Correlation Analysis in Chemistry

pp 439-540

A Critical Compilation of Substituent Constants

  • Otto ExnerAffiliated withInstitute of Organic Chemistry and Biochemistry, Czechoslovak Academy of Sciences

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The practical usefulness of a correlation equation as well as its significance in theory strongly depend on its range of validity, not only as it is defined but also as its use is restricted in practice by the number of parameters actually known. Hence the tables of parameters are of primary importance and reflect the actual state of a given field. In this chapter attention is focused on constants characterizing the substituent, which are mainly denoted by the letter σ with various subscripts and superscripts, but sometimes also by other symbols (E s, v). The correlation equation most frequently has the form (10.1). For the most common equations of this type
$$y = \rho _0 + \sum\limits_{i = 1}^A {\rho _i \sigma _i } $$
(Hammett, Taft, Yukawa-Tsuno) many tabulations of σ constants are available (e.g. Refs. 1–13), but authors usually select from the literature only one preferred value for each substituent and the extent of the table is always a compromise between two demands: reliability and completeness. Actually neither can be fully satisfied, so that the user cannot really rely on all the data on the one hand, and must always search for additional data on the other. The problem is more urgent than with the tabulation of any other constants, since the individual σ values are not only of varying precision but may even be defined and calculated in quite different ways. It follows that the main problem of this chapter will be the clear definition of how a given σ value was obtained.