Pulegone (Δ4(8)-terpenone-3), C$_{10}$H$_{16}$O, is an unsaturated ketone found in pennyroyal oil, from which it may be obtained by distillation in vacuo. It is a dextro-rotatory liquid which boils at 221–222° C. F. Tiemann (Ber., 1897, 30, p. 22) synthesized it from citronellal by converting this compound into isopulegol acetate by acetic anhydride; this ester is hydrolysed, and the isopulegol oxidized to isopulegone, which on treatment with baryta yields pulegone. Pulegone reduces ammoniacal silver nitrate on long boiling. It is reduced by hydrogen to l-menthol. When heated with water to 250° C. it yields methyl-1-cyclohexanone-3 and acetone. When methylcyclohexanone and acetone are condensed together in the presence of sodium methylate, an isomer of pulegone boiling at 215–216° C. is obtained. Pulegone combines with hydrobromic acid to form a hydrobromide, which on heating in methyl alcohol solution with basic lead nitrate is converted into isopulegone (Δ8(9)-terpenone-3) (C. Harries and G. Röder, Ber., 1899, 32, p. 3361). It is a laevo-rotatory liquid. A dextro-form (a mixture) is also obtained by the oxidation of isopulegol with chromic acid. On reduction it yields isopulegol and no menthol (cf. pulegone).
Carvone (Δ6:8(9)-terpadiene-one-2), C$_{10}$H$_{14}$O, is an unsaturated optically active ketone which is found very widely distributed in nature. The dextro-form is the chief constituent of oil of caraway, and is also found in oil of dill; the laevo-form is found in oil of spearmint and kuromoji oil. The dextro-form is obtained practically pure by the fractional distillation of caraway oil; the laevo-form from the oils containing it, by first forming its addition compound with sulphuretted hydrogen, decomposing this by alcoholic potash, and distilling the product in a current of steam. It may be synthetically prepared from limonene nitrosochloride, alcoholic potash converting this compound into l-carvoxime, which on boiling with dilute sulphuric acid yields l-carvone; similarly terpineol nitrosochloride by the action of sodium ethylate yields oxydihydrocarvoxime, which on hydrolysis yields i-carvone. On heating with phosphoric acid carvone is converted into carvacrol (1-methyl-2-oxy-4-isopropylbenzene). Carvone is closely related to phellandrene, for C. Harries and M. Johnson (Ber., 1905, 38, p. 1832), by reduction of carvone hydrobromide, obtained Δ6-terpenone-2, which with phosphorus pentachloride gives chlor-2-α-phellandrene.
Bi-Cyclic Terpene Group
A nomenclature for the bicyclic hydrocarbons was devised by A. v. Baeyer (Ber., 1900, 33, p. 3771). According to this system each hydrocarbon contains two tertiary carbon atoms, which are combined with each other three times, either directly or by means of other intervening carbon atoms, the combination forming a series of “bridges.” These bridges are distinguished by numbers, denoting the number of carbon atoms contained in them, the direct union of the two tertiary carbon atoms being designated as 0; if one carbon atom intervenes, then the number 1 is used, and so on. Thus three numbers serve as the “characteristic” for the compound. Hydrocarbons of this class with five atoms of carbon are termed “bicyclopentanes,” with six atoms of carbon “bicyclohexanes,” &c. Thus, for example, the compound (1) would
be called “bicyclo-(1·1·3)-heptane,” and (2) would be “bicyclo-(0·1·4)-heptane.”
Thujene (tanacetene), C$_{10}$H$_{16}$, is a derivative of bicyclo-(0·1·3)-hexane. The name was first given to the hydrocarbon obtained by F. W. Semmler (Ber., 1892, 25, p. 3345) on the dry distillation of thujylamine hydrochloride. It is a liquid which boils at 60–63° (14 mm.), and has been shown by L. Tschugaeff to be a monocyclic hydrocarbon, for which he proposes the name “isothujene.” The true thujene was prepared by L. Tschugaeff (Ber., 1900, 33, p. 3118) by heating the methyl xanthogenic ester obtained from thujyl alcohol. It is exceedingly unstable. The isomeric β-thujene was also obtained by the same investigator by the dry distillation of trimethylthujyl ammonium hydroxide. It boils at 150–151° C., and possesses a different rotatory power.
Sabinene, C$_{10}$H$_{16}$, also a bicyclo-(0·1·3)-hexane derivative, is found in oil of savine, from which it was first obtained by F. W. Semmler (Ber., 1900, 33, p. 1455). On shaking with dilute sulphuric acid it yields terpinenol (Δ1-terpen-ol-4) (O, Wallach, Ber., 1907, 401 p. 592).
