Bronsted relation deviations from

Remark b. Because nucleophilicity is usually recorded in organic solvents and pKa in water (or water/methanol mixtures), this can make a large difference. Answer The nucleophilicities of a series of molecules in different organic solvents are correlated with excellent correlation coefficients (r). Furthermore, correlations of each series with pKa in the case of non-ortho substituents is also very good. That the slope, a, of the Bronsted relation is an estimate of transition state location may be questioned (78T2331). Deviations from the Bronsted a seem to be well established as a measure of steric effects. 

Bell has examined some of the reasons for expecting gross deviations from the Bronsted relation and has suggested that both positive and negative deviations may occur if the transition state represents a structure considerably different in its charge distribution than the base of the conjugate acid (or vice versa). 

In its application to catalysts, the preceding paragraph suggests that pseudo-acids or bases will be ineffective catalysts compared with other acids or bases of the same strength i.e., they should exhibit negative deviations from the Bronsted relation. There are a number of isolated observations which confirm this idea. Thus Bronsted and Pedersen (15) found that the nitrourethane ion had an unexpectedly small catalytic... 

The right-hand side of Table II shows that certain catalysts exhibit large positive deviations from the Bronsted relation. This can be interpreted if we remember that both the carboxylic acids and the phenols undergo some structural rearrangement on ionization. The carboxylate... 

Apparent deviations from the Bronsted relation can also occur if the equilibrium used to define the acid-base strength of the catalyst involves subsidiary equilibria in addition to the gain or loss of a proton, and such... 

A similar equation can be derived for acid catalysis by HX and HY. The expression in brackets in (124) represents the deviation from the simple Bronsted relation caused by the presence of the subsidiary equilibria in (122) if Xi = Xj no deviation will be observed, since we shall then have Kx = Ky = K. 2i deviation exists, it is clear that the separate values of Xi and X2 cannot be derived from observations on a single catalysed reaction, though in principle they can be obtained if the deviations are measured for two reactions with different values of p. However, a simpler result is obtained if, as is often the case, either x or X2 is close to zero. Thus if X2 = 0, corresponding to an anion which exists in a single form, (124) becomes... 

---