Erlenmeyers expansion of the isosterism concept

Starting in 1932, Erlenmeyei pnblished a series of detailed studies about the isosterism concepL and particularly. 

Erlenmeyer also proposed three expansions of the isosterism concept  

To the whole group of elements present in a given column of the periodic table. Thus silicon becomes isosteric to carbon, sulfur to oxygen, etc. 

To the pseudoatoms, with the aim of including groups which at a first glance seem totally different, but which, in practice, possess rather similar properties. This is the case for the psendohalogens (e.g. C1=CN=SCN, etc.) 

To the ring eqnivalents the equivalence between —CH=CH— and —S— explaining the well-known analogy between benzene and thiophene (Table 15.3). 

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