144 HENDERSON AND SMEATON CONTRIBUTIONS TO THE ~VIII.-Cont9.ibutions to the Chcnzistry of the Teqmnncs. Part X I X . Synthesis of a m-Menthadiene from m-iso Cymenc. By GEORGE GERALD HENDERSON and THOMAS FREDERICK SMEATON. SEVERAL menthadienes have been prepared by various synthetic methods but none that belongs to t.he met'a-series and may there-fore be considered a derivative of m-isocymene had been synthe-sised until the problem was successfully attacked by W. H. Perkin, jun. and his collaborators (compare T. 1905 87 1083; 1907 91, 480; 1908 93 1876; 1910 97 2129; 1911 99 118). Sylvestrene, carvestrene and the other meta-ment*hadienes prFpared synthetic-ally by these investigators contain one ethylenic linking in the nucleus and one in a side-chain hence it appeared desirable to obtain a menthadiene of the meta-series containing two ethylenic linkings in the nucleus in order to compare its properties with those of the corresponding para-menthadienes.Starting with m-isocyrnene obtained by the action of phosphoric oxide on fenchone we first prepared misocymene-6- sulphonic-aci CHEMISTRY OF THE TERPENES. PART XIX. 145 a d converted this into the corresponding phenol 6-hydroxy-m-zio-cymene by fusion with alkali. The succeeding steps in the procma, which was similar to that previously adopted by one of us for the synthais of menthadienes from thymol and carvacrol respectively (Henderson and Boyd T. 1911 99 2159; Henderson and Schotz, T. 1912 101 2563) were briefly as follows (1) Preparation of m-menthan-6-01 (1 -rnethyl-3-isopropglcyclohexan-6-01) C,,H,,*OH, from 6-hydroxy-m-isocymene by hydrogenation in the presence of active nickel.(2) Dehydration of the m-menthanol by heating with oxalic acid with the formation of a mmenthene (l-methyl-3 - isopropylcycZohexene) CloHI8. (3) Preparation of the mmenthene dibromide C,,H,,Br,. (4) Conversion of the di-bromide into a m - menthadiene (1 -methyl - 3 - isopropylcydo-hexadiene) ClOH16 by elimination of two molecular proportions of hydrogen bromide through treatment with alcoholic potassium hydroxide. It is evident that dehydration of 1 -methyl-3-isopropylcycZo-hexan-6-01 (I) might result in the formation of either l-methyl-34sopropyl-A5-cycZohexene (11) or l-methyl-3-isopropyl-A~-cycZo-hexene (111) The dibromide of the former hydrocarbon would yield on treatment with alcoholia potassium hydroxide 1 -methyl-3-isopropyl-A4 %ycZohexadiene (IV) whilst from the latter either of two isomerides l-methyl-3-isopropyl-A~~5-cycZohexadiene (V) or l-methylene-3isopropyl-h5-cycZohexadiene (VI) might be produced in a similar manner.Direct evidence of the constitution of the hydrocarbon CloH18, which we obtained from the m-menthanol is lacking but its proper-ties are very similar to those of a m-menthene which was prepared by Knoeveaagel (Annulen 1897 297 169) by heating l-methyl-3-isopropylcycZohexan-5 -01 with phosphoric oxide at 1 10-130° and should therefore be either 1-methyl-3-isopropyl-A4-~ycZohexene or l-methyl-3-isopropyl-A~-cycZohexene : ’HMe*CH,*?HPrS CHMe-CHI,- 8H P r a j CHMe- CH,*CHPrP &H,-CH=c!H f- bH2=CH(OH)* H ~ H = c H - ~ H ~ a 146 HENDERSON AND SMEATON CONTRIBUTIONS TO THE The physical constants of the two hydrocarbons are as follows: B.p. D. n,. MD. H. and S. ......... 167-168" 0.8222 1.45683 45.61 K. .................. 167-168" 0.8197 1-45609 45-67 Comparison of the figures suggests the conclusion that the sub-stances are identical and therefore that the hydrocarbon which we obtained from I-methyl-3isopropylcycZohexan-6-01 is l-methyl-3-&opropyl-A5-cycZohexene (11). If this is the case the dibromide prepared from it is 5 6-dibromo-l-methyl-3-isopropylcycZohexane, and therefore the hydrocarbon C10H16 which the dibromide yields on treatment with alcoholic potassium hydroxide is in all proba-bility l-methyl-3-isopropyl-A4 :6-cycZohexadiene (IV).The follow-ing evidence in support of this view may also be quoted. Harries and Antoni (Annalen 1903 328 88) obtained a " dihydrocymene," Cl0HI6 by distilling the phosphate of 1 3-diamino-m-menthane, which should be either A4 :6- or A1 :5-l-met~hyl-3-isopropylcycZo-hexadiene. The properties of this hydrocarbon-boiling