Pinene, C$_{10}$H$_{16}$, derived from bicyclo-(1·1·3)-heptane, is found in many essential oils, and is the chief constituent of oil of turpentine; the l-variety is found in French oil of turpentine, the d-variety in Russian, American and Swedish oil of turpentine. Pinene is also a constituent of the oils of sage, lemon, eucalyptus, olibanum, bay, fennel, sassafras, rosemary and of valerian. The active varieties are obtained by the fractional distillation of the various oils of turpentine. The inactive variety is obtained by heating pinene nitrosochloride with an excess of aniline (O. Wallach, Ann., 1889, 252, p. 132; 1890, 258, p. 243), or better with methylaniline (W. A. Tilden). The three varieties boil at 155–156° C. Pinene readily absorbs oxygen from the air, resinous products being formed, together with small quantities of formic and acetic acids. ​Acid oxidizing agents convert it into terephthalic and terebic acids, whilst alkaline potassium permanganate in dilute solution oxidizes it to pinene glycol, C$_{10}$H$_{16}$(OH)$_2$, pinonic acid, C$_{10}$H$_{16}$O$_3$, pinic acid, C$_9$H$_{14}$O$_4$, &c., the products of the reaction varying according to the temperature (G. Wagner, Ber., 1894, 27, p. 2270; F. Tiemann and F. W. Semmler, Ber., 1895, 28, pp. 1344, 1778). Concentrated sulphuric acid converts it into camphene; and an alcoholic solution of sulphuric acid gives terpinene and terpinolene. When heated to 250–270° C. it yields dipentene; the moist halogen acids at ordinary temperature convert it into the dihalogen halides of dipentene. Dry hydrochloric acid gives pinene hydrochloride (artificial camphor), C$_{10}$H$_{17}$Cl, a white crystalline solid identical with bornyl chloride which melts at 131° C. Elimination of halogen hydride by means of a weak alkali (e.g. soap, silver acetate, &c.) converts it into camphene. Thus the conversion of pinene into its hydrochloride is probably accompanied by an intermolecular rearrangement—
Nitric acid in aqueous alcoholic solution converts it into terpin hydrate. Pinene nitrosochloride, C$_{10}$H$_{16}$NOCl, was first obtained in 1874 by W. A. Tilden (Jahresb., 1874 p. 214) from nitrosyl chloride and a mixture of pinene and chloroform. O. Wallach (Ann., 1889, 253, p. 251) prepared it by the action of acetic acid and ethyl nitrite on oil of turpentine in presence of fuming hydrochloric acid. W. A. Tilden (Jour. Chem. Soc., 1904, 85, p. 759) showed that strongly active pinene gives bad yields of the nitrosochloride, since, being bimolecular, its formation is retarded by the inversion of half of the terpene. The nitrosochloride melts at 115° C. (circa) and is a white pleasant-smelling powder. Alcoholic potash converts it into nitrosopinene, C$_{10}$H$_{16}$NO.
Bornylene, C$_{10}$H$_{16}$, derived from bicyclo-(1·2·2)-heptane, is prepared by heating bornyl iodide to 170° C. for several hours with a concentrated solution of alcoholic potash (G. Wagner, Ber., 1900, 33, p. 2121), or by decomposition of the methyl esters of the l- and d-bornyl xanthates, the former yielding d-borny1ene and the latter l-bornylene (L. Tschugaeff, Chem. Centralblatt, 1905, i., p. 94).
Camphene, C$_{10}$H$_{16}$, also a bicyclo-(1·2·2)-heptane derivative, is a constituent of the oils of citronella, camphor, ginger and of rosemary, and also of French and American oil of turpentine. It may be obtained by the action of sulphuric acid on pinene; by heating pinene hydrobromide or hydrochloride with sodium acetate or glacial acetic acid to 200° C.; or by heating bornyl chloride with aniline (O. Wallach, Ber., 1892, 25, p. 916). According to Konowalow it is best prepared by heating borneol with a diluted sulphuric acid (1·2) for about 6–8 hours, between 60–100° C., with continual shaking, a yield of about 90 per cent. being obtained. The melting- and boiling-points of camphene vary slightly according to the sources from which it is obtained, the former being about 50° C. and the latter about 159–161° C. It is known in d-, l- and i- forms. It combines with hydrochloric acid to form a hydrochloride, which on reduction with sodium and alcohol yields camphene. Many different oxidation products may be obtained from camphene by varying the conditions of experiment (J. Bredt and W. Jagelki, Ann., 1900, 310, p. 114; G. Wagner, Ber., 1890, 23, p. 2311; S. Moycho and F. Zienkowski, Ann., 1905, 340, p. 17; J. E. Marsh and J. A. Gardner, Jour. Chem. Soc., 1891, 59, p. 648; 1896, 69, p. 74).
