Hammett treatment

Hammett treatments show good correlations with large negative p values for the hydrolysis of acetals of aromatic aldehydes. This is consistent with the development of a positive charge at the carbonyl center in the rate-determining step. 

The diazonio group is a somewhat more complex substituent for such evaluations because it is charged, in contrast to the majority of substituents on which the Hammett treatment is based. The electrostatic interaction of the diazonio and other charged groups was calculated by Hoefnagel et al. (1978) and by Exner (1978). The substituent constants they obtained, including the effects of coulomb interactions, are only slightly different from those of Lewis and Johnson (1959). 

The CT values are numbers that sum up the total electrical effects (resonance plus field) of a group X when attached to a benzene ring. The treatment usually fails for the ortho position. The Hammett treatment has been applied to many reactions and to many functional groups and correlates an enormous amount of data quite well. Jaffe s review article lists p values for 204 reactions, many of which have different p values for different conditions. Among them are reactions as disparate as the following ... 

For a review of Hammett treatment of NMR chemical shifts, see Ewing, D.F. in Chapman, N.B. Shorter, J. Correlation Analy.sis in Chemistry Recent Advances Plenum, NY, 1978, p. 357. 

It would be of interest to see if our E and C parameters for the substituted phenol-base interactions can be transformed into Hammett ff and g parameters, i.e., to see how well the constraints of a Hammett treatment are adhered to by our E and C parameters derived from all types of donor-acceptor interactions. Unfortunately, the form of the... 

Triazole has values of 2.19 (as a base) and 10.26 (as an acid). A representative selection of acidity data Hammett equation is more satisfactory for acidic than basic values of triazoles <65ZC38l). For triazoles with only annular NH as acidic groups the Hammett a value is 6.00, while for 3-(or 5-) hydroxytriazoles it is 7.62. In both cases constants are most appropriate <65ZC304) this is also true for nitrotriazoles <70KGS558). 

The mechanism of decarboxylation of /3-lactones has attracted much attention. The gas-phase decomposition of 2-oxetanone is a unimolecular first-order process. It has a considerably lower energy of activation than the pyrolysis of oxetane and a much higher entropy of activation, indicating a loose activated complex (69JA7743). The ease of the reaction is greatly affected by the electronic effect of substituents at position-4, but not at position-3. The Hammett treatment of a series of rrans-4-aryl-3-methyl-2-oxetanones gave a good correlation with [Pg.374]

For reviews of the application of the Hammett treatment to unsaturated systems, see Ford Kalritzky Topsom, in Chapman Shorter, Ref. 18, pp. 269-311 Charton Prog. Phys. Org. Chem. 1973, 10. 81-204. 

Substituent effects on the acidity of pyridinium ions46,47 have been extensively studied in order to learn about the ability of a hetero-aromatic ring to transmit electronic effects and to determine how and to what extent substituents influence the protonation of an annular nitrogen atom. No other series of heteroaromatic compounds has been studied as carefully and as thoroughly as these model compounds. Yet, interpretations are far from being universally accepted. Single-parameter Hammett treatments will be considered before dual-substituent parameter (DSP) approaches. 

The DSP approach nicely answers the controversial question about which substituent parameters should be employed to correlate pKa data for 4-substituted pyridinium ions. Statistically, the best correlation is given by Eq. (9), which has values to measure the resonance contribution of a substituent, a result in keeping with chemical intuition. This correlation is statistically superior to a Hammett treatment, where both resonance and inductive effects of a group are combined into a single parameter, p or ap.53,54 Moreover, now it is possible to rationalize why a simple Hammett treatment using ap works so well. Equation (9) reveals that the protonation equilibrium is much more sensitive to an inductive effect (p, — 5.15) than to a resonance effect (p = 2.69). Hence, substituent parameters, such as erp, which are derived from a consideration of the dissociation constants for benzoic acids where resonance contributions are small serve as a useful approximation. The inductive effect is said to have a larger influence on pKa values for pyridinium ions than for benzoic acids because the distance between the substituent and the reactive site is shorter in the pyridine series.53... 