point 172-174O DiiZ 0.8423 mi,"5 1-47936-do not correspond at all closely with those of our m-menthadiene which are as follows: boiling point 169-171° DE 0.8515 rz 1-47270. ' Apparently, therefore the latter is not the ,l1:6-isomeride whilst i f the m-menthene from which it is derived has the A5-structure as appears probable formula VI is also excluded. Of the three possible formulze for the menthadiene there is none in which conjugated double bonds are absent and indeed the fact that it is not capable of uniting additively with more than one molecular proportion of bromine although containing two ethylenic linkings in its molecule shows that i t has this conjugated structure.Nevertheless the value found for its molecular refraction was normal instead of showing an exaltation as might have been expected. A t the same time it should not be forgotten that, according to Auwers and Eisenlohr (Ber. 1910 43 SO) the presence of conjugated ethylenic linkings in a compound can be established with more certainty from consideration of its molecular CI,H,,J=2 DX 0.8515. Dqo 0.8500. Molecular refraction. -. Line. n20. Found. Calculated. A. D ..................... 1.47270 44-89 45-24 - 0.36 C .....................1.46964 44.65 44-97 - 0.32 c f ..................... 1.48802 46-21 46.39 -0.18 Molecular dispersion (M,-M,) . Found. Calculated. A -7 1.56 1.426 0-134 =9.4 per cent CHEMISTRY OF THE TERPENES. PART XIX. 147 dispersion than from t,he molecular refraction and as shown in the above table the figures obtained for the m-menthadiene confirm the view expressed regarding its structure. EXPER IYENTAL. YMt =CH-gPrP C(OH):CH*CH 6-Hydroxy-m-isocymene, mAoCymene (1 -methyl-3-isopropylbenzene) was prepared from fenchone by treatment with phosphoric oxide according to Wallach’s method (Annalen 1893 275 IS$) and converted into 6-hydroxy-m-isocymene by the process described by Kelbe (Annalen 1881 210 30) with certain modifications.On warming with concentrated sulphuric acid m-isocymene yielded readily a mixture of m - is0 cy m ene 6 -sul p honi c acid and mis o oy m en e- 4 -sulphonic acid the latter only in small proportions. The sulphonic acids were converted into their barium salts in the usual way and the salt of the 4-acid which is more readily soluble in water was separated from the sparingly soluble salt of the 6-acid. The potassium salt of the 6-acid was obtained by heating the barium salt with a concentrated aqueous solution of potassium carbonate, and purified by crystallisation from water from which it separates in lustrous plates. Finally the potassium salt was fused with six times its weight of potassium hydroxide the fusion dissolved in water the solution acidified and the 6-hydroxym-&ocymene dis-tilled over in a current of steam.From the distillate it was separated partly by means of a tap funnel and partly by extrac-tion with ether dried and distilled (b. p. 231O). It is a colourless, refractive liquid the vapour of which has a very irritating effect o n the lungs. 1 -Me t h y 1-3-isopr o p y 1 cy cloh e xan- 6 - 01 (I ) . The 6-hydroxy-m-isocymene was converted into 1 -methyl-3-iso-propylcyclohexan-6-o1 by treatment with hydrogen in the preselnce of active nickel. I n order to obtain the nickel in an active form, t,he hydroxide of the metal was precipitated by the addition of potassium hydroxide to a solution of the pure nitrate the pre-cipitate washed exhaustively with water formed into a paste by admixture of granules of pumice and dried.The greater part of a long combustion tube was filled with the mixture and the oxide of nickel reduced with pure hydrogen at 280O. The 6-hydraxy-nt isocymene was placed in a boat in the front part of the tube which was maintained a t 170-180° while a steady stream of carefully G* 148 HENDERSON AND SMEATON CONTRIBUTIONS TO THE purified hydrogen was paased through. The hydrogenation pro-duct distilled slowly into the receiver as a colourless liquid. Attempts to crystallise the compound from various solvents or by cooling having proved unsuccessful it was purified by distillation under diminished pressure. 