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1楼 2012-11-14 20:20:45

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Small substituents in the ortho-position have a beneficial effect: restriction of rotation around the Ar-O bond, leads to a decrease in entropy upon approach to the transition state for the rearrangement. The relative rates of reaction between O-(o-tolyl) and O-(p-tolyl) N,N-dimethylthiocarbamates was 1.9. However, the 2,6-dimethyl analog reacts slightly more slowly, and when the size of substituents is increased from methyl to tert-butyl the rate decreases substantially, as steric compression upon approach overwhelms the favorable entropic effect