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General acid and general base catalysis of the enolization of acetone : an extensive study

University of British Columbia

The enolization of acetone is subject to both general acid and general base catalysis, eqs. [1.10] and [1.78]. The Bronsted equation relates the catalyzing power of the acid or base to the equilibrium acid or base strength of the species involved, eqs. [1.14] and [1.13]. [Formula Omitted] This work involved measuring enolization rate constants for over 130 acid and base catalysts. These include monoprotic and diprotic carboxylic acids, phosphonic acids, carboxylate monoanions and dianions and phosphonate dianions. A number of the conjugate bases of the diprotic acids, i.e. bifunctional monoanions, were also examined. A number of effects were probed by an examination of the resulting Bronsted plots. In the case of general acid catalysis, a number of sterically crowded catalysts displayed enhanced catalytic activity. That is to say, they are more effective in the enolization process than their equilibrium acid strengths would suggest. This result was evident in both carboxylic acid and phosphonic acid catalysis, but not in general base catalysis. The role of steric factors in these processes is unclear. A group of bifunctional monoanions are shown to act as general acids in the enolization process. A comparison of 5- and 2 - substituted isophthalate monoanions reveals a steric accelerating effect for the 2-substituted species, a result consistent with earlier observations. Species with polarizable substituents are better catalysts, in both acid and base catalysis, and this result is explained. A set of carboxylate dianions, whose conjugate monoanions possess no hydrogen-bonding, form a reasonable Bronsted line. A pair of dianions deviate below this line, and the degree of deviation is shown to be related to the degree of hydrogen-bonding present in the conjugate monoanions. The group of phosphonate dianions gives a curved Bronsted plot suggesting a changing transition state as the base strength of the catalyst is varied. In terms of the Hammond postulate, a more favourable proton transfer (involving a stronger base) is leading to an earlier, more reactant-like transition state i.e. the proton transfer is occurring earlier along the reaction coordinate. The curvature is analyzed in terms of Marcus Theory and gives experimental support to the concepts of transition state energetics that are encompassed in the Hammond and Marcus models. A correlation of primary isotope effects with the curved Bronsted line is presented and this further maps out the varying degrees of proton transfer in the transition state. A number of other examples of experimental evidence for the Hammond postulate are presented. Carboxylic monoprotic acids, free from both polarizability and steric factors, appear to form a curved Bronsted line. While the curvature is in the direction predicted by Marcus Theory, primary isotope effects suggest that a changing transiiton state is not a cause of the curvature. As well as the topics mentioned here, a number of other facets of the reaction are discussed, including electrostatic factors. It is shown, for example, that including catalysts of varying charge in a single Bronsted correlation must be done with caution.

Text

2010-10-19

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http://hdl.handle.net/2429/29389

Chemistry

University of British Columbia

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