Butane—The Gauche Interaction

The notions of eclipsing and gauche interactions are applicable not only to stable organic structures, but also to reactive intermediates. In fact, as described in the next Going Deeper highlight, the geometry of radicals is influenced by a conformational effect similar to those discussed for standard alkanes. 

In Chapter 1 we noted that radicals prefer planar structures, but the barrier to pyramidalization is very low. In fact, there is a conformational effect that leads to pyramidalization. Computational studies by Padden-Row and Houk have shown that even the simplest alkyl radicals are pyramidalized, and methyl radical is really the only totally planar radical. At right are shown three examples. The interpretation of these structures invokes simple conformational analysis. In each case, pyramidalization occurs in the direction that minimizes eclipsing interactions. 

For ethyl and isopropyl the effect is not large. Theory predicts, however, that the f-butyl is substantially pyramidalized, with a C-C-C angle of 117.3°. For f-butyl and the other alkyl radicals, the barrier to pyramidal inversion is quite small. One measurement suggests a barrier of roughly 0.45 kcal / mol for t-butyl radical. So, under most experimental conditions, simple alkyl radicals can be 

Number of Gauche Interactions in Variously Substituted C-C Bonds 

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