Aromatic reactivity. Part XXXII. Detritiation and desilylation in trifluoroacetic acid. The solvent isotope effect, and the influence of added water and salts

Abstract

The rates at 25·0° of detritiation of 4-tritio-m-xylene and of cleavage (desilylation) of p-chlorophenyltrimethylsilane by trifluoroacetic acid increase to a maximum (at ca. 80–83 and 83–87 mole-% acid, respectively), and then decrease as water is added. This is attributed to competition between the (rate-increasing) solvating effect of water on the polar transition state and the (rate-decreasing) lowering of the acidity of the medium as water, a base, is added. The degree of protonation of 2,4-dichloro-6-nitroaniline, and thus the –H₀ acidity function, pass through similar maxima. The maxima are less pronounced when CF₃·CO₂D–D₂O mixtures are used in place of CF₃·CO₂H–H₂O mixtures. Solvent isotope effects are large in the desilylation (k_H/k_D is 6·2 in the anhydrous acid, and 7·3 in the medium giving the maximum rate), but are smaller (correspondingly, 1·75 and 1·9) in the detritiation because of the reversibility of the initial proton transfer in this reaction. Added lithium trifluoroacetate has little effect on the rates, but lithium chloride and bromide and perchlorate have large effects which seem to result from formation of hydrogen bromide or chloride or perchloric acid. Oxonium ions seem to play no significant part, even when 40 mole-% of water is present, and the reactions are thought to involve proton transfers from the dimeric species (CF₃·CO₂H)₂ or the monohydrate CF₃·CO₂H, H₂O, to give the intermediates ArHT⁺ and ArHSiMe₃⁺, the proton transfer being wholly rate-determining in the desilylation. Initial-state stabilisation of aryltrimethylsilanes by d_π–p_π bonding is suggested as an explanation of the smallness of ring-substituent effects in desilylation compared with those in detritiation.