Fenchene, C$_{10}$H$_{16}$, a bicyclo-(1·2·2)-heptane derivative, is not found in any naturally occurring products. The hydrocarbon may be obtained by the reduction of fenchone and elimination of water from the resulting fenchyl alcohol, or by the elimination of halogen hydride from the fenchyl halogen compounds (O. Wallach, Ann., 1892, 263, p. 145; 1898, 302, pp. 371 seq.).
The above bicyclo-terpene hydrocarbons are most probably best represented by the following formulae (pinene is given above):—
Alcohol and Ketone Derivatives
Borneol (Borneo camphor), C$_{10}$H$_{17}$OH occurs in the pith cavities of Dryobalanops camphora, and in the oils of spike and rosemary; esters are found in many fir and pine oils. It may be prepared by heating camphor with alcoholic potash (M. Berthelot, Ann., 1859, 12, p. 363); or by reducing camphor in alcoholic solution with sodium (O. Wallach, Ann., 1885, 230, p. 225; J. Bertram and H. Walbaum, Jour. prak. Chem. 1894 (2), 49, p. 12). L. Tschugaeff (Chem. Centralblatt. 1905 i., p. 94) obtains pure d-borneol as follows:—Impure d-borneol (containing isoborneol) obtained in the reduction of camphor is dissolved in xylene and converted into the sodium salt by metallic sodium. This salt is then turned into the xanthate, C$_{10}$H$_{17}$OCS$_2$Na, which with methyl sulphate yields the corresponding methyl ester. The unchanged isoborneol is removed by steam distillation, which also decomposes any methyl xanthate of isoborneol that may have been formed. The residue is crystallized and hydrolysed, when pure borneol is obtained. It behaves as a secondary alcohol. Nitric acid oxidizes it to camphor and when heated with potassium bisulphate, it gives camphene. With phosphorus pentachloride it forms a bornyl chloride, identical with pinene hydrochloride.
Isoborneol is a tertiary alcohol which may be obtained by dissolving camphene in glacial acetic acid, adding dilute sulphuric acid and heating to 50–60° C. for a few minutes, the isobornyl acetate so formed being then hydrolysed (J. Bertram and H. Walbaum, loc. cit.). It crystallizes in leaflets, which readily sublime. Chromic acid oxidizes it to camphor.
Thujone (tanacetone), C$_{10}$H$_{16}$O, is found in many essential oils. Oil of thuja contains chiefly α-thujone, and oil of tansy chiefly β-thujone. Oil of artemisia and oil of sage contain a mixture of the two, whilst oil of absinthe contains principally the β-variety. The two forms may be obtained by fractional distillation of the oils, followed by a fractional crystallization of their semicarbazones from methyl alcohol. α-Thujone is laevo-rotatory and when warmed with alcoholic potash it is partially converted into β-thujone. Sodium in the presence of alcohol reduces it to thujyl alcohol, which on re-oxidation is converted into β-thujone. The β-form is dextro-rotatory and is partially converted into the α-variety by alcoholic potash. When heated to 280° thujone is transformed into the isomeric carvotanacetone (&Delta6-terpenone-2). On boiling with ferric chloride it yields carvacrol. Hot dilute sulphuric acid converts it into isothujone (dimethyl-1·2-isopropyl-3-cyclopentene-1-one-5). Thujone behaves as a saturated compound and forms a characteristic tribromide. When heated with zinc chloride it yields hydropseudocumene. According to F. W. Semmler (Ber., 1900, 33, p. 275; 1903, 36, p. 4367) it is to be considered as a methyl-2-isopropyl-5-bicyclo-(0·1·3)-hexanone-3.
Carone, C$_{10}$H$_{16}$O, is a trimethyl-3·7·7-bicyclo-(0·1·4)-heptanone-2, obtained by acting with alcoholic potash on dihydrocarvone hydrobromide (A. v. Baeyer, Ber., 1896, 29, pp. 5, 2796; 1898, 31, pp. 1401, 2067). It is a colourless oil, having the odour of camphor and peppermint, and boiling at 210° C. It is known in d-, l-, and i-forms. It does not combine with sodium bisulphite. When heated it is transformed into carvenone. It is stable to cold potassium permanganate solution, but on heating gives a dibasic acid, caronic acid, C$_5$H$_8$(CO$_2$H)$_2$, which Baeyer suggested was a gem-dimethyltrimethylene-1·2-dicarboxylic acid. This was confirmed by W. H. Perkin, junr. (Jour. Chem. Soc., 1899, 75, p. 48) who synthesized the acid from dimethylacylic ethyl ester. This ester with ethyl malonate yields ethyldimethylpropane-tricarboxylic ester, which on hydrolysis and subsequent heating is converted into ββ-dimethyl glutaric acid. The α-bromdimethyl ester of this acid when heated with alcoholic potash yields cis-, and trans-caronic acids. Eucarvone, C$_{10}$H$_{14}$O, is a trimethyl-3·7·7-bicyclo-(0·1·4)-heptene-3-one-2. O. Wallach (Ann., 1905, 339, p. 94) suggests that the ketone possesses the structure of a trimethyl-1·4·4-cycloheptadiene-5·7-one-2. Phosphorus pentachloride converts it into 2-chlorcymene (A. Klages, Ber., 1899, 32, p. 2558).
