Percentage of Enantiomer

[MissionStanford]

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"Naproxen, a nonsteroidal anti-inflammatory drug that is the active ingredient in Aleve, has a specific rotation of +66. One commercial preparation results in a mixture that has a 97% enantiomeric excess."

Question: What percent of each enantiomer is obtained from the commercial preparation?

I know you just do 97% + (3%/2) = 98.5% of one enantiomer and then 100% - 98.5% = 1.5% of the enantiomer, but my book says the following:

"97% of the commercial preparation is (+)-naproxen; 3% is a racemic mixture. Therefore the commercial preparation forms 98.5% (+)-naproxen and 1.5% (-)-naproxen."

How do we know which percentage is for (+)-naproxen versus (-)-naproxen? I don't know why it's saying that 97% of the preparation was (+)-naproxen. Where did that come from? All we know is that there's 97% enantiomeric excess, but it didn't say which enantiomer there was an excess of. Am I missing something?

 

[Gauss44]

Naproxen has specific rotation of +66. It would be a lousy "commercial preparation" if the desired product was only 1.5%.

 

[MissionStanford]

Naproxen has specific rotation of +66. It would be a lousy "commercial preparation" if the desired product was only 1.5%.

That's a good point. Didn't consider that. So I guess common sense implies that the excess is of (+)-naproxen? Also, when it says naproxen has a specific rotation of +66, that means that one configuration of naproxen (the one used in Alleve) has a specific rotation of +66 and the opposite configuration has specific rotation of -66, right?

 

[Gauss44]

That's a good point. Didn't consider that. So I guess common sense implies that the excess is of (+)-naproxen? Also, when they say naproxen has a specific rotation of +66, that means that one configuration (the one used in Alleve) has a specific rotation of +66 and the opposite configuration has specific rotation of -66 right?

Correct.