Monosubstituted benzenes, Hammett

Let us illustrate this with the example of the bromination of monosubstituted benzene derivatives. Observations on the product distributions and relative reaction rates compared with unsubstituted benzene led chemists to conceive the notion of inductive and resonance effects that made it possible to explain" the experimental observations. On an even more quantitative basis, linear free energy relationships of the form of the Hammett equation allowed the estimation of relative rates. It has to be emphasized that inductive and resonance effects were conceived, not from theoretical calculations, but as constructs to order observations. The explanation" is built on analogy, not on any theoretical method. 

Recently, a kinetic study has been made of the substitution of diazotised sulphanilic acid in the 2 position of 4-substituted phenols under first-order conditions (phenol in excess) in aqueous buffer solutions at 0 °C131a. A rough Hammett correlation existed between reaction rates and am values, with p about -3.8 however, the point for the methoxy substituent deviated by two orders of magnitude and no explanation was available for this. The unexpectedly low p-factor was attributed to the high reactivities of the aromatic substrates, so that the transition state would be nearer to the ground state than for reaction of monosubstituted benzene derivatives. 

The first recorded correlation of dipole moments with substituent constants was observed by Taft (3), who reported results for alkyl cyanides, chlorides, iodides, and tertiary amines. Kross and Fassel (31) have reported the correlations of dipole moments for 4-substituted nitrobenzenes with the simple Hammett equation. Rao, Wohl, and Williams (32) have studied the correlation of dipole moments of disubstituted benzenes with eq. (1) and of monosubstituted benzenes with the equation... 

Table IV. Correlation of Optical Hydrated Electron Production from Monosubstituted Benzene Derivatives with Hammett Substituent Constants...

Figs. 3-4. The failure of the Hammett cj-constants for the correlation of log (kjk-g) for (3) the nitration of monosubstituted benzenes—substituents with large resonance interactions are denoted by broken circles and (4) the chlorination of monosubstituted benzenes. 

A number of workers have reported the NMR spectra of mono-substituted phenyl groups and correlated the shifts vdth molecular parameters. It has been shown for organic compounds that the chemical shift of the para-carbon in monosubstituted benzenes is linearly related to the total TT-electron density at the para position in these compounds. Also the shift separation of the meta- and para-carbons appears to be linearly related to the rr-electron density on the para-carbon due to resonance interaction with the substituent 49, 156). Spiesecke and Schneider have reported a good linear relationship between the para-carbon chemical shift of monosubstituted benzenes and the Hammett, o-para constant, but no such relationship appears to exist for the other carbon chemical shifts, except between the chemical shift for the substituted carbon atom (corrected for magnetic anisotropy effects of the substituent) and the electronegativity of the substituent 210). 

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). 

The substituent constant of the Hammett equation has been related successfully to the logarithm of the activity coefficient ratio at infinite dilution for a series of m- and / -phenyl isomers. Hammett stated that a free-energy relationship should exist between the equilibrium or rate of behavior of a benzene derivative and a series of corresponding meta- and para-monosubstituted benzene derivatives. The Hammett equation may be written as... 

Table 6.62. Substituent parameters of main groups Qp Hammett constant (monosubstituted benzenes) (F14) a Charton aliphatic constant (C26)...

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