Steric Considerations

Monosubstituted Benzenes

For monosubstituted benzenes especially those substituted with large groups (t-Bu, i-Pr), the para product will predominate in most EAS reactions.  For example the bromination of t-butylbenzene would result in primarily the para product because of steric hinderance at the ortho site.

Polysubstituted Benzenes

As you would expect in the reaction of 1,4-disubstituted benzenes the electrophile will attach at the least sterically hindered site.

With 1,3-dibsubstituted benzene its very unlikely (more difficult) to react between the two substituents as in the following example.