Structure of Simpler Organic Compounds. 153 



The reaction is the same as that involved in the pre- 

 paration of benzene from benzoic acid. 



Xylenes, C 6 H 4 (CH 3 ) 2 . Three isomerides are known, all 

 of which exist in the distillate of coal tar. The m-com- 

 pound predominates in quantity over the other two. 



Mesitylene, C 6 H 3 (CH 3 ) 3 . The 1-3-5 (symmetrical) 

 compound has been already mentioned, and is of impor- 

 tance owing to the fact that it can be obtained syntheti- 

 cally from acetone (p. 141). 



Other homologues of benzene are known, such as 

 cymene, C 6 H 4 (CH 3 )(C 3 H 7 ), p-methyl ^so-propyl benzene, 

 some of which are contained in essential oils. 



Halogen Derivatives of Benzene and its Homologues. 



The action of chlorine and bromine on benzene itself 

 has been already mentioned (p. 144). Addition products 

 only form in the presence of direct sunlight. The iodine 

 substitution products cannot be obtained directly. They 

 are best obtained by the action of potassium iodide in 

 aqueous solution on a diazonium salt 



C 6 H 5 -N 2 S0 4 H + KI = C 6 H 5 I + N 2 + KHS0 4 



The action of halogens on alkyl derivatives of benzene is 

 of interest. In this case substitution of a hydrogen atom 

 can take place either in the benzene nucleus or the alkyl 

 group or so-called " side-chain." In the absence of direct 

 sunlight or in the presence of a "carrier" (see p. 145), the 

 substitution takes place in the nucleus. By the action of 

 chlorine on toluene, under such conditions, a mixture of 

 o- and p-chlor toluenes 



CH 3 CH 3 



is obtained. 





