366 CONSIDERATION OF CARBON COMPOUNDS. 



position the introduced radicals stand to one another, and while we 

 have no proof whatever in regard to this position, yet we often repre- 

 sent it graphically, as, for instance, in the following three cases, where 

 the two groups OH replace hydrogen in different positions : 



OH OH 



C. 



\ x 



A 



Ortho-position. Meta-position. Para-position. 



1.2. 1.3. L4. 



Designating the hydrogen atoms in benzene with numbers, thus : 



123456 



C 6 H H H H H H, the above 3 compounds show that in one case 

 the hydrogen atoms 1 and 2, in the second 1 and 3, in the third 1 

 and 4 have been replaced by OH. The compounds formed in this 

 way are distinguished as ortho-, rneta-, and para-compounds. 



The molecular formula of the above three compounds is C 6 H 6 O 2 , 

 apparently indicating benzene in combination with two atoms of 

 oxygen or dioxybenzene ; actually they are dihydroxy benzene. 



1 2 



Ortho-dihydroxy benzene, C 6 H 4 OHOH, is known as pyro-catechin, 



1 3 



raeta-dihydroxy benzene, C 6 H 4 OHOH, as resorcin, and para-di- 



1 4 



hydroxy benzene, C 6 H 4 OHOH, as hydroquinone. 



Benzene series of hydrocarbons. By replacing the hydrogen 

 atoms in benzene by methyl, CH 3 , a series of hydrocarbons is formed 

 having the general composition C n H 2n e- To this benzene series 

 belong : 



B. P. 



Benzene . . . C 6 H 6 ............... 80 C. 



Toluene . . . C 7 H 8 = C 6 H 5 CH 3 110 



Xylene . . . C 8 H 10 = C 6 H 4 (CH 3 ) 2 142 



Cumene . . . C 9 H 12 = C 6 H 3 (CH 3 ) 3 151 



Cymene . . . C 10 H U = (C 6 H 2 (CH 3 ) 4 ? 175 



Penta-methyl-benzene C^H^ = C 6 H(CH 3 ) 5 188 



Hexa-methyl-benzene C 12 H 18 : : C 6 (CH 3 ) 6 202 



The first four members of this series are found in coal-tar ; the 

 fifth member, cymene, C 10 H 14 , occurs in the oil of thyme ; the last 

 two have been obtained by synthetical processes. While but one 

 toluene is known, the higher members form quite a number of iso- 

 meric compounds. Cymene, found in the oil of thyme, is, for 



