Proton transfer to unsaturated carbon

In this section rate-equilibrium correlations for proton transfer to olefins and aromatic systems will be discussed. Although the kinetic behaviour varies from one unsaturated system to another some general features will become apparent. Most results for proton transfer involving unsaturated carbon have been obtained by studies of an overall reaction in which proton transfer to carbon is involved as a rate-determining step. The mechanisms of reactions of this type were discussed in Sects. 2.2.3 and 2.2.4. In these cases the rate coefficient for proton addition to form a carbonium ion is obtained. However, a few examples will be described where the equilibrium between an unsaturated system and a carbonium ion has been measured giving rate coefficients in both directions. 

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]. 

Complete kinetic data [48] have been obtained for the olefin l,l-bis(p-dimethylaminophenyl)ethylene (XXI) and its carbonium ion, viz. 

Equilibrium protonation to give the carbonium ion can be observed in 80 % (v/v) methanol—water containing buffers. Proton removal from the carbonium ion (pX ca. 2.7 in this solvent) by acetate and chloroacetate is thermodynamically favourable but occurs with rate coefficients of 19.1 and 4.7 1 mole-1 sec-1, respectively, which are well below the values which would be found for normal proton transfer. Protonation of the olefin by hydrogen ion is thermodynamically favourable but occurs slowly with rate coefficient 23 1 mole-1 sec-1. These results clearly show that protonation of olefinic carbon belongs to the category of slow proton transfers. 

Variation of Bronsted exponents with reactivity for the acid catalysed hydrolysis of vinyl ethers in aqueous solution (Reprinted with permission from A. J. Kresge et al., J. Am. Chem. Soc., 93 (1971) 413. Copyright by The American Chemical Society.) 

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One further reaction of this type studied by Kreevoy and his group [50] is the acid catalysed cleavage of unsaturated compounds containing a carbon—mercury bond, for example, allyl mercuric iodide (29). The important conclusion is that rate-determining proton transfer to unsaturated carbon occurs as a first step (A—SE2 mechanism) and as expected the reactions are catalysed by general acids. 

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... 

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