Proton acidity exponent

In contrast, for a proton transfer from a hydrogen-bonded acid occurring by the mechanism in (24), the predicted dependence of kf and on the strength of the catalysing base is quite different. In this case, when the pA bh of the base and the p -value of the hydrogen-bonded acid are closely matched, that is at Ap 0, it would be predicted that the proton in the transition state will be roughly half-transferred and Bronsted exponents a and p of around 0.5 should be observed. 

Obviously, these results cannot be explained by a very enol-like transition state. This does not mean that enol stability does not affect reactivity, but that it probably does so to a lesser extent than at first expected. Such a conclusion runs counter to the first assertions and to what is usually assumed (see e.g. Lamaty, 1976), but is in agreement with the data cited above concerning Bronsted a-exponents. Indeed, the a-value of 0.74 observed for acid-catalysed enolisation of cyclohexanone (Lienhard and Wang, 1969) corresponds to a relatively early transition state since the Bronsted / for base-promoted proton abstraction from the hydroxycarbenium ion intermediate [see eqn (3)] equals 1 — 0.74, or 0.26. As pointed out above, some data on the stereochemistry of ketonisation were accounted for by assuming an enol-like transition state. Clearly, these interpretations need to be re-examined. 

Similar Bronsted exponents, 0.94 0.02 for phenolate ions and 0.98 0.08 for secondary amines, were observed but the Bronsted plots for these two types of catalyst were separated by about half a unit in log 0 k. The values of the Bronsted exponents are close to the limiting values of unity expected for normal proton transfer. Reaction (78) is thermodynamically favourable in the reverse direction and for fully normal proton transfer the rate coefficients for recombination of the carbanions with phenols and ammonium ions should be around 101 0 1 mole"1 sec"1. Calculations using the approximate pif 21 for this acid measured [69] in dimethyl... 

The first mechanism (a) occurs if fe t < k2 and the observed rate coefficient is given by feobs = k1. The second mechanism (b) applies if fe i > fe2 and then kohs = k2 x K where K = fe1 /fe j. The two mechanisms which correspond, respectively, to a rate-determining proton transfer and a pre-equilibrium followed by a subsequent step have been discussed in detail for isotope exchange reactions in Sect. 2.2.1. The second possibility (b) is apparently favoured by Cram [120] for racemization of 2-methyl-3-phenylpropionitrile whereas Melander [119] has interpreted his results in terms of the first (a). From the variation of the rate coefficient for racemization in different solvent mixtures of methanol/ dimethylsulphoxide a Bronsted exponent j8 = 1.1 was calculated [119] using an acidity function method which will be described fully in Sect. 4.6. 

Bronsted exponents for base catalysed proton transfer from carbon acids determined by an acidity function technique in mixed solvents... 

A number of other proton transfer reactions from carbon which have been studied using this approach are shown in Table 8. The results should be treated with reserve as it has not yet been established fully that the derived Bronsted exponents correspond exactly with those determined in the conventional way. One problem concerns the assumption that the activity coefficient ratios cancel, but doubts have also been raised by one of the originators of the method that, unless solvent effects on the transition state are intermediate between those on the reactants and products, anomalous Bronsted exponents will be obtained [172(c)]. The Bronsted exponents determined for menthone and the other ketones in Table 8 are roughly those expected by comparison with the values obtained for ketones using the conventional procedure (Table 2). For nitroethane the two values j3 = 0.72 and 0.65 which are shown in Table 8 result from the use of different H functions determined with amine and carbon acid indicators, respectively. Both values are roughly similar to the values (0.50 [103], 0.65 [104]), obtained by varying the base catalyst in aqueous solution. The result for 2-methyl-3-phenylpropionitrile fits in well with the exponents determined for malononitriles by general base catalysis but differs from the value j3 0.71 shown for l,4-dicyano-2-butene in Table 8. This latter result is also different from the values j3 = 0.94 and 0.98 determined for l,4-dicyano-2-butene in aqueous solution with phenolate ions and amines, respectively. However, the different results for l,4-dicyano-2-butene are to be expected, since hydroxide ion is the base catalyst used in the acidity function procedure and this does not fit the Bronsted plot observed for phenolate ions and amines. The primary kinetic isotope effects [114] also show that there are differences between the hydroxide ion catalysed reaction (feH/feD = 3.5) and the reaction catalysed by phenolate ions (kH /kP = 1.4). The result for chloroform, (3 = 0.98 shown in Table 8, fits in satisfactorily with the most recent results for amine catalysed detritiation [171(a)] from which a value 3 = 1.15 0.07 was obtained. 

