Dextrorotatory α -pinene has a specific rotation [ α ] D 20 = + 51. 3 ∘ . A sample of α -pinene containing both the enantiomers was found to have a specific rotation value [ α ] D 20 = + 30. 8 ∘ .

Question

Dextrorotatory α -pinene has a specific rotation [ α ] D 20 ​ = + 51. 3 ∘ . A sample of α -pinene containing both the enantiomers was found to have a specific rotation value [ α ] D 20 ​ = + 30. 8 ∘ . The percentage of the (+) enantiomer present in the sample is ____ .

TestHub · Accepted Answer

The enantiomeric excess (ee) is calculated as:

$ee = \frac{o b ser v e d s p ec i f i cro t a t i o n}{s p ec i f i cro t a t i o n o f p u ree nan t i o m er} \times 100%$

$ee = \frac{+ 30. 8 ^{\circ}}{+ 51. 3 ^{\circ}} \times 100% \approx 60%$

The percentage of the (+) enantiomer is given by:

$% (+) = \frac{100 + ee}{2} = \frac{100 + 60}{2} = 80%$

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