Camphor, C$_{10}$H$_{16}$O, is a trimethyl-1·7·7-bicyclo-(1·2·2)-heptanone-2. The d-variety is found in the camphor tree (Laurus camphora), from which it may be obtained by distillation in steam. The l-variety is found in the oil of Matricaria parthenium. It crystallizes in transparent prisms which possess a characteristic odour, sublimes readily and is easily soluble in the usual organic solvents. It boils at 209° C. and melts at 176° C. (circa). The d-form may also be obtained by the distillation of calcium homocamphorate (A. Haller, Bull. Soc. Chim., 1896 (3), 15, p. 324). When heated with phosphorus pentoxide it yields cymene, and with iodine, carvacrol. Nitric acid oxidizes it to camphoric acid, C$_8$H$_{14}$(CO$_2$H)$_2$, camphoronic acid, C$_9$H$_{14}$O$_6$, and other products. It forms an oxime with hydroxylamine which on dehydration yields a nitrile, from which by hydrolysis campholenic acid, C$_9$H$_{15}$CO$_2$H, is obtained. It combines with aldehydes to form alkylidene compounds, and yields oxymethylene compounds when subjected to the “Claisen” reaction. It does not combine with the alkaline bisulphites. It is readily substituted by chlorine and bromine; and with fuming sulphuric acid forms a camphor sulphonic acid. Sodium reduces it, in alcoholic solution, to borneol. When heated with sodium formate to 120° C. it is converted into bornylamine. Caro’s acid converts it into campholid, and a compound C$_{10}$H$_{16}$O$_4$ (A. v. Baeyer and V. Villiger, Ber., 1899, 32, p. 3630). When heated with concentrated sulphuric acid to 105–110° C. it yields carvenone and 4-aceto-1·2-xylol (J. Bredt, Ann., 1901, 314, p. 371).
​A vast amount of work has been done on the constitution of the camphor molecule. The earlier investigations on the ready formation of benzene derivatives by the breaking down of camphor led to the view that the molecule was a simple six-membered carbon ring. Subsequent research, however, showed that the formula proposed by J. Bredt (Ber., 1893, 26, p. 3047), in which camphor is to be regarded as a bicyclo-heptane derivative, is correct. This formula is based on the fact that camphoronic acid yields trimethylsuccinic, isobutyric, and carbonic acids, and carbon on dry distillation, and Bredt suggested that it was an ααβ-trimethylcarballylic acid,
HO$_2$C·CH$_2$·C(CH$_3$)(CO$_2$H)·C(CH$_3$)$_2$CO$_2$H,
a conclusion confirmed by its synthesis (see below). The Bredt formula is also supported by the synthesis of r-camphoric acid by G. Komppa (Ber., 1901, 34, p. 2472; 1903, 36, p. 4332). In this synthesis ethyl oxalate is condensed with ββ-dimethyl glutaric ester, and the resulting diketoapocamphoric ester (1) is then methylated to diketocamphoric ester (2). The keto groups in (2) are converted in CH$_2$ groups as follows:—Sodium amalgam converts this ester into dioxycamphoric ester (3), which with hydriodic acid and phosphorus yields r-dihydrocamphoric acid. At 125° C. this compound combines with hydrobromic acid to form β-bromcamphoric acid, which on reduction with zinc and acetic acid yields r-camphoric acid (4):—
This series of reactions leads to a complete synthesis of camphor, since A. Haller (Comptes rendus, 1896, 122, p. 446) has shown that camphoric anhydride (1) on reduction yields campholid (2), which by the action of potassium cyanide and subsequent hydrolysis of the nitrile formed is converted into homocamphoric acid (3), the calcium salt of which yields camphor (4) on distillation:—
Thus camphor and its oxidation products are to be represented as
Camphor yields three classes of halogen substitution derivatives known respectively as α, β and π compounds, the positions being shown in the formula above. The α compounds result by direct substitution, the β and π derivatives being formed in an indirect manner. Cyancamphor, C$_{10}$H$_{15}$O·CN, is formed by passing cyanogen gas into sodium camphor, or by digesting sodium oxymethylene camphor with hydroxylamine hydrochloride (L. Claisen, Ann., 1894, 281, p. 351).
