Reaction constant effective

The term p is a reaction constant and is mathematically evaluated for a particular reaction by plotting log kjkQ against a. The slope of the straight lines is p, and reflects the sensitivity of the reaction under study to effects of substituents. The value of p is obviously affected by temperature, solvent changes, etc. 

The applicability of the two-parameter equation and the constants devised by Brown to electrophilic aromatic substitutions was tested by plotting values of the partial rate factors for a reaction against the appropriate substituent constants. It was maintained that such comparisons yielded satisfactory linear correlations for the results of many electrophilic substitutions, the slopes of the correlations giving the values of the reaction constants. If the existence of linear free energy relationships in electrophilic aromatic substitutions were not in dispute, the above procedure would suffice, and the precision of the correlation would measure the usefulness of the p+cr+ equation. However, a point at issue was whether the effect of a substituent could be represented by a constant, or whether its nature depended on the specific reaction. To investigate the effect of a particular substituent in different reactions, the values for the various reactions of the logarithms of the partial rate factors for the substituent were plotted against the p+ values of the reactions. This procedure should show more readily whether the effect of a substituent depends on the reaction, in which case deviations from a hnear relationship would occur. It was concluded that any variation in substituent effects was random, and not a function of electron demand by the electrophile. ... 

The selectivity of an electrophile, measured by the extent to which it discriminated either between benzene and toluene, or between the meta- and ara-positions in toluene, was considered to be related to its reactivity. Thus, powerful electrophiles, of which the species operating in Friedel-Crafts alkylation reactions were considered to be examples, would be less able to distinguish between compounds and positions than a weakly electrophilic reagent. The ultimate electrophilic species would be entirely insensitive to the differences between compounds and positions, and would bring about reaction in the statistical ratio of the various sites for substitution available to it. The idea has gained wide acceptance that the electrophiles operative in reactions which have low selectivity factors Sf) or reaction constants (p+), are intrinsically more reactive than the effective electrophiles in reactions which have higher values of these parameters. However, there are several aspects of this supposed relationship which merit discussion. 

Some authors use O] instead of cr as the substituent constant in such correlations.) An example is provided by the aminolysis of phenyl esters in dioxane the substrates RCOOPh were reacted with -butylamine, and the observed first-order rate constants were related to amine concentration by = k2 [amine] kj [amine]. The rate constants kz and k could be correlated by means of Eq. (7-54), the reaction constants being p = +2.14, b = + 1.03 (for A 2) and p = -1-3.03,8 = -1-1.08 (for ks). Thus, the two reactions are about equally sensitive to steric effects, whereas the amine-catalyzed reaction is more susceptible to electronic effects than is the uncatalyzed reaction. 

In the manner outlined, a few attempts have been made to apply the Hammett equation to the transmission of substituent effects in the pyridine series. In the alkaline hydrolysis of 5-substituted ethyl picolinates (5-R-2-COOEt) in 85% ethanol at 25, 35, and 45°, the reaction constants are about 60% as large as those in the corresponding benzene series the overall fit to the Hammett equation, however, is at best fair, since out of four points (R = Et, H, I, Ac) one (Ac) deviates widely. 

Imoto and co-workers have also studied the pA values of substituted thiazolecarboxylic acids and the alkaline hydrolysis of their ethyl esters, each in three relative positions (2-B-4-Y, 2-B-5-Y, and 5-II-2-Y). In the case of the pA values, the p-values are far from constant, varying from 0.83 to 2,35, This variation is likely to be due to the intervention of tautomeric equilibria and of hydrogen bonds. The /3-ratios for the three sets of ester hydrolyses are roughly constant (0,61-0.73), and, assuming that the introduction of two heteroatoms leads to cumulative (multiplicative) effects on the transmission, this result is of the same order of magnitude as the product of the and values discussed above, i.e. 1.0 and 0.6, respectively. The lowest value for the pA (0,83) for the 2-R-5-COOH series is also of the same order of magnitude. All the available reaction constants are summarized in Table VI. 

Reaction constant for the transmission of electrical effects through atom X to Y. 

