Abstract
Structure-reactivity analysis with the dual-parameter ΔE and ΔC substi- tuent constants illustrates deviations that can arise with conjugating substituents 4-X in a para position. Complications arise when the inte- raction of a 4-X substituent with the reaction site (rates or equili- bria) or with a spectroscopic measurement (NMR shift) gives rise to ad- ditional π-contributions thereby leading to a deviation in a single- parameter Hammett linear free energy plot. The deviations for the 4-X groups are referred to in the ΔE and ΔC correlations as π-plus effects. Their presence in systems that give curved Hammett plots for rates, equilibria, and 19 F chemical shifts is illustrated
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