A" substituent constant

For the nine substituents m- andp-methyl, p-fluoro, m- and p-chloro, m- and p-bromo, and m- and p-iodo, using the results for nitration carried out at 25 °C in nitromethane or acetic anhydride - (see tables 9.1, 9.5), a plot of logjoA/ j against cr+ produced a substituent constant p = —6-53 with a standard deviation from the regression line i = 0-335, 2 correlation coefficient c = 0-975. Inclusion of... 

In general, the dissection of substituertt effects need not be limited to resonance and polar components, vdiich are of special prominence in reactions of aromatic compounds.. ny type of substituent interaction with a reaction center could be characterized by a substituent constant characteristic of the particular type of interaction and a reaction parameter indicating the sensitivity of the reaction series to that particular type of interactioa For example, it has been suggested that electronegativity and polarizability can be treated as substituent effects separate from polar and resonance effects. This gives rise to the equation... 

Hansch, C, Leo, A Substituent Constants for Correlation Analysis in Chemistry and Biology, John Wiley New York, 1979... 

In this equation, klko) is the rate constant for basic hydrolysis of XCH2COOR divided by the rate constant for basic hydrolysis of CH3COOR, ( o)a is the similar rate-constant ratio for acid catalysis, and 0.181 is an arbitrary constant. The parameter CT/ is a substituent constant for a group X, substimted at a saturated... 

Hansch C, Leo A. Substituent constants for correlation analysis in chemistry and biology. New York Wiley, 1979. 

It is of interest to note that almost fifty years ago physical organic chemists began to derive the corresponding relationships between substituents and chemical reactivity. These are the well known free energy relationships such as the Hammett Equation (6) in which a is a substituent constant, p a reaction constant, kR the rate of the reaction with substituent R present and k0 the rate of reaction with a standard substituent (usually hydrogen) (33). 

Lower-case letters are used for the coefficients in equations that represent a substituent constant as a function of other substituent constants. The difference between pure and composite parameters is that the former represent a single effect while the latter represent a mixture of two or more. The percent composition of these parameters is given by ... 

The structure-reactivity relationship is a concept familiar to every organic chemist. As commonly used it refers to a linear free energy relationship, such as the Bronsted or Hammett equations, or some more general measure of the effect of changing substituent on the rate or equilibrium of a reaction. A substituent constant is conveniently defined as the effect of the substituent on the free-energy change for a control reaction. So the so-called structure-reactivity relationship is in fact usually a reactivity-reactivity relationship. 

A number of substrates having a benzylic ether moiety were reacted with 51 to afford the corresponding benzylic esters in good yields (equation 84). For evaluating the effects of p-substiments on the oxidation of a series of benzylic ethers, a competitive oxidation of p-substimted benzylic propyl ethers with 51 was carried out. The Hammett correlation plot for the oxidation reaction gave a better correlation of the relative ratio factors with the a rather than with the a+ substituent constants and afforded a reaction constant p+ = —0.57 (r = 0.99). This p+ value shows that 51 is an electrophilic species and appears to be comparable to the p+ value of —0.65 for benzylic hydrogen abstraction from dibenzyl ethers by the benzoyloxy radicaP . 

The result is a substituent constant (er) which is a numerical description of die electronic effect of a substituent relative to a hydrogen atom on the model reaction. Stated a different way, a substituent constant a is a quantitative way to... 

The problem of testing the application of a linear free-energy treatment for substitution reactions was solved by reversing the usual Hammett procedure. In the conventional approach, log (kjkn) is plotted against a. It is evident that an alternative would be to examine the application of a linear correlation by maintaining the group constant and to plot log (k/ka) versus p for different reactions. A satisfactory linear relationship would indicate constancy of the electronic contribution of the substituent. The slope of the line would define a substituent constant for the substituent in question. 

The first term of the expression (48) is the contribution of the Hammett parameters. The second term is formed of a substituent constant (48) measuring the resonance contribution and a modified reaction constant, pr. The success of this treatment is directly attributable to the introduction of a second reaction constant. This additional parameter permits the effect of the substituent, Areaction constant, p), to depend on the susceptibility of the side-chain to resonance interactions. 

Hansch constant — A measure of the capability of a solute for hydrophobic (lipophilic) interaction (see - hydrophobic effect) based on the partition coefficient, P, for distribution of the solute between octan-l-ol and water. The most general way of applying P in correlation analysis, etc. is as logP, but the behavior of substituted benzene derivatives maybe quantified by a substituent constant scale, n, which is defined in a way analogous to the Hammett a scale. There are various n scales, depending on the substrate series used as reference. See also - hy-drophilicity. 

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