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.