Question

Naproxen, a nonsteroidal anti-inflammatory drug that is the active ingredient in Aleve (p. 115), has a specific rotation of +66. One commercial preparation result in a mixture with a 97% enantiomeric excess.

a. Does naproxen have the R or the S configuration?

b. What percent of each enantiomer is obtained from the commercial preparation?

Short answer

a. Cannot be determined

b. Commercial preparation forms 98.5% (+) naproxen and 1.5% (-) naproxen.

Step-by-step solution

Definition of specific rotation-

The observed rotation can be used to calculate the specific rotation at a given temperature and wavelength using the formula-

[𝞪] = 𝞪 /l c

where, [𝞪] is the specific rotation,

T is the temperature in degrees Celsius,

l is the wavelength of the incident light (when the sodium D-line is used, l is indicated as D),

ɑ is the observed rotation,

l is the length of the sample tube in decimetres, and

c is the sample concentration in grams per 100 millilitres.

Given quantities-

specific rotation of Naproxen = +66

enantiomeric excess on commercial preparation = 97%

97% of commercial preparation is (+)-naproxen: 3% of racemic mixture.

Calculation of specific rotation of naproxen-

Enantiomeric excess = (observed specific rotation/specefic rotation of pure (+) enantiomer) × 100%

97% = (x/66)× 100

X = +64.02