1 -Me t h y l-3 -iso;~ropylcyclohexan-6 - 01 is a colourless somewhat viscous liquid with a pleasant odour. It is very sparingly soluble in water and readily so in alcohol or ether.Its physical constants are as follows boiling point 119-121°/28 mm. DZ 0.9156, $' 1.46659 MD found 47.30 calc. 47.55. l-Methtyl-3-isopropyLA~-cydohexene (11). I n order to effect dehydration the purified 1-methyl-3-isopropyl-cydohexan-6-01 was mixed with about twice its weight of finely powdered anhydrous oxalic acid and boiled for several days in a flask provided with an air condenser until the process appeared to be completed. The contents of the Aask were distilled in a current of steam when a clear oily liquid passed over and floated on the surface of the aqueous layer. The distillate was saturated with ammonium sulphate and extracted with ether the ethereal extract washed with water and dried over anhydrous calcium chloride and the ether removed by distillation.The residual liquid was dis-tilled and two fractions were collected the larger one boiling at 165-185O and the other at 185-205O. The latter wils set aside for further treatment with oxalic acid and the fraction which boiled at 165-185O was redistilled over sodium until a product of constant boiling point was obtained. l-MethyZ-3-is~propyLA5-cyclohexene is a colourless mobile liquid with an agreeable odour. Its boiling point is 167-168°/760 mm. DZ 0.8222 nD 1.45683, MD found 45-61 calc. for C,,H,8,\= 45-39. It is unsaturated, immediately decolorising solutions of potassium permanganate and of bromine. H M e-CX.,-y HPr 6 8 HBr*CHBr* CH 5 6-Dibrmo- 1 -methtyl-3-isopropylcyclohexame, A cooled solution of bromine (1 mol.) in glacial acetic acid was added gradually to a solution of the m-menthene (1 mol.) in the same solvent cooled with ice-water and stirred continuously.After addition of the bromine the solution was left for half an hour and then poured into ice-water. The dibromo-derivative settled down as a heavy oily liquid almost colourless which was washed with water until free from -tic acid and used for the next operation without further purificatio CHEMISTRY OF THE TERPENES. PART XIX. 149 l-Meth yl-3-isopropyE-A4 :6-cycloh ezadiene (IV) . The dibromo-derivative was boiled for several hours under a reflux condenser with an excess of alcoholic potassium hydroxide until no further separation of potassium bromide took place. The mixture was then distilled in a current of steam and the oily distillate separated from ths water with the aid of light petroleum.The extract was washed with water and dried over anhydrous potassium carbonate and the petroleum then distilled off. The liquid which remained was fractionally distilled over sodium in an atmosphere of dry hydrogen; no signs of decomposition were observed during the process and the distillate was perfectly colour-less. After several fractionations a product of constant boiling point was obtained. 1 -Methyl-3-isopropyl-A4 :6-cyclohezadiene (A* L6-m-menthadiene) is a colourless limpid liquid with an agreeable d o u r . It boils a t 169-171°/760 mm. and has DZ 0.8515 1.47270 M found 44.89 calc. for C,,HI6 I= 45.24. The hydrocarbon is unsaturated, at once reducing an alkaline solution of potassium permanganate, but is not 'capable of combining additively with more than one molecular proportion of bromine.With sulphuric acid it gives a red with sulphuria acid and alcohol a reddish-brown and with acetic anhydride and sulphuric acid a violet coloration. From the results of the determination of its physical constank shortly after purification and again after an interval of ssven days i t evidently undergoes change probably polymerisation on keeping. Thus the following results were obtained with a specimen which had been set aside for a week after purification Dg 0.8614 nz 1.47437, M found 44.44. The m-menthadiene appears in fact to behave in a similar way to cyclopentadiene . (compare Auwers and Eisenlohr Zoc. cat.). We take this opportunity of expressing our gratitude to Mr. W. G. Birrell for valuable assistance in the experimental part of the work and to the Research Fund Committee of the Carnegie Trust for a grant in aid of the expenses. UNIVERSITY OF GLASGOW. [Received January 15th 1920.