Question

Optical purity is defined as the ____________

(a) Ratio of rotation of pure enantiomer to rotation of mixture

(b) Ratio of rotation of mixture to rotation of pure enantiomer

(c) Product of rotation of mixture and rotation of pure enantiomer

(d) Sum of rotation of mixture and rotation of pure enantiomer

I have been asked this question in an internship interview.

The doubt is from Stereoselectivity and Stereospecificity topic in chapter Structure and Stereostructure of Molecules of Engineering Chemistry I

Answer 1

The correct choice is (b) Ratio of rotation of mixture to rotation of pure enantiomer

Explanation: Optical purity is defined as the ratio of rotation of mixture to rotation of pure enantiomer. It is expressed as a percentage. If the sample only rotates plane-polarized light half as much as expected, the optical purity is 50%.