Acid Analysis

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Science  01 May 2009:
Vol. 324, Issue 5927, pp. 569
DOI: 10.1126/science.324_569b

Acids catalyze a wide range of chemical reactions by lending a proton to stabilize a particular intermediate structure. As Macht et al. show, however, the sensitivity of chemical rates to catalytic acid strength can vary in a subtle and at times surprising manner as a consequence of the reaction mechanism. The authors specifically compared the rate dependence of three reactions—dehydrations of 1- and 2-butanol and isomerization of hexane to 2-methylpentane—to the varied acid strengths of a range of solid acid polyoxometalate and zeolite catalysts. The overall barriers to reaction increased from 2-butanol dehydration to the alkyl isomerization, with 1-butanol dehydration effectively in the middle of the range. However, both dehydrations evidenced the same sensitivity to acid strength, whereas the alkyl isomerization sensitivity was roughly three times higher. To account for this result, the authors suggest that the concentrated charge distributions in the dehydration transition states are stabilized to some extent by conjugate base charge on the catalyst—an effect enhanced as acid strength drops—and so the overall impact of catalyst acidity is reduced. Conversely, the significance of this electrostatic stabilizing interaction is diminished for the more diffuse charge distribution in the isomerization transition state, resulting in a more pronounced barrier dependence on acid strength.

J. Am. Chem. Soc. 131, 10.1021/ja900829x (2009)

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