Hammett-Brown relationship

That is, the more reactive an attacking species, the less preference it has for the para position compared to the meta. If we combine the Hammett-Brown a p relationship with the linearity between log Sf and log p and between log Sf and log m] , it is possible to derive the following expressions ... 

Physical phenomena other than rates and equilibrium constants can be correlated by Hammett-type relationships. For example, as Figure 2.4 shows, in 13C nuclear magnetic resonance spectroscopy (called Cmr) the chemical shift of the cationic carbon in 17 is correlated by Brown s cr + values.21 And the C=0... 

Brown developed the selectivity relationship before the introduction of aromatic reactivities following the Hammett model. The former, less direct approach to linear free-energy relationships was necessary because of lack of data at the time. 

A modification of the Hammett approach, suggested by Brown, called the selectivity relationship is based on the principle that reactivity of a species varies inversely with selectivity. Table 11.3 " shows how electrophiles can be arranged in order of selectivity as measured by two indexes (1) their selectivity in attacking toluene rather than benzene, and (2) their selectivity between the meta and para positions in toluene. As the table shows, an electrophile more selective in one respect is also more selective in the other. In many cases, electrophiles known to be more... 

Statistical and computational methods have been used to quantify structure-activi relationships leading to quantitative structure-activity relationships (QSAR). The concqpt of QSAR can be dated back to the work of Crum, Brown and Fraser from 1868 to 1869, and Richardson, also in 1869. Many notable papers were published in the period leading up to the twentieth century by men such as Berthelot and Jungfleisch in 1872, Nemst in 1891, Ov ton in 1897 and Meyer in 1899 (7). Professor Corwin Hansch is now regarded by many as the father of QSAR, because of his work in the development of new and innovative techniques for QSAR. He and his co-woikers produced a paper that was to be known as the birtii of QSAR, and was oititled "Correlation of biological activity of phenoxyacetic acids with Hammett substituent constants and partition coefficients" (2). 

The empirical relationship (11) which correlated the available data for toluene (Brown and Nelson, 1953) was shown (McGary et al., 1955) to be obtained by algebraic manipulation of the Hammett eq. (1). 

Figs. 24-27. The relationship between the reaction constant and log (fc/fcH) for the Hammett side-chain reactions of (24) p-methoxy substituted benzenes (25) p-methyl substituted benzenes (26) yj-fluoro substituted benzenes and (27) p-ehloro substituted benzenes. (Reproduced by permission from Brown and Stock, J. Am. Chem. Soc. 84, 3298 (1962).)... 

For the quantitative treatment of substituent effects in such reactions, Brown proposed (Brown and Okamoto, 1957) a new Hammett-type structure-reactivity relationship, the Brown equation (1), in terms of substituent constant instead of a in the original Hammett equation. 

The history of quantitative structure-activity relationships dates back to the last century, when Crum-Brown and Fraser in 1865 postulated that there ought to be a relationship between physiological activities <1> and chemical structures C. Later, Richet correlated toxicities with aqueous solubility. Around 1900, Meyer and Overton found linear relationships between the narcotic potencies of organic compounds and their partitioning behavior. In the mid-1930s, Hammett defined a reaction constant p to describe the reactivity of aromatic systems R, expressed by rate constants k (or equilibrium constants K) and a parameter o to describe the electronic properties of aromatic substituents X (1 equation 1) (see Linear Free Energy Relationships (LFER)) ... 

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