And the Hammett equation

Shen (118) suggested a formal analogy between the Hammett equation and the Price-Alfrey equation. Charton and Capato (119) have derived relationships between e and q values and the Hammett equation as follows. 

Ritchie and Sager (124) distinguish three types of reaction series according to whether the Hammett equation or the isokinetic relationship is obeyed, or both. The result that the former can be commonly valid without the latter seems to be based on previous incorrect statistical methods and contradicts the theoretical conclusions. Probably both equations are much more frequently valid together than was anticipated. The last case, when the isokinetic relationship holds and the Hammett equation does not, may be quite common, of course, and has a clear meaning. Such a series meets the condition for an extrathermo-dynamic treatment when enough experimental material accumulates, it is only necessary to define a new kind of substituent constant. 

H. H. Jaffe (1919-1989)297 was professor of chemistry at the University of Cincinnati. His research interests were quantum chemistry, the basicity of weak bases, spectroscopy, excited state chemistry, and the Hammett equation, of which he wrote a comprehensive review article in 1953.298 At one time, according to Science Citation Index, this was the most cited article in the chemical literature. 

Derived from physical organic chemistry and the - Hammett equation, this can be considered to be the first approach to modern QSAR studies. Proposed by Hansch and co-workers in the early 1960s [Hansch et al, 1962 Hansch et al, 1963 Hansch and Fujita, 1964 Hansch et al, 1965 Hansch and Anderson, 1967 Hansch, 1969 Hansch, 1971 Hansch, 1978], it is the investigation of the quantitative relationships between... 

Note Knowledge of the electronic effects of substiments in the aromatic ring, expressed by o-values, and the Hammett equation helps in planning the syntheses of polysubstimted aromatic compounds [18-20]. Orientation of electrophilic aromatic substitution is determined by the electronic properties of the present substiments. Electron-acceptor substiments (with positive o-values) deactivate ortho/para positions, resulting in the orientation of electrophilic substimtion to the meta-position. This effect is quantitatively expressed by Hammet o-values and for most usual substiments is presented in Table 5.1 [21]. 

Side-chain Reactivities.—Several papers quantitatively treating the influence of the thiophen ring on side-chain reactivity have appeared. The reaction of 5-substituted 2-thenoyl chlorides with aniline in benzene follows second-order kinetics. The values of the Hammett p-constants are greater than that for the benzoylation reaction, indicating that the thiophen nucleus is more efficient than the benzene ring in relaying the electronic effects of the substituent to the reaction centre. The rate of the reaction of 3-thenoyl chloride and of thiophen-2-sulphonyl chloride with substituted anilines has also been studied, and the Hammett equation applied to the rate... 

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