A molecule is optically active if it is non-superimposable on its mirror image (chiral).

• 2-Chlorobutane structure: $C{H}_3-\mathbf{CH}(\mathbf{Cl})-C{H}_{2}C{H}_3$
• The second carbon (C2) is bonded to four completely different groups:

1. $-H$
2. $-Cl$
3. $-C{H}_3$ (Methyl)
4. $-C{H}_{2}C{H}_3$ (Ethyl)

• Therefore, C2 is a Chiral Center (Asymmetric Carbon). It lacks any plane or center of symmetry, allowing for the existence of two enantiomers that rotate plane-polarized light.