Question

A compound is having an observed rotation of +8.4. The rotation for the pure enantiomer is calculated as +12. The composition of mixture in terms of dextrorotatory $\left(d\right)$ and levorotatory $\left(l\right)$ is:

• A. $d=70\%,l=30\%$
• B. $d=30\%,l=70\%$
• C. $d=15\%,l=85\%$
• D. $d=85\%,l=15\%$

Answer 1

The correct option is D $d=85\%,l=15\%$

$\text{Optical purity}=\text{Enantiomeric excess}$
$\%\text{Optical purity}=\frac{\text{Observed optical rotation}}{\text{Optical rotation of pure enantiomer}}\times 100\%$

$\%\text{Optical purity}=\frac{8.4}{12}\times 100=70\%$
$d-l=70\%d+l=100\%$
Therefore,

$d=85\%l=15\%$