Hammett relationships

The Flammett equation, one of the oldest and most useful of LFER, correlates the rates of reaction of a series of meta- and para-substituted aromatic compounds with a common substrate. 

Second-order rate constants A for the replacement of a ring-substituted benzoate group by hydroxide ion in a number of complexes (base hydrolysis) have been carefully determined at 25°C in 40% aqueous methanol  

The log A vs o plot is reasonably linear with no marked deviations for seven X-substituents. The value of p is 0.75 and this is much smaller than that for alkaline hydrolysis of the free 

Espenson, R. Marcec, and M. Orhanovic, Inorg. Chem., 25, 2562 (1986). (1986) American Chemical Society. 

On the other hand, reduction of (H20)5Cr(NC5H4X) by Ru(bpy)3 has an associated value for p of 1.1, the same as for the Co and Ru ammine complexes and therefore a similar mechanism. For other applications of Hammett LFER see Refs. 141-144. 

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The validity of the Hammett relationship log K/Ko = pa- has been extensively investigated for five-membered heteroaromatic compounds and their benzo analogues. The ratio Pheterocycie/Pbenzene is closest to Unity for thiophene. Judged from work on the polarographic reduction of nitro compounds, the ability to transmit electronic effects is HC=CH = S < O < NH. 

A Hammett relationship of the form ApK = 5.8am has been proposed for 4-substituted pyrazoles (74TL1609) in order to explain the effect of 4-nitro ApK = 4.5, am = 0.71) and 4-diazo groups (Apiifa = 10.0, am = 1.76). The acidity constants of a series of pyrazolidine-3,5-diones have been determined (75AJC1583) and the 4- -butyl-1,2-diphenyl derivative phenylbutazone has a pK of 4.33. 

The reaction of substituted 1-arylethyl chlorides with K OC(CH3)3 in DMSO does not follow a Hammett correlation. Instead, the reactivity order is />-N02 > p-MeO > P-CF3 > P-CH3 > H > p-Cl. What explanation can you offer for the failure to observe a Hammett relationship ... 

In the bromination of styrene, a po-+ plot is noticeably curved. If the extremes of the curves are taken to represent straight lines, the curve can be resolved into two Hammett relationships with p = —2.8 for electron-attracting substituents and p = —4.4 for electron-releasing substituents. When the corresponding -methylstyrenes are examined, a similarly curved ap plot is obtained. Furthermore, the stereospecificity of the reaction in the case of the -methylstyrenes varies with the aryl substituents. The reaction is a stereoespecific anti addition for strongly electron-attracting substituents but becomes only weakly stereoselective for electron-releasing substituents, e.g., 63% anti, 37% syn, forp-methoxy. Discuss the possible mechanistic basis for the Hammett plot curvature and its relationship to the stereochemical results. 

The form of Eq. (7-24) suggests, by comparison with Eq. (7-19), that a single interaction mechanism is operative between the substituent R and the reaction site X. That this is not necessarily true is shown as follows. Suppose two interaction mechanisms control the substituent effect. Then a Hammett relationship may be written as in... 

For a general discussion of the Hammett relationship see Section 7.2. Unfortunately the present author never received details of that work. 

It is interesting to note, however, that the equilibrium for complex formation of substituted benzenediazonium ions with 18-crown-6 follows a classical Hammett relationship as in Scheme 7-1. 

Apparent exceptions are the constants k2 for diazonium salts with the electron-withdrawing substituents 4-C1 and 3-CN. The values of k2 for these compounds are more than a factor of 10 larger than expected on the basis of Hammett relationships. Product analyses rationalize this observation whereas in all other cases products are likely to be formed by heterolytic dediazoniation, the products from the 4-chloro-and 3-cyanobenzenediazonium ions include chlorobenzene and benzonitrile, typical compounds obtained in homolytic dediazoniations. This result corresponds to the reaction products observed by Moss et al. (1982) in micellar dediazoniation, compared with the nonmicellar reaction (see Sec. 8.3). 

Just as electrophilic aromatic substitutions were found more or less to follow the Hammett relationship (with a" " instead of o see p. 692), so do nucleophilic substitutions, with cr instead of a for electron-withdrawing groups. ... 

The effect of a substituent on the aromatic substitution reaction is similar to its effect on electrophilic side chain reactions, but not precisely parallel. Thus the Hammett relationship using the usual sigma or substituent constants gives considerable scatter when applied to aromatic substitution. The scatter is probably due to an increased importance of resonance effects in the nuclear substitution reaction as compared with the side chain reactions. 

Mercury(II) reacts with organochromium complexes by electrophilic substitution. Rate constants have been reported for Hg2+ attack at a series of alkylchromium complexes with the macrocyclic ligand 1,4,8,11-tetrazacyclotetradecane, CrR(H20)([14]aneN4). The Hammett relationship established for a series of meta and para substituted benzyl analogues is consistent with attack of the Hg2+ at a-carbon (89). 

However, the effects on the rates in nonpolar solvent systems is dramatically larger and often of apparent contradiction. For example, the Hammett rho value for the oxidation of substituted methyl cinnamates and cinnamic acids by tetrabutylammonium permanganate in methylene chloride solutions is positive (33,49). See Figure lb. However, a rho value of converse sign (-0.6) is obtained from a Taft plot (Figure Ic) for the oxidation of vinyl ethers in aqueous tetrahydrofuran (33,50). For many other compounds the Hammett relationships are no longer linear, but concave upward ... 

Recently, a direct kinetic study on the amination of substituted phenyhnagnesium bromides using Af,Af-dimethyl O-mesitylenesulfonyl hydroxylamine 3b as amination reagent has been reported by Erdik and Ate Ulkii . Rate data, the Hammett relationship and activation entropy support an Sn2 displacement of the carbon nucleophile on the electrophilic nitrogen. These results are consistent with the competition kinetics for electrophilic amination of substituted phenyl Grignard reagents with O-methylhydroxylamine In. ... 

Theoretical and spectroscopic studies of the protonation of tetrazoles have been discussed in Sections 4.17.2.2 and 4.17.3. Protonation of 5-substituted tetrazoles, 1,5-disubstituted tetrazoles and 2,5-disubstituted tetrazoles in aqueous H2SO4 at H2SO4 concentrations up to 96% occurs at N-4 <82KGS142l>. The tetrazoles are weak bases with basic pAl values of about —3 to —4 <87AHC(4i)i87>. A basic pXj value of —3.0 has been measured for tetrazole in aqueous H2SO4. Hammett relationships have been reported for the basic pX values of 5-substituted tetrazoles... 

The l parameter for each ligand L was obtained through a statistical analysis of the reported redox potentials of the large number of complexes with the Ru redox couple and the possible ligands, as given by Eq. 17 (alternatively in some cases using Hammett relationships [68-71]). 

Table 8.6 Hammett Relationships for Quantifications of Aromatic Substituent Effects on the Toxicity of Various Acids ...

Quantitative Structure-Reactivity Considerations Hammett Relationship... 

Illustrative Example 13.6 Estimating Hydrolysis Rate Constants Using the Hammett Relationship Bronsted Relationship... 

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