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Chapter 41 Suggested solutions for Chapter 41 369 The Hammett p value of -2.0 gives very definite evidence that π participa tion does not occur. If early work of It did, the closure of the unstable three-membered ring would undoubtedly be the slow step and a and group, J. Org. Chem., 1952, 17, positive charge would be formed inside the benzene ring itself. This would give a much larger p 1511; 1954, 19, 957. alue of something like -5.0 or more. One reason participation does not occur is that the starting material is planar and the p orbitals in the benzene ring cannot point in the right direction to interact with the σ* orbital of the N-O bond. They are orthogonal to it. Problem 4 Between pH 2 and 7, the rate of hydrolysis of this thiol ester is independent of pH. At pH 5 the rate is proportional to the concentration of acetate ion [AcO⁻] in the solution and the reaction goes twice as fast in as in D₂O. Suggest a mechanism for the pH-independent hydrolysis. Above pH 7, the rate increases with pH. What kind of change is this? D NaOAc SEt oH Purpose of the problem You might have started your Time for you to try your skill at interpreting pH-rate profiles. answer by sketching the pH-rate profile (p. 1104). Suggested solution The second part of the question is easily dealt with. In alkaline solution the rate of hydrolysis of this ester simply increases with pH and we have the normal specific base-catalysed reaction in which hydroxide ion attacks the carbonyl group. 0 0 0 rate-determining step e SEt SEt oH oH Fast in comparison with attack on But this is no ordinary ester. The leaving group is a thiol (pKₐ about 8) not the usual alcohol CH₃CO₂Et but still the slow step. about 16) and so the thiolate anion is a much better leaving group than Also. the CF₃ group is very electron-withdrawing so nucleophilic attack on the carbonyl group will be unusually fast. This is why there is a region of pH-independent hydrolysis not found with EtOAc (p. 1104). You could have suggested that acetate was a nucleophilic or a general base catalyst but the solvent deuterium This is a much simplified description of a series of isotope effect suggests that it is a general base. experiments described by L. R. Fedor and T. C. Bruice, J. Am. Chem. Soc., 1965, 87, 4138. 0 rate-determining step 0 0 general base catalysis SEt SEt F₃C OH + EtS OH H H OAc Problem 5 In acid solution, the hydrolysis of this carbodiimide has a Hammett p value of -0.8. What mechanism might account for this? N Ar Ar ArNH₂ N H2O