An example of steric effects is steric hindrance. This is when a large group in a molecule makes reactions not work. For example, an S2 reaction does not happen on carbon atoms that have three substituents. Steric hindrance however can also be a positive thing. Sometimes, chemists would like a reaction to happen in one place rather than another one. A bulky group can block a position. It can also change the shape such that only one diastereomer from a reaction is made.