π-Camphor sulphonic acid results from the action of fuming sulphuric acid on camphor (F. S. Kipping and W. J. Pope, Jour. Chem. Soc., 1893, 63, p. 573). Camphoroxime, C$_{10}$H$_{16}$O:NOH, was first prepared by E. Nägeli (Ber., 1883, 16, p. 497).
l-Camphor is formed by the action of nitric acid on l-borneol (W. J. Pope and A. W. Harvey, Jour. Chem. Soc., 1901, 79, p. 76). r-Camphor melts at 178–179° C. (for its preparation see A. Debierne, Comptes rendus, 1899, 128, p. 1110; W. A. Noyes, Amer. Chem. Jour., 1905, 27, p. 430).
Camphoric acid. Four optically active and two inactive forms of this acid are known. The most important is the d-form, which is produced by the oxidation of d-camphor with nitric acid. it crystallizes in plates or prisms which melt at 187° C. Potassium permanganate oxidizes it to oxalic acid and Balbiano’s acid, C$_8$H$_{12}$O$_5$, together with small quantities of camphanic, camphoronic and trimethyl succinic acids. It yields two series of acid esters, the allo-esters (1), formed by the partial saponification of the neutral esters, and the ortho-esters (2), formed by heating the anhydride with alcohols or sodium alcoholates.
l-Camphoric acid results on oxidizing l-borneol or matricaria camphor. It melts at 187° C. r-Camphoric acid is formed on mixing alcoholic solutions of equimolecular quantities of the d- and l-acids, or by oxidizing i-camphor. It melts at 202–203° C.
Camphoronic acid, C$_9$H$_{14}$O$_6$. From a study of its distillation products J. Bredt (Ber., 1893, 26, p. 3049) concluded that this acid was an ααβ-trimethylcarballylic acid, a conclusion which was confirmed by its synthesis by W. H. Perkin, junr., and J. F. Thorpe (Jour. Chem. Soc., 1897, 71, 1169):—Aceto-acetic ester is condensed with α-bromisobutyric ester, the resulting hydroxytrimethyl glutamate (1) converted into the chlor- and then into the corresponding cyan-trimethyl glutamate (2), which on hydrolysis with hydrochloric acid yields camphoronic acid (3) and some trimethyl glutaconic acid:—
Fenchone, C$_{10}$H$_{16}$O, is trimethyl-(2·7·7)-bicyclo-(1·2·2)- heptanone-3. It occurs in d- and l-forms, the former in oil of fennel and the latter in oil of thuja. It may be obtained from these oils by treating the fraction boiling between 190–195° C. with nitric acid and distilling the product in a current of steam. The fenchones are pleasant-smelling oils which boil at 192–193° C., and on solidification melt at 5–6° C. They do not combine with sodium bisulphite. They dissolve unchanged in cold concentrated hydrochloric and sulphuric acids, and are very stable; thus the monobromfenchone is only formed by heating the ketone with bromine to 100° C. under pressure (H. Czerny, Ber., 1900, 33, p. 2287). On oxidation with potassium permanganate it yields acetic and oxalic acids together with dimethylmalonic acid. By the action of hot concentrated sulphuric acid it yields acetyl-ortho-xylene,
CH$_3$CO(4)C$_6$H$_3$(CH$_3$)$_2$(1·2)
(J. E. Marsh, Jour. Chem. Soc., 1899, 75, p. 1058). When heated with phosphorus pentoxide to 115–130° C. it forms metacymene. Since it does not yield any oxymethylene compounds, it cannot contain the grouping—CH$_2$·CO—in the molecule.
Hydrocarbons, C$_{10}$H$_{18}$, of the Terpene Series
Menthene, C$_6$H$_8$(CH$_3$)(C$_3$H$_7$), is methyl-1-isopropyl-4-cyclohexene-3. It is obtained by the action of anhydrous zinc chloride or copper sulphate on menthol (J. W. Brühl, Ber., 1892, 25, p. 142), by boiling menthyl chloride with aniline (G. Wagner, Ber., 1894, 27, p. 1636), by heating menthyl chloride with potassium phenolate (L. Masson, Ber., 1896, 29, p. 1843), and by the dry distillation of the methyl ester of menthyl xanthate (L. Tschugaeff, Ber., 1899, 32, p. 3333). It is a colourless liquid which boils at 167–168° C. When strongly heated with copper sulphate it yields cymene. According to Tschugaeff, the xanthate method alone gives a pure menthene of the above constitution, the menthene obtained from the dehydration of menthol being a cyclohexene-4; and the one obtained by O. Wallach (Ann., 1898, 300, p. 278) from l-menthylamine being a cyclohexene-2.
Carvomenthene, C$_6$H$_8$(CH$_3$)(C$_3$H$_7$), is probably methyl-1-isopropyl-4-cyclohexene-1. It is prepared by heating carvomenthyl bromide with quinoline, or by heating carvomenthol with potassium bisulphate to 200° C. It is a liquid which boils at 175–176° C.
