Hammett equation—continued substituent constants, determination

The need for substituent constants of groups for which experimentally determined values are unavailable continually recurs. To supply this need methods of estimation of substituent constants are required. The first successful attempt to estimate substituent constants is due to Charton (50). The basis of this method is that a data set with substituents of the type ZW, where Z is variable and W is constant, can be treated in two equivalent ways. The system has the form ZWGY. The substituent can be taken to be Z, and the skeletal group to be WG or, the substituent can be assumed to be ZW, and the skeletal group to be G. In that case, let the composite substituent constant, ot,zw represent the effect of the ZW group, and the composite substituent constant, Hammett equation, some quantity Q which is to be correlated is given by. 

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