THE GROWTH OF CHEMICAL IDEAS 127 



It was Kekule's pupil W. Koerner who gave the experi- 

 mental proof of this relation. He made the three isomeric 

 dibrombenzenes and the mononitro compounds derived from 

 them; and he found that the number of mononitro com- 

 pounds derived from each dibromo compound was that 

 which would be prophesied by the Kekule formula, and thus 

 he identified the position of the bromine atoms in the differ- 

 ent dibromo derivatives. 



These theories of molecular constitution supplied the 

 chemists with the map and compass by which they could 

 penetrate that tangled forest of organic chemistry. They 

 could understand the difference between structural isomers; 

 that is, compounds of identical composition and molecular 

 weight but different chemical behavior. The first of these 

 isomers had been discovered by Wohler and von Liebig in 

 the pair cyanic acid and fulminic acid. Such isomerism was 

 now understood as being caused by a different linking of the 

 atoms in the molecule. A little later it became possible to 

 distinguish between isomers that differed only as the left 

 hand differs from the right. The organic chemists soon 

 evolved methods by which they could determine the posi- 

 tion of different groups in the molecule and could build 

 molecules according to plan. 



As early as 1849, E. Frankland had been able to synthesize 

 hydrocarbons of the methane series. If, for instance, ethyl 

 iodide was heated with zinc, zinc iodide was formed, and 

 the two ethyl groups united to form butane. Frankland, 

 indeed, discovered the zinc alkyls and used them in syn- 

 thetic operations, an early suggestion of the most important 

 Grignard reaction, in which magnesium is employed instead 

 of zinc. 



In 1877, C. Friedel and J. M. Crafts at the Sorbonne dis- 

 covered the reaction that is known by their names. In it, 

 alkyl groups can be introduced by treating a compound such 

 as benzene with an alkyl chloride in the presence of anhy- 

 drous aluminum chloride. About the same period also, the 

 value of the reactive methylene group was recognized, and 



