Steric hindrance at a given atom in a molecule is the congestion caused by the physical presence of the surrounding ligands, which may slow down or prevent reactions at the atom.
eg. 1:
In 1, the carbonyl carbon is bonded to two hydrogen atoms. In 2, it is bonded to a hydrogen atom and a methyl group. Since the methyl group is larger than the hydrogen atom, steric hindrance is greater at the carbonyl carbon in 2 than that in 1.
eg. 2:
In 1, the nitrogen atom is bonded to three hydrogen atoms; in 2, it is bonded to three methyl groups. A methyl group is larger than a hydrogen atom. Thus, the steric hindrance at the nitrogen atom in 2 is greater than that in 1.
eg. 3:
In 3, C1 is doubly bonded to a carbon atom and singly bonded to two hydrogen atoms, whereas C2 is doubly bonded to a carbon atom and singly bonded to two ethyl groups. An ethyl group is larger than a hydrogen atom. Thus, the steric hindrance at C2 is greater than that at C1.
see also steric strain
