an example of steric effects is steric hindrance. this is when a large group in a molecule makes reactions not work. for example, an sn2 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.