2-Nitro-phenol acidity constant

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... 

A nitro group substituted in phenol should increase the acid constant by virtue of the inductive effect of the electronegative group (with N+ attached to the ring) moreover, in the ortho and para positions there would occur an additional resonance effect, due to the contribution of structures such as the following ... 

These place a positive charge on the oxygen atom of the unionized molecule, and so cause it to repel the proton. On analysis of the experimental values for Ka at 25°C it is found that the inductive effect of a nitro group increases Ka by a factor of about 45, and the resonance effect in the ortho and para positions gives another factor of about 22. The acid constant of a nitrophenoi can be found approximately by multiplying that for phenol, l.t X 10 10, by the factor 45 for every meta nitro group and 1000 for every ortho or para nitro group in the molecule. The comparison of the values calculated in this way with those found by experiment is shown in Table 8-1. ... 

The simpler equation (4) embodies the Ostwald conception with the difference that Km is not the true dissociation constant but the apparerd constant of the indicator since it represents the product of the true dissociation constant and the equilibrium constant for the normal and aci-forms. The latter equilibrium favors the normal compound in the case of p-nitrophenol so that this substance appears to be a very weak acid. With o-nitro-phenol, however, the existence of the aci-form is favored so that this compound behaves as a stronger acid. The ratio of aci to normal is so large in the case of picric acid that relatively much of the aci- or ionogen form, as compared with the pseudo-compound, is present in aqueous solution. Consequently this substance is a rather strong acid. As the apparent dissociation constant increases, the intensity of the yellow color of aqueous solutions must likewise grow because more of the aci-form will be found in solution. This statement can be confirmed easily. Picric acid in water solutions is yellow, but colorless in organic solvents due to the predominance of the pseudo-form. 

This phenomenon is not possible in p-nitrobenzoic acid hence, p-nitrophenol is a stronger acid with respect to p-nitrobenzoic acid than is expected on the basis of a comparison of substituents in which this resonance delocalization is not an important factor. It was, therefore, recommended that Op = 1.27 be used for p-nitro derivatives of phenols and anilines, rather than the Op = 0.78 given in Table 7-10. These enhanced sigma constants, symbolized a, apply primarily to electron-withdrawing groups in reactions aided by low electron density at the reaction site. 

The relative sizes of the Hammett p and Bronsted a constants will determine the relative rate of 5-nitrosalicylamide. If intramolecular base catalysis applies, then 5-nitrosalicylamide should hydrolyse more rapidly, since the nitro group will increase the susceptibility of the amide bond to attack by hydroxide ion and increase the efficiency of the phenolic hydroxyl as a general acid catalyst. The value of Jtobs at the plateau region was found to be 18 times smaller for the 5-nitrosalicylamide than for salicylamide a mechanism of intramolecular general base catalysis is, therefore, the preferred mechanism. 

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... 

The chemical information available through LFER is primarily the reaction constant p, but this value depends upon the substituent constants selected for the construction of the LFER. The u values available are ct, ct", ct" or and Ui, these quantities are listed, for many substituents, in Tables 7-1, 7-3, 7-4, 7-6, and 7-7. A reasonable approach is to plot log k against the substituent constant defined by a standard reaction that is expected to be most like the reaction under study. It is also reasonable to plot log k against several of the ct quantities, seeking the best correlation. [In choosing between two types of substituent constants, it is necessary to make use of substituents for which the two scales (say ct and rr, for example) are not themselves correlated, for otherwise both LFERs will be acceptable. ] The ct or o constants should be applicable to reactions that do not combine reaction sites and para substituents of the + and — type (push-pull systems capable of through resonance) for example, one would not expect ct" or o to provide good correlations for reactions of phenols or anilines substituted with nitro or cyano or for reactions of benzoic acids substituted with amino or methoxy. 

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