Optical Light Rotation

[Clinicall]

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The R enantiomer of compound X rotates light +10 degrees. If a solution is made of 75% the R enantiomer and 25% the S enantiomer, what is the expected rotation of light for the solution?

Answer Below







I know that if it is a racemic mixture it won't rotate at all, so since the R has 3/4 the mixture and the S has 1/4, I can subtract 1/4 from 3/4 to get 1/2. So I take 10 x 1/2 and get a rotation of +5.

Is this the correct approach?

 

[amberkre]

You can do it that way, or use the optical activity equation for each enantiomer, solve for specific rotation and add the relative rotations together:

rotation observed = rotation measured X concentration
1) rotation observed (R) = (+10) X (.75) = +7.5
2) rotation observed (S) = (-10) X (.25) = - 2.5

rotation observed (R +S) = (+7.5 - 2.5) = +5

 

[Clinicall]

That is much easier, thanks!