122 REPORT— 1902. 



It has also been obtained from chlorhexahydrobenzene (from petroleum) 

 by eliminating hydrogen chloride/ or by treatment with zinc ethyl.^ 



Tetrahydrobenzene is a colourless liquid, resembling petroleum, with a 

 slight odour of garlic. On long standing it resinifies ; it gives a yellow 

 colour with sulphuric acid, and yields a liquid dibromide, crystalline 

 nitrosochloride, and nitrosate. Physical properties, see infra.^ 



Substituted tetrahydrohenzenes. — In 1896 Knoevenagel ■* described a 

 series of hydrocarbons supposed to be dihydrobenzenes, but which, at a 

 later date,^ were proved to be tetrahydrobenzenes. They were prepared 

 by eliminating the elements of water from hydroxyhexahydrobenzenes 

 (see pp. 124, 129) under the influence of phosphorus pentoxide. 



.CHj . CHOH .CH = CH 



\CH2 . CH . CE, \cHj . CH . CH, 



Optically active raethyltetrahydrobenzenes have been described by 

 Zelinsky.'^ 



Dihydrobenzene, CgHg. — This hydrocarbon was first prepared by 

 Baeyer '^ from 1 : 4-dihydroxyhexahydrobenzene (quinite) (p. 124), which 

 with hydrobromic acid gives 1 : 4-dibromohexahydrobenzene. The latter 

 readily loses two molecules of hydrogen bromide, yielding dihydrobenzene, 

 for which, as Baeyer points out, there are two possible formulie : — 



/CHj . CHj yCHo . CHrt 



HO . CH \CH0H Br , CH ^CHBr 



\CH„ . CHj \cHj . CHj 



„CH - CH .CHj - CH 



CH ^CH or CH ^CH 



\cHj . CHj '^CH - CH^ 



Markownikoff,^ who prepared dihydrobenzene from dichlorhexahydro- 

 benzene, obtained from light petroleum,^ considers Baeyer's hydrocarbon 

 to be a mixture, as its tetrabromide on crystallisation yields fractions of 

 different melting-points. 



Dihydrobenzene is a liquid resembling benzene, but having a marked 

 odour of garlic : like the terpenes, it resinifies on exposure to air, and gives 

 a blue colour with sulphuric acid. It rapidly decolorises potassium 

 permanganate, gives only traces of a crystalline nitrosochloride, a saturated 

 hydrobromide and tetrabromide.'" 



Siibstituted dihydrobenzenes can be obtained as follows : — 



(i.) From substituted 1 : 4-dihydroxyhexahydrobenzenes by the above 

 method. ' ' 



' Markownikoff, Annalen, 1898, 302, 27 ; Fortey, J.C.S., 1898, 73, 941. 

 - Kursanoff, Ber., 1899, 32, 2974. 



' Fortey, supra; also Hartley and Dobbie, J.C.S., 1900, 77, 84. 

 ■• Annalen, 289, 131. * Annalen, 1897, 297, 113, 



" Ber., 1902, 35, 2492. " Annalen, 1894, 278, 88. 



*> Ibid., 1898, 302, 29. « See also Fortey, J.C.S., 1898, 73, 944, 



■" Baeyer, Annalen, 1894, p. 95. 

 " Baeyer, Ber., 1892, 26, 2122 ; and 1893, 26, 232. 