The nucleophilic properties of pyridine nitrogen have been assessed in studies on pyridine-catalyzed Schotten-Baumann reactions133 and aromatic sulfonyl chloride hydrolyses,134 from which highly accurate Bronsted and Hammett treatments arise. There is some doubt as to the mechanism of this reaction.45 Equations (13) and (14) accurately express the reactivity of 3- and 4-substituted pyridines toward ethyl iodide (using rate for equilibrium constants),133 while rates of N-methylation of 2-substituted pyridines have been used to estimate both steric and electronic effects.136... 

The Hammett treatment provides a correlation of much experimental data. Tables 26-6 and 26-7 contain 38 substituent constants and 16 reaction constants. This means that we can calculate relative k or K values for 608 individual reactions. To illustrate, let us suppose that we need to estimate the relative rates of Reaction 16 of Table 26-7 for the para-substituents R = OCH3 and R = CF3. According to the p value of 4.92 for this reaction and the cr values of p-OCH3 and p-CF3 in Table 26-6, we may write... 

Hydration of several 1,2,3-triones including indane derivatives (70 Scheme 4) has been studied in dioxane-water mixtures.1053 Monohydration gives a 2,2-diol (71) forward rates and equilibrium constants have been measured over a wide range of solvent composition. Based on activation parameters, kinetic isotope effects, a Hammett treatment, and a second-order rate dependence on water, two water molecules are suggested to play distinct roles, one as nucleophile, the other as general acid-base, similar to dialdehydes.105b,c... 

The methylation of A -phcnyl hydroxylamine with methyl arenesulfonates in DMSO occurs on the O atom, in contrast to methylation in methanol, where /V-alkylation occurs.63 Rate data were obtained for various systems pertinent to the problem of alternative sites of alkylation and Hammett treatments were applied. 

In the conventional Hammett treatment, the electronic influences of the substituents, <7, are assumed to be constant. To test the adherence of a reaction to the Hammett eq. (1), it is customary to plot the log (fc/fcH) values against the a-constants. A linear relationship establishes that the reaction obeys the equation. The slope of the straight line defines the reaction constant p for the process. 

An examination of the data for substitution in the 2-position of fluorene (Fig. 32) reveals a reasonable correlation. Some scatter is observed but no more than is generally encountered in an application of the Hammett treatment to data covering a wide range of reactivity. Certainly, there is no evidence for a pronounced curvature of the kind found in the related treatment of the data for biphenyl (Figs. 29 and 30). In contrast to substitution in the para position of biphenyl, substitution in the structurally equivalent 2-position of fluorene conforms to the Extended Selectivity Relationship (see, however, p. 147). 

Phenanthridine is a fairly weak base (pKa, in water, 4.52 + 0.01 at 20°).247 The effect of substituents has not been examined systematically (and predictions based on a Hammett treatment appear somewhat unreliable in the phenanthridine series).267 The available data are in line with qualitative predictions based on the electronic... 

For the compounds obeying the Hammett equation, the distribution pattern for the OH attack can be estimated by assuming that the position on the ring with the maximum C7 value has the least probability, while the one with the minimum has the greatest probability. In our recent work > on cinnamate derivatives where the addition of the OH radical to the olefmic double bond is an additional pathway, a value of-0.3 was estimated from the Hammett plot (Fig. 5). This value is in agreement with those reported earlier for several substituted benzenes (p" = -0.52 to -0.4). However, the Hammett treatment for reactions with rate constants close to diffusion-controlled ones may not be satisfactory due to limited variation in kinetic data and large experimental uncertainty. In contrast, better correlation is expected in the corresponding gas phase reactions. 

Attempts have been made with some success to apply Hammett treatments to reactions and reactivity of imidazoles. Complications due to the natures of the ring nitrogens and tautomerism, however, have created difficulties. 

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