Nitro group substituent constants

Because the difference in the shieldings between the two solvents for one nitrobenzene derivative is constant at almost 4.5 ppm, there is no evidence of any serious influence of substituents on the range and direction of solvent-induced variation in the shielding of the aromatic nitro group. 

V. SUBSTITUENT CONSTANTS OF THE NITRO GROUP FROM THE APPLICATION OF MODERN EXPERIMENTAL AND THEORETICAL... 

There is a continual tendency for the values of cr/ and aR (and other substituent chemical shifts for a series of mono-substituted benzenes in very dilute solution in cyclohexane, carbon tetrachloride or deuteriochloroform were the basis for a redefinition of the aR scale and some amendment of a R values. However, the value for the nitro group was confirmed as 0.15. 

In the anion-radicals of nitro compounds, an unpaired electron is localized on the nitro group and this localization depends on the nature of the core molecule bearing this nitro substituent. The value of the hyperfine coupling (HFC) constant in the electron-spin resonance (ESR) spectrum reflects the extent of localization of the unpaired electron values of several nitro compounds are given in Table 1.1. 

A simple case where the general a constants in Table 8.5 do not succeed in correlating acidity constants is when the acid or base function is in direct resonance with the substituent. This may occur in cases such as substituted phenols, anilines, and pyridines. For example, owing to resonance (see Fig. 8.4), a para nitro group decreases the pKa of phenol much more than would be predicted from the o para constant obtained from the dissociation of p-nitrobenzoic acid. In such resonance cases (another example would be the anilines), a special set of o values (denoted as oJpara) has been derived (Table 8.5) to try to account for both inductive and resonance... 

When the reaction site comes into direct resonance with the substituent, the a constants of the substituents do not succeed in correlating equilibrium or rate constants. For example a />-nitro group increases the ionization constant of phenol much more than would be predicted from the ov ND2 constant obtained from the ionization of />-nitrobenzoic acid. The reason is readily understood when one realizes that the />-nitrophenoxide ion has a resonance structure (11) in which the nitro group participates in through-resonance7 with the O-. The extra stabilization of the anion provided by this structure is not included in the ap NOs constant... 

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