Camphane, C$_7$H$_9$(CH$_3$)$_3$, is 1·7·7-trimethyl-bicyclo-(1·2·2)-heptane. It is prepared by the action of sodium and alcohol on pinene hydriodide, or by reducing the hydriodide with zinc in acetic acid solution. It is a crystalline solid which melts at 153° C. and boils at 160° C.
Olefine Terpenes
Myrcene, C$_{10}$H$_{16}$, was first isolated by F. B. Power and C. Kleber from oil of bay (Schimmel & Co., Bulletin, April 1895, p. 11); it is also found in oil of sassafras leaves. It is obtained from bay oil by shaking the oil with a 5 per cent. solution of caustic soda, followed by fractionation in vacuo. It boils at 67–68° C. (20 mm.), and polymerizes when heated for some time. When oxidized by potassium permanganate it yields succinic acid. By the action of glacial acetic acid in the presence of dilute sulphuric acid, a liquid is produced, which on hydrolysis yields myrcenol, C$_{10}$H$_{18}$O, an alcohol which is probably an isomer of linalool (P. Barbier, Comptes rendus, 1901, 132, p. 1048). The hydrocarbon is probably to be considered as being (CH$_3$)$_2$C:CH·(CH$_2$)$_2$·C(:CH$_2$)·CH:CH$_2$ (Enklaar, Bulletin of Rouse-Bertrand fils, Nov., 1906, p. 92). Ocymene is an isomer which can be extracted from the leaves of the basil. Enklaar (loc. cit.) represents it as (CH$_3$)$_2$C:CH·CH$_2$·CH:C(CH$_3$)·CH:CH$_2$. Anhydro-geraniol, C$_{10}$H$_{16}$, the first olefine terpene isolated, was prepared in 1891 by F. W. Semmler; it is formed when geraniol is heated with potassium bisulphate to 170° C.
Alcohols, Aldehydes and Ketones
d-Citronellol, C$_{10}$H$_{19}$OH or CH$_3$·C(:CH$_2$)·(CH$_2$)$_3$·CH(CH$_3$)·(CH$_2$)$_2$OH, or 2·6 dimethyl-octene-1-ol-8 occurs in Réunion geranium oil and was first prepared by F. D. Dodge (Amer. Chem. Jour., 1889, 11, p. 463) by reducing the corresponding aldehyde (d-citronellal). It is an odorous oil, which boils at 117–118° C. (17 mm.). Oxidation by chromic acid mixture converts it into citronellal, whilst ​a more drastic oxidation with potassium permanganate yields acetone and β-methyladipic acid.
l-Rhodinol, C$_{10}$H$_{19}$OH or (CH$_3$)$_2$C:CH·(CH$_2$)$_2$·CH(CH$_3$)·(CH$_2$)$_2$·OH, or 2·6 dimethyl-octene-2-ol-8, occurs in the essence of geranium and of rose. It is a structural isomer of citronellol (P. Barbier and L. Bouveault, Comptes rendus, 1896, 122, pp. 529, 673; Bull. Soc. Chim., 1900, [3], 23, p. 459), and its inactive form has been synthesized from ethyl heptenone. It is an oil of strong rose odour, which boils at 110° C. (10 mm.). Chromic acid mixture oxidizes it to rhodinal and rhodinic acid, whilst by drastic oxidation it yields acetone and β-methyladipic acid.
Geraniol, C$_{10}$H$_{17}$OH, or (CH$_3$)$_2$C:CH·(CH$_2$)$_2$·C(CH$_3$):CH·CH$_2$OH, 2·6 dimethyl-octadiene-2·6-ol-8, is found in the oils of geranium, citronella, neroli, petit-grain, spike, ylang-ylang, and in Turkish and German rose oil. It is prepared from the oils by treating them with alcoholic potash and then fractionating in vacuo. The geraniol fraction is then mixed with freshly dried calcium chloride and the mixture allowed to stand in vacuo at a low temperature, when the compound C$_{10}$H$_{18}$O·CaCl$_2$ separates out, This is washed with absolute ether and finally decomposed by water, when pure geraniol is liberated (O. Jacobsen, Ann., 1871, 157, p. 232; J. Bertram and E. Gildemeister, Jour. prak. Chem., 1897 (2), 56, p. 507). It may also be prepared by reducing the corresponding aldehyde (citral) with sodium amalgam. It is a colourless, pleasant-smelling oil, which boils at 230° C. Oxidation converts it into citral and geranic acid, (CH$_3$)$_2$C:CH·(CH$_2$)$_2$·C(CH$_3$):CH·CO$_2$H. By shaking it with 5 per cent. sulphuric acid it yields terpin hydrate, and when heated with concentrated alcoholic potash to 150° C. it is converted into dimethylheptenol (P. Barbier, Comptes rendus, 1899, 128, p. 110). Geraniol may be converted into linalool by distilling a faintly alkaline solution of acid geranyl phthalate with steam.
