Hammett a constant

The most reliable method of preparing benzofuroxans is by decomposition of o-nitrophenyl azides. Decomposition can be achieved by irradiation, or more usually by pyrolysis temperatures between 100° and 1.50° are commonly used. Refluxing in glacial acetic acid is the recommended procedure for 4- or 5-sub-stituted 2-nitrophenyl azides, but with 3- or 6-substituted compounds higher boiling solvents are usually necessary. Quantitative studies on the reaction rate have been made, and a cyclic transition state invoked, an argument which has been used to account for the greater difficulty of decomposition of the 6-substituted 2-nitrophenyl azides. Substituent effects on the reaction rate have also been correlated with Hammett a constants, ... 

The influence of substituents in Ar1 is characterized by negative p values (plot based on ordinary Hammett a constants)208. Reactions involving other types of phosphorus nucleophiles (e.g. PPh2Alk)208 and ArS02 ions as nucleophiles have also been studied209. The work has been extended to the study of a-azidosulfones but the reaction does not turn out to be analogous210. There is base-catalyzed abstraction of hydrogen ion from the a-carbon atom. 

Having considered the main individual investigations of S02Me and SOMe in respect of their ordinary Hammett a constants and of the exalted constant a", we must examine... 

A situation similar to that in acetyl phosphate is also encountered in benzoyl phosphate76 . Electron-attracting substituents on the phenyl ring accelerate the hydrolysis of the dianion (a linear relationship exists between log khydrol and the Hammett a constants with q = 1.2 and the linear log ki,j,drol./pKa relationship is the same as for the phosphoric monoaryl ester dianions65 . On the other hand, hydrolysis of the monoanion is influenced only slightly by substituents in the phenyl ring. These observations can also be rationalized in terms of the decomposition mechanism to the POf ion formulated for 116 and 117. 

The relationship between the structure of 1,2,4-thiadiazolidines and their H NMR spectral solvent effects has been studied by measurement of the NMR chemical shift differences (Av) of 39 derivatives in various solvents (CgDg, CCU) for methyl or methylene groups attached to an sp2-hybridized nitrogen, Av correlates linearly with Hammett a constants and for those attached to an sp3 4-hybridized nitrogen, with Taft a° constants <1982AHC285>. 

Values of these parameters are reported in Table 4. Estimated values of Hammett a constants can be calculated from the relationships25 ... 

Substitution of Si by Ge has no obvious effect on reduction but is clearly accompanied by easier oxidation. The oxidation reaction was not identified. In the silane series, values of reduction Ep are linear with Hammett a constants for substituents on the aryl ring. No parallel test on germanes has been reported. 

Series 28 contains a diverse range of electronic effects on the benzoyloxy ring. A plot of log TA100 against Hammett a constants for all but the 4-methoxy 28b (toxic) and 4-phenyl 28c (significantly more hydrophobic) was linear with negative gradient (Fig. 29).48... 

Relative activation enthalpies (Aif) in Table 2 were converted to o% kx k ) at 298 K, and were plotted against Hammett a constants. Here, we used enthalpies, because the size of the entropy and hence the free energy depend much on low frequencies, which are less reliable than higher frequencies, especially for compounds with weak interactions such as TS (8). The use of free energy (AG ) gave similar correlations with more scattered points. As for the Hammett o constant, we used dual-parameter o constants in the form of the Yukawa-Tsuno equation (LArSR equation) (9) as defined in eq 3. Here, the apparent a constant (aapp) has a variable resonance contribution parameter (r), which varies depending on the nature of the reaction examined for t-cumyl... 

The correlations presented in Figs 4 and 5 are in stark contrast to the disorder shown in a plot of ogkc/ku against the Hammett a constants for... 

At constant pH, the parameter p is dependent on the electronic effects of the nitrosoarene substituent(s). Using only definite acceptor substituted nitrosoarenes, a Hammett correlation on the a scale was obtained25. Separate investigation of fc2RSH and fc2(rearr) for a wider selection of nitrosoarenes reacting with GSH revealed reasonable correlation with Hammett a constants only for fc2RSH (p = +1.4). However, Utrearr) was found to fit better on the Hammett a+ scale (p+ = —3.5)30 [for further discussion of Utrearr) see Section II.D.l.b]. 

This preparative scheme leads to only 30% yield due to the side reactions between the meto-astatoaniline diazonium salt and astato-phenol, which cannot be eliminated even by continuous extraction of the product with n-heptane (167). All the astatophenols synthesized to date have been identified by either HPLC (99,104) or TLC (160,166,167). Their dissociation constants (KJ have been established from extraction experiments by measuring the relative distribution of compounds between aqueous borax buffer solutions and n-heptane as a function of acidity. On the basis of these derived values, the Hammett a-constants and hence the field (F) and resonance (R) effects have been estimated for these compounds (167) (see Table VI). The field effect for astatine was found to be considerably weaker than that for other halogens the resonance effect was similar to that for iodine (162). 