It has been claimed [114] that since a limiting Bronsted exponent is observed for the ionization of chloroform this carbon acid is fully normal in its proton transfer behaviour. This behaviour is explained by arguing that the carbanion is tetrahedral and the charge is localized on carbon [114]. Hence results are obtained which are similar to those for oxygen and nitrogen acids which also ionize to give a base with the electrons localized on one atom. Phenylacetylene shows similar behaviour and the carbanion may also possess an electron pair localized on carbon [143]. The results for all carbon acids will be compared in Sect. 5 and this point will be discussed in more detail. 

The expected change in Bronsted exponent with change in reactivity is illustrated by the results [49] shown in Table 9 for the hydrolysis of vinyl ethers (mono alkoxy-activated olefins) which occurs by initial slow protonation of olefinic carbon as in mechanism (28). The value of R which is the catalytic coefficient for an acid of pK 4.0 calculated from results for carboxylic acids with pK around 4.0 is taken as a measure of the reactivity of the system. The correlation of a with reactivity is scattered but the trend is in the expected direction. The results are quite similar to those shown for the ionization of ketones in Table 2. For the proton transfers shown in Table 9 the Bronsted exponent has not reached the limiting value of zero or unity even when reaction in one direction is very strongly thermodynamically favourable. The rate coefficient in the favourable direction is probably well below the diffusion limit, although this cannot be checked for the vinyl ethers. Non-limiting values for the Bronsted exponent have also been measured in the hydrolysis of other vinyl ethers [176]. 

The mechanisms of two other reactions described in Sect. 2.2 involve slow proton transfer to unsaturated carbon. The general acid catalysed cleavage of vinyl mercuric halides [42, 50] for example, allyl mercuric iodide, CH2=CHCH2HgI (XXII), gives Bronsted exponents around 0.7. Linear Bronsted plots are obtained with carboxylic acid catalysts but, as observed in other A—SE 2 reactions, general acids of different structural types (for example, hydronium ion or bisulphate ion) show substantial deviations. Bronsted catalysis of the hydrolysis of diazo compounds (N2 =CR X) has been studied by the groups of Albery and Kreevoy. With... 

Recently a new method has been developed for analysing rate-equilibrium data for proton transfer reactions (Marcus Theory) [200], Although the theory has not been tested extensively, it seems to have received fairly wide acceptance. This new treatment leads to various parameters which are useful in understanding results for carbon acids and offers an explanation for some anomalies in Bronsted plots such as curvature and Bronsted exponents outside the range 0 < a or j3 < 1. 

The small isotope effects observed in proton transfer from cyanocarbon acids to various bases shown in Table 3 (for example feH/feD = 1.46 for proton transfer from malononitrile to water) are compatible with an extremely product-like transition state in which the proton is almost fully transferred [113] (Sect. 4.3). Similar conclusions may be reached from the small isotope effects observed for chloroform (feH/feD = 1.41 0.01 [114] and 1.11 0.05 [171]) and phenylacetylene (kH/kD = 0.95 0.09 [143]) for reaction with hydroxide ion, and for reaction of disulphones with water (feH/feD = 2.2 0.1 [65]). In all these cases the magnitude of the Bronsted exponent is close to the limiting value of unity as expected for a product-like transition state. 

Subtract the of the acid from the P abH of the base to get the exponent of Teq. If the proton transfer K q is equal to or greater than 10 the proton transfer is within the useful range. 

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