Hammett equation pyridines

The only reaction series involving a reaction at a heteroatom which has been extensively studied is the basicity of substituted pyridines. The Hammett equation was first applied to this reaction simultaneously by Brown and co-workers and by Jaffe and Doak. A sampling... 

Chapman and co-workers have investigated the nucleophilic displacement of chlorine in various chloronitropyridines by three pyridines. In each of these series of three compounds, an excellent correlation is observed, but, again, longer series would be extremely desirable. Similarly, the nucleophilic attack of a series of four pyridines on propylene oxide follows the Hammett equation with high pre-... 

Pyridine bases are well known as ligands in complexes of transition metals, and it might well be anticipated that the equilibrium constants for the formation of such complexes, which are likely to be closely related to the base strength, would follow the Hammett equation. Surprisingly, only very few quantitative studies of such equilibria seem to have been reported, and these only for very short series of compounds. Thus, Murmann and Basolo have reported the formation constants, in aqueous solution at 25°, of the silver(I) complexes... 

Finally, two sets of physical properties have been correlated by the Hammett equation. Sharpe and Walker have shown that changes in dipole moment are approximately linearly correlated with ct-values, and Snyder has recently correlated the free energies of adsorption of a series of substituted pyridines with u-values. All the reaction constants for the series discussed are summarized in Table V. 

In the manner outlined, a few attempts have been made to apply the Hammett equation to the transmission of substituent effects in the pyridine series. In the alkaline hydrolysis of 5-substituted ethyl picolinates (5-R-2-COOEt) in 85% ethanol at 25, 35, and 45°, the reaction constants are about 60% as large as those in the corresponding benzene series the overall fit to the Hammett equation, however, is at best fair, since out of four points (R = Et, H, I, Ac) one (Ac) deviates widely. 

The first reaction series to be considered are the basicities of the various quinolines. Baciocchi and Illuminati have demonstrated that the pK values of quinolines substituted in the B-ring follow the Hammett equation well if ApK, i.e., the difference between the pK values of substituted and unsubstituted compounds, is plotted against a, the quinoline points fall on the same line as the pyridine points, as shown in Fig. 5, so that the p-values for the two series are identical. 

These constants, K toK/, may be estimated by use of the Hammett equation. Estimation of 1 and K 4 involves application of the methods outlined in Section II, A, i.e., application of substituent constants for and N+H to the Hammett equation for the acid-base equilibria of benzoic acids. Estimation of A2 and involves application of the method used in Section III,A, i.e., the p-value for the basicity of substituted pyridines, with cr-values for COOH and COO . Provided the necessary a- and p-values are known, this procedure permits the calculation of four independent, or virtually independent, estimates of Krp. A check on the method is available from the relationships shown in Eq. (16) which is readily obtained by multiplication of Eq. (12) and (14) and of Eq. (13) and (15). 

A number of correlations of ionization potentials for substituted benzenes (40-42), benzyl (43), phenoxy (44), and alkyl (45) radicals and substituted pyridines (46) with the simple Hammett equation have been reported. Charton (47) has studied the application of the extended Hammett equation to substituted ethylenes and carbonyl compounds. The sets studied here are reported in Table II (sets 2-10 and 2-11). Results of the correlations are set forth in Table 111. The results obtained are much improved by the exclusion of the values for X = C2 H3, Ac, F, H and OAc from set 2-10 (set 2-lOA) and the value for X = H from set 2-11 (set 2-11 A). The composition of the electrical effect corresponds to that found for the Op constants as is shown by the pR values reported in Table IV. 

Values of ka and b are correlated by the Hammett equation for pyridines and for triaryl phosphines. Very large negative p values were found for pyridine reactions especially (35). Key values are highlighted in Table II. 

While it has been known for some time that pyridine-1-oxides are protonated on oxygen (Gardner and Katritzky, 1957), amino-pyridine-1-oxides have attracted more attention recently. Using the Hammett equation, it was concluded that the cation of 3-amino-pyridine-1-oxide was an equilibrium mixture of O- and N-protonated forms in the ratio 8 1 (Jaffe, 1965), A similar conclusion was reached for 3-dimethylaminopyridine-l-oxide (Forsythe et al.,... 

Clearly, the multifarious factors at work on SCS in pyridines as well as other aromatics can be matched by the more complex manifestations of the Hammett equation and its extensions to provide some sort of correlation, but no systematic overall correlation or related theory has emerged as yet. 

In Table II are given recent values reported for pyridine o constants. These values show a high degree of consistency and conformity, estimated within the usual accuracy of Hammett equation correlations. This... 

In fact this is a specific example of a general contention116 that in all heterocyclic systems Eq. (3) rather than the straightforward Hammett equation is the more correct. Equation (15), however, gives no indication that electron acceptors influence pK values of pyridinium ions by induction alone, although this circumstance has been carefully authenticated,104,109 but it does give voice to the contention that a correlation with o+ would be more realistic, to allow for the presence of canonical 19 in y-substituted pyridines for groups of +R character. Evidence... 

Pyridines are also well known as ligands in transition metal complexes, and if the equilibrium constants for the formation of such complexes can be related to base strength, it is expected that such constants would follow the Hammett equation. The problem has been reviewed,140 and a parameter S, formulated which is a measure of the contribution of the additional stabilization produced by bond formation to the stabilization constants of complexes expressed in terms of a.141 The Hammett equation has also been applied to pyridine 1 1 complexation with Zn(II), Cd(II), and Hg(II) a,/3,y,<5-tetraphenylporphins,142 143 the a values being taken as measures of cation polarizing ability. Variation of the enthalpy of complexation for adducts of bis(2,4-pentanediono)-Cu(II) with pyridines plotted against a, however, exhibited a curved relationship.144... 

The o values of rings annelated to pyridine in systems such as quinoline, isoquinoline, benzoquinolines, acridine, and phenanthridine and determined by the basicity of the ring nitrogen atom are predicted186 and found to be small.33,34 This is an important point stressed and elucidated previously,181 and demonstrating the validity of the basic assumptions of the Hammett equation. 

A detailed analysis of substituent effects on the pKa values of imidazoles and tetrazoles as well as benzimidazoles and naphthimidazoles has been made.308 The ortho effect is shown to parallel that of 2-substituted pyridines and quinolines47 190 and application of the Hammett equation to the tautomerism of these systems is also considered. The equation also satisfies the effect of substituents on the basicity of the nitrogen in the 1-position for 1-pyrazolines.309... 

The reaction of vanadium salts (VC14, V202(S04)2, V(OCOMe)4, VO(acac)2) and porphyrin gives a complex which, after work-up, is isolated as a very stable oxovanadium(IV) species, VO(Por) (for VO(OEP), V—O = 1.620(2)A, V—= 2.102A, A(N4) = 0.543 A).17 -21 In the presence of a large excess of nitrogenous ligand such as pyridine and piperidine, it forms a six-coordinate complex with a small equilibrium constant (K = 10 I-10 21 M-1).18 The effect of / substituents on the association constant (Kx) is expressed by the Hammett equation (equation 4). 

Evaluation of steric effects can also be made by separating electronic from steric effects with the help of linear free energy relationship and appropriate parameters. Applications of the Hammett equation to heterocycles have been reviewed (64AHC(3)209 76AHC(20)1) and the influence of substituent effects on the basicity and N-alkylation of pyridines, which have been by far the most widely studied, shows the difficulties in this approach. Jaffe and Jones (64AHC(3)209) reported a good correlation between pKA of 3- and 4-substituted pyridines and Hammett a parameters ([Pg.179]

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