2-chlorobutane is an optically active molecule. A compound to be said as optically active is only if it has to have an asymmetric chiral carbon centre. A carbon is said to be chiral if the have different substituents attached. Here the carbon numbered as 2 in the molecule is chiral as it has different substituents [-c2H5, cH3, H, Cl] attached to it. It gives rise to enantiomers by rotating the plane polarized light in different directions (R) and (s) of then compound.