Hammett reaction constant values

A common ground that is explicitly or implicitly defended in the majority of studies on Mizoroki-Heck reactions is that the limiting stage for the whole cycle is the oxidative addition step. By this criterion, the most important substrates, aryl halides, are subdivided into very reactive (aryl iodides and electron-deficient aryl bromides), less reactive (all other aryl bromides and electron-deficient aryl chlorides) and very unreactive (all other aryl chlorides). As evident as this classification may seem, it is not based on any solid proof. Indeed, if it were really so important, the oxidative addition step should have been characterized by very strong dependence on substituent effects in these substrates. However, this has not been observed in either Mizoroki-Heck reactions or in any other palladium-catalysed reaction of aryl hahdes. The Hammett reaction constant values p, whenever measured, are rather modest in valne [5]. Such values could hardly have accounted for the well-known enormous distance between the reactivity of, for example, a typical activated substrate 7 and a typical deactivated substrate 8 (Figure 2.1). 

In the common parlance of physical organic chemists such phrases as product-like or reactant-like transition states are common. The degree of resemblance of transition states to either reactants or products is usually assessed for reaction series obeying the linear-free-energy principle on the basis of suitable reaction constants, such as Bronsted a- and (3-values, and Hammett reaction constants p. The question is inherently more complex for cyclisation reactions, since they are not expected to follow the linear-free-energy principle. 

The data do not support a reaction as simple as this, however, the rate law implicates a second PyO at this stage. Similar experiments were extended with the use of a series of ring-substituted pyridine N-oxides, RC5H4NO, as the substrates. Correlation of the values of kcat against ctr gave a particularly large and negative Hammett reaction constant, = —3.84. This is so because PyO enters in three steps of the scheme, each of which is improved by electron donation. 

Solvolysis and isomerization may either proceed through a common ion-pair reaction intermediate, or the isomerization reaction may proceed by a separate concerted reaction pathway that avoids formation of this intermediate kcon Scheme 10). Hammett reaction constants of = —4.9 and pjt = —5.5 for reactions of X-4-0(S)CPb were calculated from the data in Table 1. The larger negative value... 

The Hammett reaction constants p = 2.7138 and 3.0157 were determined for addition of water to meta-substituted l-(4-methoxyphenyl)ethyl carbocations (80-X, Scheme 44A) and 1-4-methoxybenzyl carbocations (p-Me-1 +-X), respectively. The value of p = 2.7 for addition of water to 80-X is 36% that of the value of = 7.6, the slope of Hammett-type plots equilibrium constants for addition of water to 80-X to form the alcohol.138 This shows that the addition of water proceeds through a transition state where partial bond formation to the nucleophile results in a 36% change in the interaction between the m-substituent and the cationic benzylic carbon, compared with the complete loss of this interaction at the water adduct. Scheme 44A therefore proceed through a transition state in which there is a 36% change in the interaction between the m-substituent and positive charge at the benzylic carbon due to partial bond formation to the water nucleophile. 

It should be noticed that from the activity data measured at a certain fixed pH, the correlations of equivalent quality are obtained both for the neutral and ionized forms. There are interrelations a priori between Equations 32 and 33 such as p — p = pA and c — c = pH — pKAstd, where pA is the Hammett reaction constant for the ionization equilibrium and pKA8td is the value of a standard compound. Thus, it is only possible to predict the molecular form responsible for the activity by comparing Equations 32 and 33 derived from data obtained at various pH. Moreover, the optimum value of log KA, log KA°, for the apparent activity of ionizable congeners can be derived by setting the derivative of either Equation 32 or 33 equal to zero as shown in Equations 44 and 45. 

That C -X heterolysis is less developed in the transition state for thermolysis of esters than alkyl halides is supported by recent studies on substituted 1-phenylethyl chlorides . At 608°K a Hammett reaction constant of—1.36 was observed, this being much smaller than that of—4.95 at 318°K observed for the solvolysis of the same substrates in 80% aqueous acetone. Although part of the difference between these latter two values is attributable to the difference in reaction temperature, the results suggest that C -X bond breaking is less developed in pyrolyses than in solvolytic reactions. 

