Destroyer OC #192

[diene]

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A compound is 70% optically pure R. Which is true?

The answer is 85% R; 15% S

How do we know the that remaining 30% will be racemic? Why can't it be 30% optically pure S? Is there some rule that I am missing out on?

 

[diene]

bump

 

[spikers220]

If the product is 70% optically pure then that means that only 70% of the sample rotates light in that specific direction. The rest has to be a 15/15 mix.

Example:

You can have 70% R and 30% S, but the answer would be 40% optically pure R - because the other 60% doesn't rotate light, and is a racemic mix.

Hopefully this makes sense...

 

[diene]

If the product is 70% optically pure then that means that only 70% of the sample rotates light in that specific direction. The rest has to be a 15/15 mix.

Example:

You can have 70% R and 30% S, but the answer would be 40% optically pure R - because the other 60% doesn't rotate light, and is a racemic mix.

Hopefully this makes sense...

Sorry I am still a little confused. In your example, why did you say 40% R?

 

[n27s]

When it says a compound is 70% pure R, your brain naturally tries to interpret that as meaning the other 30% must be S, however the key here is the word "pure". When a compound is 70 % pure R, it means the other 30% is a racemic of R and S. So half of this 30% will be R (15%) and the other half will be S (also 15%). Now you add all the R's (original pure R (70%) and R from racemic (15%) giving you a total of 85% R and leaving 15%S.

Try to think of the pure sample as what has already been separated and the remainder as a racemic. Or idk if I just confused you further.