Question

Butane $-2,3-$ diol has how many optically active stereoisomers?

• A. 3
• B. 4
• C. 2
• D. 1

Answer 1

Answer: (C)

Substances possess the property to rotate the plane of polarized light are called optical active.
$$\begin{gathered} {\text{CH}}_3-{\text{CH}}_{\text{|} \\ \text{OH}}^{\star }-{\text{CH}}_{\text{|} \\ \text{OH}}^{\star }-{\text{CH}}_3 \end{gathered}$$
In it two asymmetric carbon atoms and a plane of symmetry are present. So, optically active isomers $=2^{n-1}=2^{2-1}=2$
( $n=$ Number of asymmetric carbon atoms)

$d$ and $l$ are the non-superimposable mirror image of each other and can rotate polarized light. Meso is optically inactive because it contain plane of symmetry.