A solution containing 0.08 g/mL of a pure R enantiomer in a 1 dm polarimeter rotates plane polarized light by +4°.What is the specific rotation of the S enantiomer?

Question

Quizplus · Accepted Answer

The specific rotation of a substance is defined as the angle of rotation caused by 1 g/mL of the substance in a polarimeter tube of 1 dm length. Since the given solution contains only the R enantiomer and its specific rotation is +4°, we can use the following formula to calculate the specific rotation of the S enantiomer:

[solution] × [length of tube] = ([concentration in g/mL] × [specific rotation]) × [length of tube]

0.08 × 1 = (x × -4) × 1
x = -0.02

Therefore, the specific rotation of the S enantiomer is -20°. However, some sources define specific rotation in terms of the observed rotation of the enantiomeric mixture (i.e. the racemic mixture), which would give a specific rotation of -40° for the S enantiomer. Both answers are acceptable, but the one closest to -20° is D.

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