Steric factors in organic reactions have to do with the "bumping" of the van der Waals radii of individual atoms of a molecule. Steric hindrance will prevent some reactions from occuring at all. For example, tertiary halides will not undergo Sn2 displacement, even when treated with very strong nucleophiles. The three carbon atoms plus their associated hydrogen atoms simply take up too much space for the nucleophile to be able to attack the substrate carbon atom from behind the halide.

Steric factors also affect conformations of molecules. Cyclohexanes with t-butyl substituents will exist almost exclusively in a boat conformation with the t-Bu substituent in an equatorial position.

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