Hammett reactivity constants

For meta- and para-substituted aromatic carbonyl compounds, a linear relationship exists between the earbonyl absorption frequency and the Hammett reactivity constant. - - - A relationship between the carbonyl stretching vibration frequency of aromatic carbonyl compound and the pAT " of the corresponding aromatic carboxylic acid has been demonstrated. Correlations with other parameters, such as electronegativities,"" ionization potentials, Taft a values, half-wave potentials, etc., have also been made." For aromatic compounds with ortho- substituents, a combination of factors may be important, such as chelation, steric effects, and field effects (dipole interactions through space). 

Fig. 1—2 shows a Hammett diagram for 14 different imidazolides of benzoic acids with a wide range of substituents upon which the reactivity is strongly dependent for example, the difference in rate constants between jV-(4-nitrobenzoyl)imidazole and iV-(4-dimethylaminobenzoyl)imidazole under the same reaction conditions amounts to a factor of about 3000. The Hammett reaction constant p = + 1.85 for the series shown in Fig. 1—2 indicates clearly that the hydrolysis is following a nucleophilic addition-elimination reaction path. 

There are numerous other transmission mechanisms for substituent effects. Those connected with unsaturated frameworks, such as mesomeric effects and correlations with it charge densities and Hammett-type reactivity constants, and intramolecular hydrogen-bonding effects are beyond the scope of this article and are discussed elsewhere (1,8,25,57). 

The polar effect was at first invoked to explain various directive effects observed in aliphatic systems. Methyl radicals attack propionic acid preferentially at the a-position, ka/kp = 7.8 (per hydrogen), whereas chlorine " prefers to attack at the /3-position, ka/kp = 0.03 (per hydrogen). In an investigation of f-butyl derivatives, a semiquanti-tative relationship was observed between the relative reactivity and the polar effect of the substituents, as evidenced by the pK, of the corresponding acid. In the case of meta- and / ara-substituted toluenes, it has been observed that a very small directive effect exists for some atoms or radicals. When treated by the Hammett relation it is observed that p = —0.1 for H , CeHs , P-CH3C6H4 and CHs . On the contrary, numerous radicals with an appreciable electron affinity show a pronounced polar effect in the reaction with the toluenes. Compilation of Hammett reaction constants and the type of substituent... 

The various factors that influence reactivity in these types of molecules are clearly illustrated in a study193 of their reactions with methyl iodide and p-nitrophenyl acetate, giving rise in certain cases to deviations from Hammett-type plots. Thus, rates for isoquinoline, pyrimidine, and pyridazine fit reasonably well on to the pK — log Krci (Mel) plot (Fig. 3) and thus conform to the Hammett reaction constant... 

Because of the bulk of comparable material available, it has been possible to use half-wave potentials for some types of linear free energy relationships that have not been used in connection with rate and equilibrium constants. For example, it has been shown (7, 777) that the effects of substituents on quinone rings on their reactivity towards oxidation-reduction reactions, can be approximately expressed by Hammett substituent constants a. The susceptibility of the reactivity of a cyclic system to substitution in various positions can be expressed quantitatively (7). The numbers on formulae XIII—XV give the reaction constants Qn, r for the given position (values in brackets only very approximate) ... 

The first adequate examination of the applicability of the Hammett equation to aromatic substitution was made by Roberts and his associates (1954). New data on the nitration of the halobenzenes together with the results for other substituted benzenes (Ingold, 1953) revealed that the rates for nitration in the meta position were, indeed, correlated satisfactorily by the Hammett rr-constants. Para substituents, in particular groups directing to the ortho and para positions, exhibit important deviations from the line defined by the meta reactivities (Fig. 3). It was suggested that these deviations were the consequence of significant resonance interactions in the transition state. At the same time, de la Mare (1954) examined the application of the Hammett parameters for the correlation of the relative rate data for non-catalytic chlorination. The poor agreement achieved (Fig. 4) prompted his conclusion that variable resonance interactions in the... 

There are principally two different approaches of correlating experimental rate data of electrophilic substitution with reactivity indices (1) Correlating the index with the rate data of a given reaction, e.g. bromination. For example, a satisfying correlation of Dewar reactivity numbers with the log of rate constants of the bromination of benzene, naphthalene (1- and 2-position), biphenyl (4-position), phenanthrene (9-position), and anthracene (9-position) has been observed [55]. In correlations of this type the reactivity index corresponds to the reactivity constant in the Hammett equation while the slope of the linear correlation corresponds to the reaction constant (see also Sect. 3) (2) correlating the index with experimental a values. 

The structure-reactivity relationship of acidic organophos-phorus compounds is well demonstrated by mono-esters of p-substi-tuted phenylphosphonic acids. The acidity of these organic acids increased as the polar nature of the substituents enhanced. A linear free energy relationship exists between the pKa value and the Hammett cr constants in acidic p-substituted phenylphospho-nates. When these structure parameters are plotted either against the t POO" asym. or against the 31p chemical shift of their dicyclo-hexylammonium salts straight lines resulted in both cases. 

This procedure has been extensively applied in correlating side-chain reactivities283-289 and spectroscopic quantities.290 In correlating electrophilic substitution reactivity data, the Hammett a constants must, of course, be replaced by the Brown and Okamoto a+ constants.185... 

The exchange of equation 175, proceeds nearly quantitatively at 80-90 °C in both aprotic and polar protic solvents, including water, in the presence of metallic copper or copper(I) compounds368 in over 90% yield. The exchange reaction is first order both in the copper mono-iodide and in the iodo-organic compound. The reactivity of the compounds investigated correlated with the Hammett-Taft constants. The exchange proceeds probably by the mechanism illustrated by equation 176 367,371. 

A number of correlations have been drawn between 1 JSiH and parameters related to bonding and reactivity. The magnitude of 1 JSiH was found to correlate with Hammett a constants in phenyl and benzyl substituted silanes and disilanes144,145, and aryloxy- and (arylthio)dimethylsilanes146 and with Taft polar substituent constants a in silanes R1R2R3SiH (R = alkyl, Ph, H)147. 

Chemical shifts for aryltin compounds are given in Table XV. Attempted correlations of C chemical shifts with Hammett a constants work best with C(l) for the para and meta phenyltin derivatives (203). It must be realized however that such correlations of reactivity parameters (such as o, determining reactivity and chemical shielding are similar. 

The use of a Hammett substituent constants in QSAR analyses has been prominent since the early studies by Hansch and Fujita (1964) in medicinal chemistry. The electron-withdrawing/releasing potency of substituents on an aromatic system has been formalized by the Hammett equation (Hammett, 1940). The relative reactivities among homologous series of compounds under defined conditions - for example, the logarithms of the rate and equilibrium constants for the hydrolysis of substituted benzoic acid esters (rate constant k) and the ionization constants (K) of the corresponding acids, were found to be linearly related ... 

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