Hammett a substituent constant

The Hammett a substituent constants would be of little value if they only applied to the dissociation of substituted benzoic acids. Fortunately, this is not the case. For a very wide variety of reactions or equilibria involving a substituted phenyl group, if a plot is made of log k or log K against the Hammett <7 substituent constant, a straight line is obtained, showing that a relationship of type (3.8) holds ... 

The rate of thermal rearrangement of 3-aryl-2,2-dimethylmethylene-cyclopropanes has been studied as a function of aryl substiments. Some of the data are given below. Examine the rate data for correlation with the Hammett a-substituent constants. What conclusion do you draw about the mechanism ... 

Hammett correlations also permit some insight into the reactivity and selectivity of electrophiles in aromatic substitution reactions. In general, the standard Hammett a substituent constants give poor correlations with reactions involving electrophilic aromatic substitution. The values, which reflect an increased importance of direct... 

In Scheme 7-1 kx and kn refer to the rate constants for a benzene derivative, in our case the benzenediazonium ion, bearing a substituent X in the 3- or 4-position, and the corresponding unsubstituted benzene derivative respectively. The term p is Hammett s reaction constant for the reaction, and o is Hammett s substituent constant which is, at least in principle, independent of the nature of the reaction but different for the 3- and 4-positions. A plot of log kx (or log kx - log kH) versus o should give a straight line. Its slope (positive or negative) corresponds to the reaction constant p. Equilibrium relationships are treated analogously. 

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). 

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. 

As a rule, H-acid catalysis lowers stereospeciflcity of the Diels-Alder reactions. Besides, kinetic parameters of these reactions correlates to the Hammett substituent constants. In the case of the cation-radical mechanism, correlation of kinetic parameters to the Hammett-Brown substituent constants gives the best accordance. 

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 corrosion inhibiting properties of an organic molecule may be modified by structural changes that alter the availability of electrons. A good correlation has been observed between the relative corrosion inhibiting ability of certain methyl-substituted pyridines and Hammett-like substituent constants (65MI11501). 

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. 

The rate constants for the reactions of eaq with aromatic compounds depend very strongly on the substituent. Benzene itself has a relatively low reactivity, k = T4 x 107 M -1 s 1, phenol and aniline are even less reactive, but substituents such as CN, Br, and N02 increase the rate constant almost to the diffusion controlled limit. The rate constants for several series of mono- and disubstituted benzenes were correlated with Hammett s substituent constants a and showed good linear relationships. 

The effect of substituents on the rate constant for reaction of H with benzene was also examined. Correlation with Hammett s substituent constants was possible and showed that the hydrogen atom behaves as a slightly electrophilic reactant (Neta, 1972a, and references therein see also Brett and Gold, 1971, 1973 and photochemical studies by Pryor et al., 1973). 

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. 

A linear correlation (equation 19) has been found between the logarithm of the yield Q (%) of the stabilization product and the value of the Hammett para-substituent constants dp by taking... 

Dialkyl, alkyl aryl, and diaryl trithiocarbonates are reduced in DMF to the radical anion, which loses a thiolate ion [146-148]. The rate of the cleavage was measured with CV for substituted diphenyltrithiocarbonates, the rate constant of the cleavage was found to vary linearly with Hammett s substituent constant a. The anion radicals of mono- and... 

The S-N bond distances exhibited are 2.10-2.50 A, which are within the sum of the van der Waals radii of nitrogen and sulfur (3.35 A). A linear relationship is observed between the and 13C NMR chemical shifts of the N-methyl group and the Hammett <7m-substituent constants. 

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