Hammett a values

A comparison of Hammett a values of the new activating groups, such as amide and —CF2—, with that of conventional activating keto groups shows them to be electronically similar. 

A good correlation with ordinary Hammett a values was based on 16 well-behaved substituents, and p-SOMe conformed well to this. Various other substituents showed deviations which were attributed to enhanced + R effects. These included p-SPh and this was explained in terms of 7t(pd) bonding, which was thus taken to play no part in the effect of p-SOMe on the methyl hyperfine splitting. More recently several 4-substituted benzyl radicals of the type RSO C6H4CH2 (n — 0,1 or 2 R = Me, Ph, Tol, COMe or OMe) have been examined by ESR spectroscopy249. The ability to delocalize spin density onto the substituent decreases in general as n increases and the effect of R depends on the oxidation state of sulfur. These authors have devised a new scale of substituent effects (sigma dot... 

Activation energies and entropies of activation are in the same region as those for Sn2 reactions of neutral A-methylaniline. Large negative AS values are again in accord with increased charge separation with attendant solvation in the transition state. The rate constants at 298 K correlate with Hammett a values with p — 1.13 (r — 0.985) in keeping with a development of benzoate character in the transition state. 

The one-electron oxidation of iV-benzylphenothiazine by nitric acid occurs in the presence of /i-cyclodextrin, which stabilizes the radical cation by incorporation into its cavity. The reaction is inhibited by adamantane, which preferentially occupies the cavity. Novel Pummerer-type rearrangements of / -sulfinylphenyl derivatives, yielding /7-quinones and protected dihydroquinones, and highly enantioselective Pummerer-type rearrangements of chiral, non-racemic sulfoxides have been reviewed. A comprehensive study has demonstrated that the redox potential for 7- and 8-substituted flavins is linearly correlated with Hammett a values. DFT calculations in [3.3.n]pro-pellanes highlight low ionization potentials that favour SET oxidative cleavage of the strained central C-C bond rather than direct C-H or C-C bond attack. Oxidations and reductions in water have been reviewed. ... 

The effects of the 1,2,3-triazole and 1-amino-1,2,3-triazole heterocycles on the chemical shift of the carbon atoms in the phenyl ring of compounds (40) and (41) (X = H) are compared with the value for benzene. With regard to the effects of substituent X para and meta) on the 1,2,3-triazole carbons, C(4) is deshielded by electron-withdrawing groups, whereas C(5) is shielded. Reasonable correlations with Hammett a values are observed for the chemical shifts of both C(4) and C(5) of (41) <86MRC53>. The chemical shifts of C(4) and C(5) in 5-amino-l-aryl-l,2,3-triazoles are shifted upheld by 2-5 ppm compared with the corresponding 5-anilino-1,2,3-triazoles. In 5-anilino-... 

Triazole has values of 2.19 (as a base) and 10.26 (as an acid). A representative selection of acidity data Hammett treatment to the pXa values of triazoles. The 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 kinetics and mechanism of the phosphorus-catalysed dimerization of acrylonitrile to give 1,4-dicyanobut-l-ene and 2,4-dicyanobut-l-ene have been studied.114 The reactions of aryhminodimagnesium (138) with //-substituted p-cyanobenzophenones, l-cyano-9-fluorenenone, o-, m-, and p-dicyanobcnzcnes, and o-, m-, and p-nitrobenzonitriles have been examined.115 The effect of pressure on the reaction of 3 -methyl- l-(4-tolyl)triazene (139) and benzoic acid in chloroform and acetonitrile has been studied.116 The effect of acids on the rate of urethane formation from alcohols and isocyanates in the presence of alkyltin carboxylates has been examined.117 A Hammett a value has been reported for the amidine group N=CHNMe2 and used for the prediction of the basicity of sites in bifunctional amidines.118... 

The situation for the ordinary Hammett a values based on measurements of the dissociation constants is unsatisfactory. The experimental results do not seem to be very reproducible as between one set of authors and another18. The value of am appears to be about 0.40, and that of op to be about 0.5 (see also Hansch and coworkers86). These are certainly in the region expected for CHO rather than CH(OH)2, so it may well be that the hydration equilibrium is more in favour of CHO in aromatic aldehydes than in compounds in which the group is attached to a saturated carbon atom. 

Reactivities and activation parameters for pyrolytic unimolecular first-order elimination reactions of V-acetylurea, V-acetylthiourea, A.A -diacctylthiourea and N-acetylthiobenzamide have been interpreted with reference to those for other amide derivatives.55 The first-order rate constants for pyrolysis of RCONHCSNHQfkR (R = Me, R = H R = Ph, R = H, 4-N02, 3-Cl, 4-C1, 4-Me) have also been measured at 423-500 K and correlated with Hammett a ° values to give p = 1.99 at 450 K.56... 

Kinetic data for the ortho position azo coupling of substituted benzenediazonium cations with three activated naphthalene derivatives give good correlations with Hammett a-values of the substituents66 and the data indicate a transition state near in energy to that of the Wheland intermediate. 

Extended Selectivity Relationship. c Adopted Hammett a value. 

Fig. 3. Plot of the log of the rate constants for addition of perfluoro-n-octyl radicals to some para-substituted styrenes Hammett a values of the substituents...

Attempts have been made to correlate displacements of the absorption bands of parent aromatic compounds by substituents with Hammett a-values derived from the effects of the substituents on ground state kinetic and equilibrium properties. For example,... 

Figure 1.61 Percentage inhibition for meta substituted methyl pyridines as a function of Hammett a values 1. pyridine 2. 3-methylpyridine 3. 3,5-dimethyl pyridine...

The elimination of C2H4 from n-butyrophenones (117) has been found to yield a rough correlation with the Hammett a values for the substitu-... 

Recently Nelson et al. have shown that the chemical shifts of the carbon atoms of monosubstituted phenyl rings are proportional to the CNDO/2 calculated total charge densities (178). It was concluded that it is possible to estimate the total charge density at the meta- or para-carbon atoms for a monosubstituted benzene to 0.005 charge unit simply by measuring the NMR spectrum. A similar, but less accurate linear relationship exists for all the carbons in the phenyl ring. There is also a linear relationship between the chemical shift for the para position and the Hammett a value for the substituent. This linear relationship has previously been reported for phenylphosphorus derivatives (193). 

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