Here again the sensitivity of the betaine dye to changes of substituents - as given by its different chemical reactivity - is solvent-dependent. This sensitivity is lower in the more polar solvents, and vice versa. For example, the Hammett reaction constant p for this alkylation reaction, measured in polar acetonitrile as solvent, is with -0.15 considerably smaller thati the corresponding p-value measured in the less polar solvent chloroform, which amounts to -0.40(26). 

The Hammett reaction constant (p values) for DA reactions are small showing relatively low sensitivity of the reaction to substituents. Since reactions involving charge centers, e.g., a zwitterion, generally show high sensitivity (large value), it may be assumed that no truly zwitterionic species exists as an intermediate. (However, low partial charges cannot be ruled out.)... 

Fig. 4. Dependence of Hammett reaction constant on pH for values of first reduction wave of series of nitrobenzene derivatives in solutions with various contents of ethanol [31] 1) 0 2) 10 vol. % 3) 20 vol.% 4) 40 vol. %...

Pseudo-first-order rate constants (k i ) for intramolecular carboxylic-acid-catalyzed conversion of N-substituted phthalamic and related acids to phthalic anhydride (PAn) gave negative p (Hammett reaction constant) and p (Taft reaction constant). But the value of fw the formation of PAn from N-melhoxyphthalamic acid is about 10-fold smaller than the corresponding for the formation of PAn from A-hydroxyphthalamic acid under similar conditions. The nearly 10-fold larger value of k s for A-hydroxyphthalamic acid than that for A-methoxyphthalamic acid is attributed to the occurrence of intramolecular dipole-dipole interaction through transition state TSg in the catalyzed cleavage of A-hydroxyphthalamic acid. 

It may be noted that the values of Hammett reaction constants (p) for SA-catalyzed hydrolysis of A-arylphthalamic acids and A-arylbenzamides are -1.2 and + 0.56, respectively, which indicate the occurrence of different reaction mechanisms in these reactions. 

A plot against Hammett s cr-constants of the logarithms of the rate constants for the solvolysis of a series of Mz-substituted dimethylphenylcarbinyl chlorides, in which compounds direct resonance interaction with the substituent is not possible, yielded a reasonably straight line and gave a value for the reaction constant (p) of — 4 54. Using this value of the reaction constant, and with the data for the rates of solvolysis, a new set of substituent parameters (cr+) was defined. The procedure described above for the definition of cr+, was adopted for... 

The numerical values of the terms a and p are defined by specifying the ionization of benzoic acids as the standard reaction to which the reaction constant p = 1 is assigned. The substituent constant, a, can then be determined for a series of substituent groups by measurement of the acid dissociation constant of the substituted benzoic acids. The a values so defined are used in the correlation of other reaction series, and the p values of the reactions are thus determined. The relationship between Eqs. (4.12) and (4.14) is evident when the Hammett equation is expressed in terms of fiee energy. For the standard reaction, o%K/Kq = ap. Thus,... 

Given in Table 4.5 in addition to the Hammett equation are ct and substituent constant sets which reflect a recognition that the extent of resonance participation can vary for different reactions. The values are used for reactions in which there is direct resonance interaction between an electron-donor substituent and a cationic reaction center, hereas the a set pertains to reactions in which there is a direct resonance interaction between the substitutent and an electron-rich reaction site. These are cases in which the resonance conqionent of the... 

Table 7-2 lists 15 reactions whose rates are correlated by the Hammett equation. Besides the reaction constant p, the table gives a value for k° (from the fitted line), which provides all the information needed to estimate k for any member of the series, if the corresponding o is available, by means of Eq. (7-24). Note that kP in Table 7-2 is generally not identical to the experimental value of k for the ct = 0 member of the series, because this experimental point may deviate from the regression line. 

Equation (7-51), the Taft equation, provides good correlations for many aliphatic reactions. The scope of this relationship is illustrated by Table 7-10. The reaction constant p has been interpreted along the lines described for the Hammett p values. 

A completely different approach has been taken by Hine, who has considered that the substituent and reaction center are not really distinct, both being substituents in a benzene nucleus, and has then related substituent and reaction constants. Although of considerable theoretical interest, Hine s work has little bearing on practical applications of the Hammett equation since he starts from the premise of unique, single-valued substituent constants. This premise is invalid whether we are utilizing the naive approach with three separate, well-defined sets or the more refined methods with a continuous range of para values. 

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