Hammett plots/data resonance

Similarly, those reactions that are strongly assisted by withdrawal of electrons from the reaction site, such as nucleophilic aromatic substitution, give a poor fit to a Hammett plot for the substituents that are capable of withdrawing electrons by delocalization (—N02, —N2 , —C=N, and so on). An example is Reaction 16 in Table 26-7. To correlate reactivity data with structures where strong resonance effects operate, different sets of substituent constants are required.1... 

Addition reactions of ArS to alkynes produce cis- and trans-olefins, which seem to be controlled kinetically [42]. To understand stereospecific addition processes, kinetic studies are important. Some representative data obtained by the flash photolysis method are shown in Table 5 [36]. For each pair of acetylene and olefins (Scheme 8), the slopes of the Hammett plots for the rate constants with different substituents in ArS do not vary much, suggesting that the reactivities of these acetylenes and olefins are mainly determined by the resonance stability of the transition state of the reaction, but not by the polar nature of the transition state. Thus, the resonance stability of the C atom-centered radicals in... 

A plot against Hammett s cr-constants of the logarithms of the rate constants for the solvolysis of a series of Mz-substituted dimethylphenylcarbinyl chlorides, in which compounds direct resonance interaction with the substituent is not possible, yielded a reasonably straight line and gave a value for the reaction constant (p) of — 4 54. Using this value of the reaction constant, and with the data for the rates of solvolysis, a new set of substituent parameters (cr+) was defined. The procedure described above for the definition of cr+, was adopted for... 

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