摘要:

J. CHEM. SOC. PERKIN TRANS. 2 1992 1855 The Brsnsted Acid-catalysed Hydrolysis of Benzoic Anhydride in Aqueous Solutions. Evidence for two Mechanisms and the Effect of Dioxane on their DetectabiIity Derek P. N. Satchella and Wasfy N. Wassefb a King's College London, Strand, London WC2R 2LS, UK Ain -Shams University, Heliopolis, Cairo, Egypt The hydrolysis of benzoic anhydride enjoys two mechanisms of acid catalysis in 60% (v/v) dioxane- water as solvent, one at low and another at high values of [H,O+]; the former becomes effectively undetectable in a purely aqueous solvent. Implications are discussed. At 25 "Cin 60% (v/v) dioxane-water as solvent, acid catalysis of the hydrolysis of benzoic anhydride is easily detectable at low Ot values of [H,O+]: the observed first-order rate constant, kobs, increases ca.40-fold between 0 and 3.0 mol dm-3 perchloric acid.' The details of the acidity and temperature dependences of kobf suggest an A2 mechanism of catalysis.' We find that this catalysis is effectively undetectable in purely aqueous solutions of strong acids: with sulfuric acid there occurs an approximately 50% increase in kobsbetween 0 and 3.0rnol dm-3 acid, while with perchloric acid there is a comparable decrease. Such small changes are attributable to salt effects.2 However, a powerful catalysis is detected at appreciably higher concentrations (Fig. 1). This is very probably of the A1 variety since a plot (Fig. 2) of log kobs-log [H,O+] uersus X (the excess acidity4) gives mt >2 (based on m* N 0.6 for this 0-base4) and the plot of log kobsuersus H, has dope 1.0.Re-examination of the behaviour in 60% dioxane-water over a wider range of acid concentrations than hitherto, supports the previous work, but shows (Fig. 3) that the initial increase in kobs wat low values of [H,O+] (which is proportional to [H,O+]) is 0 1 2 3 followed by the type of rapid acceleration found in purely X aqueous solutions (and that this occurs at a somewhat lower Fig. 2 Plot of log kobs -log [H30f] versus excess acidity [H,O+] value). Our values ofk,,, and AS$(ca. -100J K-' mol-') when [H,O+] <3.0 rnol dm-, are in satisfactory agreement 10 0 0 5 0 7 I .-c E 1 $ 00 9 0 0 oo0 0 0.01 0 00.005 00 0 0 0 0 2 4 6 0 5 10 [H30+]/moldrn'-3 [H2SO$stoich/m~ldm" Fig.3 Hydrolysis of benzoic anhydride in 60% (v/v) dioxane-water Fig. 1 Hydrolysis of benzoic anhydride in aqueous sulfuric acid at mixtures containing perchloric acid at 25 "C. [AnhydrideIinitia,= 25 "C.[Anhydride],,,,,, = 5 x mol dm-3. 5 x moldm-3. 1856 with the earlier work,' and we concur that an A2 mechanism of catalysis is occurring in this region. The sharp change in acidity dependence when [H,O+] 54 rnol dm-3 suggests that an A1 mechanism then quickly becomes dominant. When [H30+] = 6.0 mol dm-3, ASs --2 J K-' mol-', which is indicative of an A1 mechanism. The suggested change in mechanism is also supported by the behaviour ofp-toluic anhydride: at 25 "C this anhydride displays a kinetic pattern analogous to that of benzoic anhydride in 60% dioxane-water, but (significantly) leads to similar (or smaller) kobs values when [H,O+] <2.5 mol dm-3 (A2 region) and (>six-fold) larger values when [H,O+] =5.0 mol dmP3 (A1 region).Changes in mechanism of acid catalysis with changes in acid concentration have not been found previously for carboxylic anhydrides in any sol-vent. In our comparison of solvents, the effect of increasing the water content is greatly to increase the rate of spontaneous hydrolysis (for benzoic anhydride at 25 "C and [H30+] N 0, kobs 2: 0.3 x lo-' dm3 mol-' s-' in 60% dioxane-water and 3 x dm3 mol-' s-' in water) but to decrease the relative importance of the catalytic routes, such that the A2 route becomes effectively undetectable in water (30%dioxane shows intermediate behaviour).This result, other recent work of ours with acyl fluorides and aryl isothiocyanates,6 and certain earlier ~tudies,~.' all suggest that the addition to water of increasing amounts of a co-solvent unable to donate protons via hydrogen bonds, will increase the importance of acid-catalysed J. CHEM. SOC. PERKIN TRANS. 2 1992 mechanisms of hydrolysis relative to any spontaneous hydro- lysis, and sometimes reveal the presence of catalytic mechanisms not evident from the study of purely aqueous solutions. The generality of this result, not obviously predictable, has not been so apparent before. Studies of changes in acid-catalysed mechanism with changes in solvent acidity have so far been confined largely to aqueous sulfuric acid; we suggest that the use of mixed solvents-often avoided for fear of complications-may provide particularly sensitive media for such studies.References 1 C. A. Bunton and S. G. Perry, J. Chem. SOC.,1960,3070. 2 C. A. Bunton, J. H. Fendler, N. A. Fuller, S. G. Perry and J. Rocek, J. Chem. SOC.,1963,5361. 3 R. A. Cox and K. Yates, Can. J. Chem., 1979,57,2944. 4 R. A. Cox and K. Yates, J. Am. Chem. SOC.,1978,100,3861. 5 R. E. Motie, D. P. N. Satchell and W. N. Wassef, J. Chem., SOC., Perkin Trans. 2, 1992, 859. 6 V. B. Joseph, D. P. N. Satchell, R. S. Satchell and W. N. Wassef, J. Chem. Sac., Perkin Trans. 2,1992,339. 7 C. W. L. Bevan and R. F. Hudson, J. Chem. SOC.,1953,2187. 8 R. A. Cox, Ace. Chem. Rex, 1987,20,27. Paper 2/04878G Received 10th September 1992 Accepted 28th September 1992

ISSN:1472-779X    

DOI:10.1039/P29920001855

出版商:RSC    

年代:1992

数据来源: RSC