It has been found that the chemical shift of the benzylidene carbon, Sct, in ring-substituted benzaldoximes (Scheme 2) correlates well with Hammett a constants. On the other hand, it is known that the chemical shift of that atom in the NMR spectra of benzaldoximes depends mainly on the substituents inductive effect and its resonance effect is of reduced importance. Multiparameter correlations of Scr with the inductive and resonance substituents constants are of better quality. It is known that the chemical shifts of para-substituted benzaldoximes are linearly dependent on Hammett a constants. Correlations between the NMR chemical shifts of the oximino oxygen and a, a and aj substituent constants for substituted benzaldoximes are poor. These results show that coplanarity of Ar—CH=NOH creates difficulties in transmission of the substituents effect, especially the resonance effect, to the CH=NOH group. ... 

Nucleophilic attack by iV-methylaniline is favoured by electron-withdrawing groups on the amide and acyloxyl side chains. A series of / ara-substituted Af-acetoxy-Af-butoxy-benzamides (138) (Table 6) gave a weak but positive Hammett correlation with a constants (p = 0.13, r = 0.86) °. The analogous reactions of pyridine with para-substituted phenacyl halides in methanol afforded a similar Hammett correlation a, p = 0.25) . The bimolecular rate constants for the limited series of Ai-benzoyloxy-A-benzyloxybenzamides (139) in Table 6 correlated strongly with Hammett a constants (p = 1.7, r = 0.97) °. Stabilization of developing carboxylate character supported the computed charge redistribution in the transition state ... 

Rates of reaction were moderately fast. Glutathione reacted rapidly in DMS0-rf6/D20 ( 2 ° ca 2 X 10 lmol s ) and a series of A-benzoyloxy-A-benzyloxybenzamides (139) and (L)-cysteine ethyl ester in methanol-(f4 reacted with low a values (8-16 kcal mol ) and negative AS (—19 to —43 calK mol ), similar to their reaction with anilines. Bimolecular rate constants at 298 K (Table 8) correlated positively with Hammett a constants with slightly lower sensitivity p =. as opposed to p =. l for A-methylaniline). ... 

The relevant data for the dicyclopentadienyl metals are given in Table V (109) only in the case of ferrocene and nickelocene are the results both well-defined and comparable, and the values of 0are in agreement with the electron configurations [ ]( )4( 2i/)° and f ]( 2 )4( 2w)2 in that nickelocene is the more easily oxidized. More recently, it has been found (110) that qualitative agreement is observed between the oxidation potentials of a series of para-substituted phenylferrocenes and the Hammett a constant of the substituent, although in this case polarographic measurements at a... 

Chemical shifts of 4-methyne protons in condensed 4//-pyran derivative 630 correlate well with Hammett a constants for substituents X polar substituent effects operating in molecules 630 were discussed.462 The spectra of some 13C-IabeIed 2//-pyrans were also measured.309... 

In Figure 2.2a, in which Hammett a constants are plotted, there is only a scatter of points but in Figure 2.2b straight line is obtained. 

Thiophenoxide ion reacts with PhC=CC02Me in DMF containing 0.5% MeOH to give a mixture of ( )- and (Z)-products PhC(SPh)=CHC02Me. The rate constant depends on the MeOH concentration, indicating a third-order reaction. The plot of log 3 vs Hammett a constants varies from 0.42 to 0.77, depending on the temperature. The activation parameters and p values are consistent with a concerted mechanism.79... 

Log k/k0 for acid dissociation of a series of substituted ethyl benzoates vs. the Hammett a constant of the substituent groups. (From Tinsley, I.J., Chemical Concepts in Pollutant Behavior, John Wiley Sons, New York, 1979. With permission.)... 

The linear correlation between the AEn values for meta- and para-substituted ben-zylic anions and the Hammett a constants has suggested that n conjugation may be used to explain the influence of substituents which have n orbitals.1... 

Fig. 1. The relationship between log (k/k ) for the methanolysis of J-menthyl benzoates with methoxide in methanol and the Hammett a-constants.

Hammett s equation (Hammett, 1940) correlates the rate of reactions with the electronic nature of the substituent in aromatic compounds. The Hammett a constants characterize the conjugation eifect and the inductive effect of the substituent for side-chain reactions. 

Berger and Wolfe (1996) reported a correlation of hydrolysis data for 12 sulfonylurea herbicides. The use of bond strength or Hammett a constants was impossible because of the complex structures of the compounds. The hydrolysis pathways for this class of compounds are also more complex, but the use of quantum mechanical parameters provided the detailed structural information needed to develop a useful correlation. As a result of the many different functional groups, several reaction pathways are available depending on the substituents. Also, there is a complicating pH effect on the pathways and the kinetics of hydrolysis as shown by product studies. The 12 herbicides used in this study are listed in Table 13.4, and the pseudo first-order hydrolysis rate constants are given in Table 13.5. Figure 13.2 shows the basic structure of these compounds. 

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