Plant Qrowth'Suhstances 



Positioning and Nomenclature 



The indole molecule consists of one five-membered 

 (pyrrole) ring (composed of one NH and four CH groups) 

 having two CH groups in common with a benzene ring. 

 Numbering of substituents in the pyrrole ring usually com- 

 mences with the N group as i , proceeding around the pyrrole 

 ring away from the benzene ring. The pyrrole nucleus some- 

 times bears the letters a and p in place of 2 and 3, substituents 

 attached to the nitrogen atom then having the prefix N. 

 The practice of using Greek letters to denote position in 

 the indole nucleus is likely to lead to confusion, especially in 

 considering compounds, like tryptophan (3-indole-a-amino- 

 propionic acid), in which Greek letters also serve to indicate 

 positions in a side-chain.^ The prefix Pr has also been used to 

 indicate substitution in the pyrrole ring. The use of this 

 prefix, which does not affect the numbering as given above, is 

 almost extinct. 



Most of the naturally occurring indole derivatives are 

 substituted in the 3-(P) ring position. 2-substituted derivatives 

 are perhaps less often synthetized than the 3-substituted, 

 though 2-carboxy-3-methylindole (2-carboxy-skatole ; isomeric 

 with indole-3 -acetic acid) was made by Wislicenus and Arnold 

 as long ago as 1887. Recently, indole compounds substituted 

 in the benzene ring have been synthetized and tested for 

 activity upon plant growth. The prefix bz- is used to indicate 

 substitution in the benzene nucleus, numbering then proceed- 

 ing round the benzene ring consecutively with the number- 

 ing of the pyrrole ring. The atoms common to both rings are, 

 as usual, not numbered. 



The ring nomenclature of the indolyl and related com- 

 pounds is not standardized. Sometimes the termination -ole 

 is used, sometimes -olyl ; in the case of relevant derivatives of 

 naphthalene and other higher hydrocarbons, the tendency 

 seems to be to retain the ring name unchanged. This con- 

 fusion is unfortunate, but no greater degree of precision can 

 be expected while the Chemical Society (Smith, 1936) write 



^ The author has used the a- and ^- notation in discussing the naphtha- 

 lene compounds, instead of using the more modern i- and 2-. 



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