Hammett reaction parameter

Kinetics of reaction of the di- and trisubstituted acids 7a, 7c and 7d with anthocyanins were moderately enhanced compared to 7e (Figure 5). With the amino moiety in 7b, however, rate of formation was accelerated by a factor larger than 100 (data not shown). These observed reactivities coincide with the expected order of reaction rates for para-substituent effects when electron-deficient transition states are involved, as given by Brown s af constants 27,28). It is known that electron-donating substituents can accelerate reactions involving the olefinic double bond of cinnamates. Negative Hammett reaction parameters p have been obtained,... 

These results are used to calculate the following Hammett [Pg.394]

Many equilibrium and rate processes can be systematized when the influence of each substituent on the reactivity of substrates is assigned a characteristic constant cr and the reaction parameter p is known or can be calculated. The Hammett equation... 

The rate of the Schmidt reaction of para-substituted benzoic acids is governed by the electron-releasing character of the substituent, the value of the Hammett p parameter being the same as that needed to correlate product ratios in the related reaction of 1,1-diarylethylenes.807 The latter reaction is believed to go by a similar mechanism. 

In conclusion, for the SN2 reactions of primary alkyl derivatives, the author is of the view that the Bronsted parameter a does not constitute measure of transition state charge development, and is unlikely to represent a meaningful measure of transition state geometry. As we shall see in the following section, however, for reactions described by three or more configurations, the Bronsted parameter or Hammett p parameter may provide a relative measure of transition state charge development (Pross, 1984). 

The cyclic mechanism is probably seldom a fully concerted (E2) process, and the different timing of individual electron shifts results in a transition towards the El or ElcB mechanisms (cf. Sect. 2.1.1). The choice of the mechanism depends on the reactant structure as well as on the catalyst nature. As an indicator of the mechanism, either the degree of stereoselectivity (see refs. 68, 121, 132 and 141) or the value of the reaction parameter of a linear free energy relationship, e.g. p or p constants of the Hammett and Taft equations (cf. ref. 55), may be used. 

HA compounds is not necessary for the formation of a polyester. Nevertheless, an acceleration effect of HA compounds on the rate of copolymerization was detected later 36 57 74), even for systems in which proton donors are directly bound to monomers 67). This effect is not the sum of the contributions from the tertiary amine and the proton donor but even stronger. Hence, proton donors display a cocatalytic effect. Concerning the effect of HA compounds Tanaka and Kakiuchi 36) established a linear correlation between Hammett s ct constants and the logarithm of the gelation time for various substituted derivatives of benzoic acid, benzyl alcohol and phenol, and positive reaction parameters q were found in all cases. This means that electron-withdrawing substituents increase the effect of HA compounds, or their effect becomes more pronounced with increasing hydrogen atom acidity. 

The cathodic process (reduction) has been studied in different experimental conditions being the potential of the one-electron reaction correlated with other structural parameters, i.e., with the reduced-product lowest vacant orbital energy [86], with the substituent Hammett a parameter [87], or with the biological activity (see below). Table 4 shows reduction potentials for some QDO and PDO derivatives in different experimental conditions [88,89]. 

The absolute rate constants for ene-addition of acetone to the substituted 1,1-diphenyl-silenes 19a-e at 23 °C (affording the silyl enol ethers 53 equation 46) correlate with Hammett substituent parameters, leading to p-values of +1.5 and +1.1 in hexane and acetonitrile solution, respectively41. Table 8 lists the absolute rate constants reported for the reactions in isooctane solution, along with k /k -, values calculated as the ratio of the rate constants for reaction of acetone and acctonc-rff,. In acetonitrile the kinetic isotope effects range in magnitude from k /k y = 3.1 (i.e. 1.21 per deuterium) for the least reactive member of the series (19b) to A hA D = 1.3 (i.e. 1.04 per deuterium) for the most reactive (19e)41. Arrhenius plots for the reactions of 19a and 19e with acetone in the two solvents are shown in Figure 9, and were analysed in terms of the mechanism of equation 46. 

Model studies discussed in previous chapters show that the reactivity of cations and alkenes are very strongly affected by inductive and resonance effects in the substituents. Correlation of the rate constants of addition of benzhydryl cation to various styrenes with Hammett substituted benzhydryl cations to a standard alkene (2-methyl-2-pentene) gave also good correlation and p+ = 5.1 [28]. The large p value signals difficult copolymerizations between alkenes, even of similar structures. Thus, in contrast to radical copolymerization which easily provides random copolymers, cationic systems have a tendency to form either mixtures of two homopolymers or block copolymer (if the cross-over reaction is possible). 

Scheme 33. Kinetic studies of the cation radical Diels-Alder reactions of aryl vinyl sulfides with cyclopentadiene and of aryl propenyl ethers with 2,3-dimethyl 1,3-butadiene. In both solvents, both extended reaction series have rates which correlate excellently with the Hammett a parameters and poorly with the Hammett-Brown o-+ values. The preference is statistically significant at or above the 95 % confidence level.

In a similar attempt to determine the nature of the rate-determining step in the reaction of aryl halides with magnesium, the kinetics of the reaction of substituted aryl bromides with magnesium and with tri-/i-butyltin hydride in ethereal solvents were examined [81c], using competitive kinetics techniques, and were correlated with the substituent effect using Hammett cr parameters. A Hammett plot log iK/K)=f(( x) then provided the value of the Hammett constant p for the reactions of aryl halides with magnesium and with tri-u-butyltin hydride in various solvents (Fig. 4 and Table 11). 

An example of application of the Hammett relation is shown in fig. 7.25. Data for the rate constant for bromination of substituted benzeneboronic acids in 20% aqueous acetic acid show an excellent linear correlation with the Hammett ax parameter for both para and meta substituents. The reaction parameter pH for this system is -4.52. The Hammett equation has been shown to successfully describe kinetic and equilibrium data for over 300 aromatic reactions [49]. 

Reaction parameter in the Hammett equation Reaction parameter in the Taft equation Substituent parameter in the Hammett equation Substituent parameter in the Hammett equation for reactions in which a positive charge is generated on carbon atom adjacent to aromatic nucleus... 

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