Linear Hammett correlation

The classic reaction catalyzed by peroxidases is the one-electron oxidation of phenols. The guaiacol assay for peroxidase activity, an assay that derives from the earliest observation of peroxidase activity [70], involves the one-electron oxidation of guaiacol to a free radical that undergoes subsequent radical-radical combination to give a colored dimeric product (Fig. 5.13) [71]. The ability of substituted phenols to act as substrates and reduce HRP Compound I is controlled by the electronic nature of the substituent [72], At pH 7.0, the linear Hammett correlation observed between the substituent a value and the rate of Compound I reduction adheres to the equation log kx/ku = 6.92a, where kx is the rate of the... 

Examination of the measured second order rate constants for the dye oxidations shows that the reaction is significantly favoured by electron-donating substituents the 4-methoxylated compound (9) reacts 274 times faster than the 4-nitro analogue (3). Furthermore, the excellent linear Hammett correlation of log(second order rate constant) with and the p value of-1.66 confirm this conclusion and show that all the substrates are oxidised by the same mechanism. However, the size of the p value is larger than that obtained from the earlier oxidation of phenols (p -1.10) and is larger than expected for a mechanism involving H atom abstraction in the rate determining step. A possible explanation is that the substituents have two effects on the reaction which reinforce each other they influence H atom abstraction by the electrophilic oxidant, as noted previously this is favoured by electron-donation, and they shift the position of the azo-hydrazone equilibrium. 

The oxidative addition of the palladium(O) complexes generated from Pd(OAc)2 and P(p-Z-C6H4)3 follows a linear Hammett correlation with a negative slope (p = -2.8) establishing, as expected, that the more basic the phosphine, the more reactive the palla-dium(O) complex. ... 

Linear Hammett correlation observed in the formation of benzal-dehyde semicarbazone at pH 1.75. (Reproduced from reference 204. Note that the vertical axis is a log scale.)... 

Some investigators try to observe a Hammett correlation without measuring the ko value for the unsubstituted compound (i.e., the substituent is hydrogen). Show that a plot of log k versus cr should give a linear Hammett correlation with the same slope one would obtain by plotting log (fc/fco) versus a. 

Show that these data give a linear Hammett correlation and calculate the value of p. Do you think this value of p supports the authors conclusion that the transition structure for step 1 is like the arrangement indicated in Figure 6.89(b) ... 

Rate data for the ozonolysis of a series of substituted styrenes in CCI4 at 25°C are summarized in Table 6.14. Do the data show a linear Hammett correlation with either tr, cr, or meta substituents in all three correlations.) If so, what is the value of p at 25°C What do these results suggest about the reaction mechanism Specifically, does the addition of ozone appear to be electrophilic or nucleophilic in nature ... 

Functionalization of polythiophenes with aryl groups has attracted much interest because aryl terminals can be further functionalized. A series of 3-phenyl- and 4-phenylthiophene substituted on the phenyl ring with donor or acceptor groups has been electropolymerized [208] in order to tune the potentials for p and n doping. Monomer oxidation potential and polymer redox potentials were found to follow a linear Hammett correlation. 

Linear Hammett correlations, however, will not be obtained if the rates of the two steps are comparable so that no single step is rate determining, or if there is a shift in rate-determining step since. 1 different p values will be obtained before and after the shift. This was found to be the case in semicarbazone formation. When the reaction was studied at pH 3-9 a sharp break occurred at a = 0 (Figure 4) By using a steady-state approximation and solving for... 

Although some non-linear Hammett correlations may be adequately explained in terms of a shift in rate-determining step with a change in a, not all such deviations can be so justified. There is, however, another reason for the curvature in plots observed in condensation reactions involving aromatic carbonyl compounds. Such carbonyl compoimds are stabilized by para substituents capable of electron donation by resonance due to stabilization of canonical form 32. The a constants were derived to indicate the effect of... 

Pd°(dba)2 + nP(p-Z-Csll4) mixtures behave as for PPh3 (Scheme The values of the equilibrium constants (Eq. 4 of Scheme 3) have been determined as a function of the Z substituent. For a given value of n(n> 2), the equilibrium between Pd°(dba)L2 and Pd°L3 lies more in favor of Pd°(dba)L2 when the phosphine is more basic (Hammett correlation for with p = + 4). Similarly, when n = 2, Pd°L2 is less easily released from Pd°(dba)L2 (Eq. 2 of Scheme 3) when the phosphine is more basic. The oxidative addition involves Pd°L2 as the reactive species (Schemae 4), which is more reactive when the phosphine is more basic. Thus, two opposite effects are at hand when the basicity of the phosphine is varied, so that the resulting overall rate constants do not follow linear Hammett correlations. Instead, bell-shaped curves are obtained whose maximum value and position depend on n, that is, on the phosphine concentration (Figure... 

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