Proton transfers to cyanocarbon bases

Besides the outer sphere electron transfers, we have identified (Albery, 1975d) another class of reactions that exhibit Case III behaviour, and this example is proton transfer to cyanocarbon bases. These reactions were studied by Long and co-workers. First, by using tritium, they measured the fractionation factor for the tritium as it was pulled off the carbon as in [1], The results 

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Simple electron transfer at electrodes Proton transfer to cyanocarbon bases... 

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. 

General base catalysed halogenation of carbon acids often involves a rate-determining proton transfer from carbon, and halogenation is a technique which is frequently used for obtaining rates of proton transfer. The technique has been developed particularly by Bell [34] to measure rates of proton transfer from ketones. Other carbon acids have been studied, however, for example, esters [35], sulphonates [36], nitro-paraffins [37], and cyanocarbons [38]. Under chosen conditions which may vary depending upon the substrate, the mechanism, illustrated for acetone in eqns. (19) and (20),... 

A value of kH/kD = 1.4 was obtained [114] for the rate of proton transfer compared with deuteron transfer from chloroform to hydroxide ion and this result is similar to the values determined earlier for several haloforms [164, 166]. In the most recent work [171(b)] a value kH /kD = 1.11 0.05 was determined for chloroform. These values are close to those observed for reaction of cyanocarbon acids (though a different base catalyst is involved) and in Sect. 4.3 it was argued that isotope effects as low as these are expected for a transition state in which proton transfer is almost complete. The isotope effect for proton transfer from chloroform was measured using a new and useful method [114]. It can be shown that the ratio of initial rates of uptake of tritium for the first ten per cent of reaction from tritiated water into CHC13 and CDC13 is identical to the primary isotope effect for proton loss (feH /fcD). The procedure can be used for measuring isotope effects on proton transfer from carbon acids to hydroxide ion or buffer catalysts and is more convenient than other methods. Other methods which have been used, for example, involve the comparison of rates of detritiation and dedeuteration or the comparison of rates of bromination for isotopically different acids (RCH and RCD) [113]. 

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