Substituent effects Swain-Lupton treatment

The Swain-Lupton treatment was a reaction against the proliferation of scales of polar substituent constants. The authors maintained that the polar effect of any given substituent could be adequately expressed in terms of just two basic characteristics a field constant and a fixed resonance constant Swain and Lupton maintained that the correlation analysis of chemical reactivity data and spectroscopic data of aromatic systems could be carried out satisfactorily in terms of and 9. cf the four cri -type parameters introduced for the DSP equation), meta and para series being dealt with separately, as in the case of the DSP equation. The assumptions involved in establishing the and 9 . scales provoked much criticism. Nevertheless, the treatment achieved fair success when applied to chemical reactivity data and some spectroscopic data, particularly The most... 

Swain-Lupton equation A dual-parameter approach to the correlation analysis of substituent effects, which involves a field constant (F) and a resonance constant (R). The original treatment was modified later. The procedure has been considerably applied, but also much criticized. 

The successful decomposition of a values into field and resonance components could eliminate the need for several sets of o values. The Swain-Lupton system must treat meta- and para-substituted compounds as separate reaction series, with differing values for r and / for a meta versus para placement of the substituent. The reason is that resonance interactions are usually stronger in the para series. There must also be an additional parameter for each reaction, since the relative sensitivity to resonance and field effects differs from reaction to reaction. Swain and Lupton have observed satisfactory correlation for over forty reaction series using and This treatment also provides an indication of the relative importance of resonance and field interactions. The mathematical manipulations are, of course, more complex than in the simple Hammett equation. The Swain-Lupton correlations are carried out by a computer program that provides a best-fit correlation in terms of /, r, and The computation also yields percent resonance by comparing the magnitude of/ andr. 

Another approach to the treatment of the variability of resonance and field effects was devised by Swain and Lupton. Their approach is to partition substituent effects into pure resonance and field contributions. The substituent constant would then be expressed as a sum of the field and resonance contributions. 

Analogous DSP treatments, but with differences with regard to the basis of the calculations, were proposed by Charton (1981) and by Swain s group (Swain and Lupton, 1968 Swain et al., 1983). The potential significance of polarizability effects resulted in a triple substituent parameter treatment (Taft, 1983). A slightly different... 

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