6.] 



CYMENE 



29 



(Fendler, Arch. Pharm. 238, 671 : see 

 also Ch. Centr. 1900, 2, 574). 



NOTE : The cymene said in the older works 

 to be a constituent of so many plant oils is no 

 doubt some other hydrocarbon and was re- 

 corded before the discovery of dependable 

 chemical methods for identifying cymene. It 

 is probable also in many cases that the cymene 

 was produced by the action of the reagents 

 employed upon some constituent of the oil and 

 was not pre-existent as such. 



Cymene is found in the urine of 

 dogs after repeated doses of sabinol 

 (Hildebrandt, Ch. Centr. 1901, 1, 53). 



SYNTHETICAL PROCESSES. 



One of the generators of cymene in 

 some of the synthetical processes is 

 benzene, and as this hydrocarbon is also 

 the generator of large numbers of other 

 synthetical products its syntheses are 

 here introduced : 



Syntheses of Benzene. 



[1.1 From acetylene (see under meth- 

 ane [1; A]). The latter polymerises 

 under the influence of heat with the 

 formation of benzene (Berthelot, Comp. 

 Rend. 63, 479 ; Bull. Soc. [2] 7, 303 ; 

 Ann. Chim. [4] 9, 469). 



NOTE : Generators of acetylene (see under 

 methane [1 ; T ; U ; &c.]) thus become generators 

 of benzene. 



Carbon monoxide combines with 

 potassium at 80 to form a salt of 

 hexahydroxybenzene, the latter being 

 liberated on treatment of the salt with 

 hydrochloric acid (Brodie, Journ. Ch. 

 Soc. [a] 12, 269; Ann. 113, 358; 

 Nietzki and Benckiser, Ber. 18, 1834). 

 Hexahydroxybenzene gives benzene 

 (and diphenyl) on distillation with zinc 

 dust. 



Mellitic acid= benzenehexacarboxylic 

 acid can be obtained by oxidising char- 

 coal or other forms of carbon by alkaline 

 permanganate (Schulze, Ber. 4, 802 ; 

 806), by fuming nitric acid (Dickson 

 and Easterfield, Proc. Ch. Soc. 14, 163), 

 by the electrolysis of dilute acid or 

 alkali with carbon electrodes (Bartoli 

 and Papasogli, Gazz. 11, 468; Ch. 

 Centr. 1881, 327), by alkaline hypo- 

 chlorite (Ibid. Gazz. 15, 446), or by 



heating with strong sulphuric acid 

 (Verneuil, Bull. Soc. [3] 11, 121; 

 Comp. Rend. 132, 1340). Mellitic 

 acid gives benzene when distilled with 

 soda-lime (Baeyer, Ann. Suppl. 7, 5). 



Certain metallic carbides, e.g. of 

 barium, give benzene when heated to 

 600-800 with an equimolecular weight 

 of the metallic hydroxide (Bradley and 

 Jacobs, Germ. Pat. 125936 of 1898; 

 Ch. Centr. 1902, 1, 77). 



[II.] Methane [l] and heptane [2] 

 give benzene among the products 

 formed by passing through a hot tube 

 (for heptane see Worstall and Burwell, 

 Am. Ch. Journ. 19, 815). 



[III.] From ethyl alcohol [14] through 

 ethylene (see under methane [l; D]). 

 The latter gives benzene among the 

 products formed by passing through 

 a hot tube (Berthelot, Bull. Soc. [2] 

 > 456 > Norton and Noyes, Am. Ch. 

 Journ. 8, 362). 



NOTE : Generators of ethylene thus become 

 generators of benzene. (See under methane 

 [1 ; S ; T ; V, &c.]). 



Or from ethyl alcohol through chloro- 

 form, bromoform, or iodoform (see under 

 methane [l ; D]). Chloroform gives 

 acetylene by passing the vapour over 

 heated copper (Berthelot, Comp. Rend. 

 50, 805) or by the action of potassium 

 or sodium amalgam (Kletzinsky, Zeit. 

 [2] 2, 127 ; Fittig, Ibid.). Iodoform 

 gives acetylene by the action of finely 

 divided silver, zinc, or the copper-zinc 

 couple (Cazeneuve, Comp. Rend. 97, 

 1371 ; Bull. Soc. [2] 41, 1 06). Bromo- 

 form gives acetylene by similar treat- 

 ment (Ibid. [3] 7, 70; see also Nef, 

 Ann. 308, 329). Acetylene gives ben- 

 zene by polymerisation as under I 

 above. 



NOTE : Generators of chloroform and iodo- 

 form given under methane thus become through 

 acetylene generators of benzene. 



Or from ethylene through ethylene 

 bromide and acetylene by the action of 

 alcoholic potash on the latter (Miasni- 

 koff, Ann. 118, 330 ; Sawitsch, Comp. 

 Rend. 52, 157; Ann. 119, 184; Sa- 

 banejeff, Ann. 178, in), or by the 

 action of potassium isobutylate on the 

 bromide (Forcrand, Ann. Chim. [6] 11, 



