The unimolecular mechanism of solvolysis

Crunden and Hudson80, by comparison of solvolysis rates in formic acid and aqueous acetone (although the same authors have stated that this may be coincidental), obtained a value of 40% SN1 participation in aqueous acetone (65%) they also estimated the contribution of the SN1 route to the solvolysis of substituted benzoyl halides (Table 6). It must, however, be emphasized that the nature of the intermediate trapped in the work of Gold et al81 could either be the acylium ion RCO+ or the hydrated acylium ion [RC0H20]+. 

The positive entropy of activation (+5,6 eu, see Table 2, p. 219)57 for the hydrolysis of dimethylcarbamoyl chloride is an indication of a reaction 

Bevan and Hudson86 have reported that the hydrolysis of benzoyl fluoride is acid-catalysed and follows the [H30+], whereas other unhindered benzoyl halides do not. A parallel situation is found with the benzyl fluoride and chloride. Satchell87 has likewise demonstrated the hydrolysis of acetyl fluoride and chloride to be acid-catalysed. 

The existence of acylium ions in the reactions of acyl halides with Lewis acids has been amply demonstrated88,89, but they have not so far been directly observed in hydrolysis reactions. To achieve direct observation of an acylium ion, it must, at best, be generated into a non-reactive environment if its rate of formation is slow, or be detectable in small concentrations if it is destroyed rapidly. The limiting factors are, therefore, the rate and mechanism of heterolysis and the stability and concentration of the acylium ion formed. In this 

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