Question

How do we know if a solution of a chiral compound is optically active?

Answer 1

How else but by experiment? You need (i) to find a chiral carbon, i.e. `CR_1R_2R_3R_4`; this stereoisomer must (ii) be present in quantity.

Explanation:

From the above, we need first to find a chiral carbon in your compound. This is a necessary but not sufficient condition. This optical antipode, this stereoisomer, must be present in quantity, for its solution to be optically active.

If there is a `50:50` mix of stereoisomers, then a solution of this mixture will NOT rotate plane-polarized light. A solution of the 1 stereoisomer, will rotate plane-polarized light.