Nerol, C$_{10}$H$_{17}$OH, was obtained in 1902 from neroli oil by A. Hesse and O. Zeitschel (Jour. prak. Chem., 1902 (2), 66, p. 481); it also is found in petit-grain oil. It boils at 226–227° C. (755 mm.), and has a distinctive rose odour. It is inactive and is to be regarded as a stereo-isomer of geraniol. It does not form a compound with calcium chloride. It combines with four atoms of bromine to form a characteristic tetrabromide. It is formed (along with other products) by the action of acetic acid on linalool (O. Zeitschel, Ber., 1906, 39, p. 1780) and also by the reduction of citral-b.
Linalool, C$_{10}$H$_{17}$OH, or (CH$_3$)$_2$C:CH·(CH$_2$)$_2$·C(CH$_3$)(OH)·CH:CH$_2$, is 2·6-dimethyloctadiene-2·7-ol-6. d-Linalool was first found in coriander oil, and l-linalool in oil of linaloe. It is also found in oil of bergamot, petit-grain, lavender, neroli, spike, sassafras leaves and lemon, either in the free condition or as esters. It is a pleasant-smelling liquid which boils at 197–199° C. (according to its source). The inactive variety can be prepared from geraniol, this alcohol on treatment with hydrochloric acid yielding a mixture of chlorides, which when digested with alcoholic potash are transformed into i-linalool (F. Tiemann and F. W. Semmler, Ber, 1898, 31, p. 832). It is oxidized by chromic acid to citral. When shaken for some time with dilute sulphuric acid it yields terpin hydrate.
Citronellal, C$_{10}$H$_{18}$O, is the aldehyde of citronellol. It is a constituent of many essential oils, and was first discovered in citronella oil by F. D. Dodge (Amer. Chem. Jour., 1889, 11, p. 456); it is also found in eucalyptus oil and in lemon-grass oil. It is a dextro-rotatory liquid which boils at 203°–204° C. It is readily reduced by sodium amalgam to citronellol, and oxidized by ammoniacal silver oxide to citronellic acid. Potassium permanganate oxidizes it to acetone and β-methyladipic acid. It forms a dimethyl acetal, C$_{10}$H$_{18}$(OCH$_3$)$_2$, which on oxidation with potassium permanganate yields a dioxydihydro-citronellaldimethyl acetal,
CH$_3$·C(CH$_2$OH)(OH)·(CH$_2$)$_3$·CH(CH$_3$)·CH$_2$·CHO,
which must possess the above composition, since on further oxidation by chromic acid it yields a keto-aldehyde of the constitution CH$_3$CO(CH$_2$)$_3$·CH(CH$_3$)·CH$_2$·CHO (C. D. Harries and O. Schauwecker, Ber., 1901, 34, p. 2981); this reaction leads to the formulation of citronellal as a dimethyl-2·6-octene-1-al-8. Citronellal is readily converted into an isomeric cyclic alcohol isopulegol (Δ8(9)-terpenol-3) by acids or acetic anhydride (F. Tiemann, Ber., 1896, 29, p. 913). It combines with sodium bisulphite, giving a normal bisulphite and also a mono- and dihydrosulphonic acid.
Geranial (citral), C$_{10}$H$_{16}$O, is the aldehyde corresponding to geraniol. It occurs in the oils of lemon, orange, lemon-grass, citronella, bay, verbena, and in various eucalyptus oils. It may be obtained from the oils by means of its bisulphite compound, provided the operation is carried out at low temperature, otherwise loss occurs owing to the formation of sulphonic acids. Synthetically it may be produced by the oxidation of geraniol with chromic acid mixture, or by distilling a mixture of calcium formate and calcium geraniate. Its aldehyde nature is shown by the facts that it forms an alcohol on reduction, and that on oxidation it yields an acid (geranic acid) of the same carbon content. The position of the ethylene linkages in the molecule is proved by the formation of addition compounds, by its products of oxidation (acetone, laevulinic acid), and by the fact that on warming with potassium carbonate solution it yields methyl heptenone and acetaldehyde (F. Tiemann, Ber., 1899, 32, p. 107). On fusion with potassium bisulphate it forms para-cymene. It combines with β-naphthylamine and pyruric acid, in alcoholic solution, to form the characteristic citryl-β-naphthocinchonic acid, C$_{23}$H$_{23}$NO$_2$·1/2H$_2$O, which is useful for identifying citral. The crude citral obtained from essential oils is a mixture of two ethylene stereoisomers which are designated as citral-a and citral-b (F. Tiemann and M. Kerschbaum, Ber., 1900, 33, p. 877). Citral-a boils at 110–112° C. (12 mm.) and citral-b at 102–104° C. The structural identity of the two forms has been confirmed by C. Harries (Ber., 1907, 40, p. 2823), who has shown that their ozonides (prepared from the citrals by the action of ozone on their solution in carbon tetrachloride) are quantitatively decomposed in both cases into acetone, laevulinic aldehyde and glyoxal. Lemon-grass oil contains 73 per cent. of citral-a and 8 per cent. of citral-b. Citral combines with sodium bisulphite to form a normal bisulphite compound, a stable dihydrosulphonate, an unstable dihydrosulphonate and a hydromonosulphonate (F. Tiemann, Revue gén. de chim. pure et appl., 1, 16, p. 150). Citral condenses readily with acetone, in the presence of alkalis, to form pseudo-ionone (see Ionone, below).
