Substituent constants, for

Table 3-1 gives substituent constants for several groups. 

Table 3-1. Substituent constants for various groups to be used in Eqs. (1) and (2) (

This work already showed that substituent constants of one reaction can only be transferred to another reaction when similar effects are operating and when they are operating to the same extent. In order to find a broader basis for the transfer-ability of substituent constants, they were split into substituent constants for the resonance effect and those for the inductive effect. 

Herein is the rate constant for a dienophile with substituent x ko is the corresponding rate constant for unsubstituted 2,4c Ox is the substituent constant for substituent x and p is the reaction constant, defined as the slope of the plot of log (k / ko) versus Ox. The parameter p is a measure of the sensitivity of the reactions towards introduction of substituents. Figure 2.3 and Table 2.4 show the results of correlating the kinetic data for the reaction of 2.4a-e with 2.5 with a. ... 

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

These constants, K toK/, may be estimated by use of the Hammett equation. Estimation of 1 and K 4 involves application of the methods outlined in Section II, A, i.e., application of substituent constants for and N+H to the Hammett equation for the acid-base equilibria of benzoic acids. Estimation of A2 and involves application of the method used in Section III,A, i.e., the p-value for the basicity of substituted pyridines, with cr-values for COOH and COO . Provided the necessary a- and p-values are known, this procedure permits the calculation of four independent, or virtually independent, estimates of Krp. A check on the method is available from the relationships shown in Eq. (16) which is readily obtained by multiplication of Eq. (12) and (14) and of Eq. (13) and (15). 

Sets of homo- and hetero-nuclear substituent constants for the alkoxy-dechlorination of 4-chloroquinolines have been recently obtained from preliminary values of the p-constants and can be extended and improved on the basis of the more extensive data now available. A detailed discussion on substituent constants would extend beyond the scope of this review. In general, it can be stated that unless enhanced resonance interaction occurs with the reaction center in the transition state (i.e., para-, cata-, amphi-N02 groups) or with the aza... 

The special substituent constants for + R para-substituents are denoted by a, and those for — R para-substituents are denoted by a+ 54. They are based respectively on the reaction series discussed above. Selected values are given in Table 1. Characteristic a or a+ values are sometimes distinguished for meta-substituents also, but only for a minority of substituents which show very marked + R or — R effects do these differ significantly from ordinary a values. The range of applicability of the Hammett equation is greatly extended by means of a and cr+, notably to nucleophilic (by a ) and to electrophilic (by cr+) aromatic substitution. 

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... 

Calculated from um and op values estimated as in Assumed equal to substituent constants for CO,Et. 

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

Figure 1. Relation among Log k of Dehydrochlorination Reaction, Median Lethal Dosage to Mosquito Larvae, and Substituent Constant for Trichloroethyl Compounds...

Hansch, C., Leo, A. (1979) Substituent Constants for Correlation Analysis in Chemistry and Biology. Wiley-Interscience, New York. Hansch. C., Leo, A. J., Hoekman, D. (1995) Exploring QSAR, Hydrophobic, Electronic, and Steric Constants. ACS Professional Reference Book, American Chemical Society, Washington, D.C. 

Iwasa, I., Fujita, T., Hansch, C. (1965) Substituent constants for aliphatic functions obtained from partition coefficients. J. Med. Chem. 8, 150-153. 

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