What is steric hinderance? (Parts of molecules bump into each other due to the repulsive interaction between occupied orbitals. It slows down some chemicals reactions and affects shapes of molecules that tend to avoid such interactions. - GS). And how does it apply to Sn1 and Sn2 reactions? (As opposed to SN1 reactions, SN2 reactions are very sensitive to steric repulsion. - GS).