A synthetic chemist mixes a pure dextrorotatory () enantiomer with its corresponding pure levorotato — Isomerism and Stereochemistry Chemistry Question
Question
A synthetic chemist mixes a pure dextrorotatory ($d$) enantiomer with its corresponding pure levorotatory ($l$) enantiomer. If the specific rotation of the pure '$d$' isomer is uniquely documented as $+8^\circ$, but the observed specific rotation of the resulting un-separated mixture is exactly $+4^\circ$, what is the calculated optical purity (enantiomeric excess) of the final mixture in percentage (%)?
💡 Solution & Explanation
Optical Purity (Enantiomeric Excess, ee) is defined mathematically as the ratio of the observed specific rotation of the mixture to the specific rotation of the pure enantiomer, converted to a percentage: $\text{Optical Purity (\%)} = \left | \frac{[\alpha]_{mixture}}{[\alpha]_{pure}} \right | \times 100$. Given $[\alpha]_{mixture} = +4^\circ$ and $[\alpha]_{pure} = +8^\circ$. $\text{Optical Purity} = (4 / 8) \times 100 = 50\%$.