The least squares value for the p constant obtained by this procedure is +6.2 it wiU be obviously subject to change as more meta and epi substituents become available. Only the cata-NO group was excluded from the above plot because it causes a strongly enhanced resonance effect in nucleophilic substitution (Section IV,C, l,a) and an anomalous effect of uncertain origin in the dissociation of carboxylic acids. It can be assumed that the reaction constant for 4-chloro-... 

Figure 7. Effect of temperature on methanation reaction constants (10)...

Before we close this section we make reference to an extended form of the Hammett equation in which the substituent constant and the reaction constant are separated into contributions from the field effect (F) and the mesomeric effect (R). This procedure was suggested by Taft in 1957 for 4-substituted benzene derivatives. It is called a dual substituent parameter (DSP) equation (Scheme 7-2). 

These results demonstrate that, within experimental error, the corresponding reaction constants for the two reactions, solvolysis and rearrangement, are the same. In other words, the two reactions have the same dependence on substituent effects, which is consistent with Scheme 8-10 because the transition state for rearrangement is identical to the first transition state in the mechanism of solvolytic dediazoniation. 

The symbol k or K is the rate or equilibrium constant, respectively, for a side-chain reaction of a meta- or para-substituted benzene derivative, and k° or K° denotes the statistical quantity (intercept term) approximating to k or K for the parent or unsubstituted compound. The substituent constant a measures the polar (electronic) effect of replacing H by a given substituent (in the meta- or para-position) and is, in principle, independent of the nature of the reaction. The reaction constant p depends on the nature of... 

In a steady-state process, a gas is absorbed in a liquid with which it undergoes an irreversible reaction. The mass transfer process is governed by Fick s law, and the liquid is sufficiently deep for it to be regarded as effectively infinite in depth. On increasing the temperature, the concentration of reactant at the liquid surface CAi falls to 0.8 times its original value. The diffusivity is unchanged, but the reaction constant increases by a factor of 1.35. It is found that the mass transfer rate at the liquid surface falls to 0.83 times its original value. What is the order of the chemical reaction ... 

Effect on rate assuming no change in mechanism is caused steric factors upon substitution at C and rise to Cp have not been considered. The rate reductions are geared to substiment effects such as those giving rise to Hammett reaction constants on p- and a-aryl substitution. 

The dump temperature of the compound was varied by changing the mixer s rotor speed and fill factor while keeping the other mixing conditions and the mixing time constant. Under the assumption that the final dump temperature is the main parameter influencing the degree of the sUanization reaction, the effect of the presence of ZnO on the dynamic and mechanical properties of the compound was investigated. ZnO was either added on the two-roll mill or in the mixer. 

R.M.McCready et al.[l] demonstrate that the adding of O.IM NaCl to 0.33 M ammonia hydroxide enhances the deesterification more than twice. The adding of other chlorides, such as KCl, MgCb and CaCb also enhances the reaction. The effectiveness of the ions in relation to the deesterification and rate constant is arranged in the following order Ca>Mg>Na, K. The equal values of pH and ion strength prove that only the specific cation effect is responsible for the enhanced velocity. 

Modification of the FT-IR analysis techniques to analyze coatings under isothermal cure conditions provides the data needed to determine the rate constants for each reaction. An effective method to generate the rate constants from the experimental data has been found and will be described. 

The general reaction occurring in hydrodesulfurization has been described in Section 2.1.1. The most studied model compound is DBT. The reactivity towards hydrogenation of the phenyl substituents already mentioned (Section 2.1.1) is also observed in the hydroprocessing of sulfur compounds. The reactivity towards hydrogenolysis of the C-S bond masks the effects associated to aromatics hydrogenation. The DBT reaction network is sketched in Fig. 8 the pseudo-first-order reaction constants measured by Houalla [68] have been included. 

Perhaps the most informative studies (Anvia and Bowden, 1990) of leaving groups are of the alkaline hydrolysis of 3-substituted phenyl 2-acetyl- and 2-benzoyl-benzoates [20], The Hammett reaction constants p are ca. 0.50 and 1.48 for the 2-acetyl and 2-benzoyl esters respectively, as. shown in Table 2. These were compared with various limiting models in a similar manner to the effective charge model (Williams, 1984,1992). This comparison indicates that all simple phenyl esters of the latter types hydrolyse by a mechanism... 

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