The compounds of the citral series are readily converted into cyclic isomers by acids, the ring closing between the first and sixth carbon atoms in the chain. Two series of such compounds exist, namely the α and β series, differing from each other in the position of the double linkage in the molecule. The constitution of the α-series is determined by the fact that on oxidation they yield isogeronic acid, which can be further oxidized to ββ-dimethyladipic acid; the β-series in the same way yielding geronic acid and αα-dimethyladipic acid. The cyclocitrals themselves cannot be obtained direct from citral by the action of acids, since under these conditions para-cymene results, but they are prepared by boiling citrylidenecyanacetic ester with dilute sulphuric acid and subsequent hydrolysis of the cyclic ester with caustic potash (F. Tiemann, Ber., 1900, 33, p. 3719), or citral may be condensed with primary amines to the corresponding aldehydeimino compounds, which are then isomerized by concentrated acids, the amine group being hydrolysed at the same time (German Patent, 123747 (1901)).
Ionone, C$_{13}$N$_{20}$O. By condensing citral with acetone F. Tiemann (Ber., 1893, 26, p. 2691) obtained pseudo-ionone (1), an oil of boiling-point 143–145° C. (12 mm.), which on boiling with sulphuric acid is converted into a mixture of the isomeric α- and β-ionones (2 and 3)
α-Ionone is an oil which boils at 127–128° C. (12 mm.) and possesses a characteristic violet odour. The β-compound boils at 128–129° C. (10 mm.) and possesses a similar odour. They are largely used in perfumery. An isomer of ionone is irone, the odoriferous principle of the iris root. It boils at 144° C. (16 mm.). When heated with hydriodic acid and phosphorus it yields the hydrocarbon irene, C$_{13}$H$_{18}$ (F. Tiemann, loc. cit.).
Sesquiterpenes
Cadinene, C$_{15}$H$_{24}$, is found in the oils of cade (from the wood of Juniperus oxyeldrus), cubeb, patchouli, galbanum, cedar-wood and juniper. It may be obtained by fractionating oil of cade, converting the crude hydrocarbon into its dihydrochloride and decomposing this by boiling with aniline. It is an oil which boils at 274–275° C. and decomposes on exposure. Caryophyllene is found in oil of cloves and in oil of copaiba balsam. Various other sesquiterpenes have been described, e.g. zingiberene (from essence of ginger), cedrene (from oil of cedar-wood), santalene (from oil of sandal-wood), humulene and clovene.
Of the sesquiterpene alcohols pure santalol, C$_{15}$H$_{26}$O, has been obtained from essence of sandal-wood by conversion into the acid phthalic esters and saponification of these by potash (Schimmel & Co., Bulletin, April 1899, p 41). A mixture of two alcohols is thus obtained, one boiling at 165–167° C. (13 mm.) and the other at 173° C. They are distinguished by their different optical activities, one being practically inactive, the other strongly laevo-rotatory (see also M. Guerbet, Comptes rendus, 1900, 130, p. 417; Bull. Soc. Chim., 1900 (3), 23, p. 540). Caryophyllene alcohol is obtained from oil of cloves; by elimination of water it yields clovene, C$_{15}$H$_{24}$, a liquid which boils at 261–263° C.
Many di- and tri-terpenes have been described, but as yet are not thoroughly characterized.
References.—Gildemeister and Hoffman, The Volatile Oils (Milwaukee, 1900); R. Meldola, The Chemical Synthesis of Vital Products (London, 1904); F. W. Semmler, Die aetherischen Oele (Leipzig, 1906); G. Cohn, Die Riechstoffe (Brunswick, 1904); J. M. Klimont, Die synthetischen und isolirten Aromatica (Leipzig, 1899); and F. Heusler, Die Terpene (Brunswick, 1896). For camphor see A. Lapworth, Brit. Assoc. Rep. for 1900, and O. Aschan, Die Konstitution des Kamphers (Brunswick, 1903). (F. G. P.*)