Hammett p values, for reactions

TABLE 7. Bimolecular rate constants, deuterium isotope effects and Hammett p-values for reactions of 1,1-diarylsilenes (56 R = H)87 with MeOH, t-BuOH and AcOH in acetonitrile at 23 °C... 

Table 2.4. Solvent effect on the Hammett p-values for the Diels-Alder reaction of2.4 with 2.5 catalysed by Cu(N03)2 at 25 - C.

Table 2.7. Hammett p-values for complexation of 2.4a-e to different Lewis-adds and for rate constants (kcat) of the Diels-Alder reaction of 2.4a-e with 2.5 catalysed by different Lewis-acids in water at 2.00 M ionic strength at 25°C.

For E2 eliminations in 2-phenylethyl systems with several different leaving groups, both the primary isotope effect and Hammett p values for the reactions are known. Deduce from these data the relationship between the location on the E2 transition state spectrum and the nature of the leaving group i.e., deduce which system has the most El-like transition state and which has the most Elcb-like. Explain your reasoning. 

Table 13 The secondary a-deuterium and primary nitrogen KIEs and Hammett p values for the SN2 reaction between sodium thiophenoxide and benzyldimethyl-phenylammonium nitrate at different ionic strengths in DMF at 0°C. ...

TABLE 15. The secondary alpha deuterium and primary leaving group nitrogen kinetic isotope effects and the Hammett p values for the ion-pair and free ion Sat2 reactions between benzyldimethylphenylammonium nitrate and sodium para-substituted thiophenoxides in methanol at 20 °C... 

In the one-step symmetry-allowed mechanism, with little charge separation in the activated complex, the Hammett p-values for p-XCgH4- substituted substrates are very small. As anticipated, the reaction has a negative volume of activation. 

There was no evidence of a second-order term in amine, nor did amine self-association account for the non-linear behaviour. Hammett p values (for variation of RNHSO2) determined for formation of the complex [S.amine] (p = 1.64) and for expulsion of the anion ( ONp) (pacyi = -1-78) are consistent with an E cB process and uncomplicated by any steric effects of bound amine in the complex. The value of Pacyi is identical with that reported previously for ElcB reaction of the same esters in 50% acetonitrile-water and much greater than for their 2-type reactions in chloroform. Consequently, an ElcB mechanism involving extensive S-O bond cleavage with the formation of a A(-sulfonylamine, ArN=S02, is supported. 

Hubbard and Kirsch (1972) have recently proposed that histidine may act as a nucleophile in a-chymotrypsin acylation reactions of esters having a good leaving group (jO-nitrophenol). This suggestion was based on a similarity in p-value for acylation by p-substituted nitrophenyl and dinitrophenyl benzoates and nucleophilic attack on these compounds by imidazole, in contrast with less positive p-values for hydroxide ion catalysis. Hammett p-values for hydrolysis of substituted phenyl esters are given in Table 6 and show little apparent trend. The values for hydroxide ion and alcoholate ions are... 

Arnett and coworkers later examined the reaction of lithium pinacolone enoiate with substituted benzaldehydes in THE at 25 °C. The determination of the heat of reaction indicated that the Hammett p value for the process is 331. Although the aldol reaction was instantaneous in THF at 25 °C, the reaction with o- or p-methylbenzaldehyde could be followed using a rapid injection NMR method in methylcyclohexane solvent at —80 °C. Application of Eberson s criterion based on the Marcus equation, which relates the free energy of ET determined electrochemically and the free energy of activation determined by kinetics, revealed that the barriers for the ET mechanism should be unacceptably high. They concluded that the reaction proceeds via the polar mechanism . Consistent with the polar mechanism, cyclizable probe experiments were negative . The mechanistic discrepancy between the reactions of benzaldehyde and benzophenone was later solved by carbon kinetic isotope effect study vide infraf. ... 

Measurement of Hammett p values for the hydroxide and phenoxide attack on 4-nitrophenyl esters of substituted benzoic acids shows that there is a large change in hybridization at the carbonyl carbon atom and the transition state for the concerted reaction which has a substantial tetrahedral geometry corresponding to a change from sp2 to sp2 at the carbon.3... 

This p value is the third finding which indicates thiocarbonyl protonation. The Hammett p value for protonation of met a - and para-substituted benzamindes is —0.92157 and the value for protonation on the oxygen of iV-benzoylthiourea is expected to lie close to this value. However, the p value for the protonation of thiocarbonyl would be expected to be considerably smaller. The p value of —0.42 for the S-protonation of V-benzoylthioureas is rather high in comparison with lruns -cinnamate ions in water (25 °C p = —0.466) and substituted 3-phenylpropionate ions (p = —0.212)168. The found p value may indicate that the hydrogen-bonded form shown in Figure 18 plays an important role in the reaction. 

The Hammett p values for the YN2 reaction between diphenylamine and para-substituted benzyl bromides in methanol decrease from 0.64 to 0.10 as the reaction... 

Electrophilic attack on alkenes by bromine often goes through three-membered ring cyclic bromoni-um ions and we can sometimes tell that this is so by studying the stereochemistry. Here are two reactions of styrenes that look very similar—a reaction with bromine and one with PhSCl. With no further information, we might be tempted to assume that they both go by the same mechanism. However, the Hammett p values for the two reactions are rather different. 

The Hammett p value for the following reaction is -4.8. Explain this in terms of a mechanism. If the reaction were carried out in deuterated solvent, would the rate change and would there be any deuterium incorporation into the product What is the silicon-containing product ... 

Laird and Parker studied54 the reactions of benzylamine with m- and / -substituted styrene oxides in ethanol and showed that the Hammett p value for attack of amine at the primary /1-carbon is positive and opposite in sign to that for attack of amine at the a-benzylic carbon. Thus, the regiochemistry of addition of amines to substituted styrene oxides (cf. Scheme 13) is dependent on the nature of the phenyl substituent. In the reaction of styrene oxide with benzylamine, 83% of product arises from attack at the primary carbon and only 17% arises from attack at the benzyl carbon. In the reaction of 3,4-dimethylstyrene oxide with benzylamine, only 19% of product arises from attack of amine on the primary carbon and 81 % arises from attack of amine at the benzylic carbon. 

The map of the Hammett p values for the ElcB reaction is shown in Scheme 1. 

AU the nitroxyl adducts I-V were generated [1 ] pulseradiolytically in aqueous solution from nitrobenzenes and the corresponding a-hydroxyethyl, o.Jl-dihydrox-yethyl, 6-methyluracil-6-yl, 6-methyl-/-cytosinc-6-yl and 6-methyldihydrouracil-6-yl radicals. Production of a-hydroxyethyl, a,3-dihydroxyethyl, 6-rncthyluracil-6-yl, 6-methyl-/-cytosine-6-yl and 6-methyldihydrouracil-6-yl radicals were described elsewhere [1—4]. The heterolysis rate constants, and all activation free energy changes were taken from those references. The LFER plot for the heterolysis reaction of the nitroxyl adducts 1 and V is linear (Fig. 1) (r = 0.99). The slope of this plot, which should be equal to the ratio of the Hammett p values for the heterolysis of the two, adducts 1 and V, results as 1.0, in agreement with the individually determined p values (both 1.5). Also the solvent isotope effect (SIE) on of adduct V, that is... 

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