The specific rotation of pure enantiomer is known from the literature to be -20. The measured value of a solution containing A plus its enantiomer B (specific rotation must be +20) is -40. What is the ee (enantiomer excess) of this sample?
To calculate the enantiomer excess (ee) of a sample, you need to know the specific rotation values and the quantities of each enantiomer present.
In this case, we have the following information:
Specific rotation of pure enantiomer (A): -20
Specific rotation of solution containing A and B: -40
To calculate the ee, we can use the formula:
ee = (observed rotation - expected rotation) / (observed rotation + expected rotation) x 100
First, let's calculate the expected rotation. Since the specific rotation of pure enantiomer A is -20, and we know the specific rotation of the solution is -40, we can deduce that half of the solution is made up of enantiomer A.
So, we can calculate the expected rotation as follows:
Expected rotation = (specific rotation of A + specific rotation of A) / 2 = (-20 + -20) / 2 = -40 / 2 = -20
Now, we have the expected rotation (-20) and the observed rotation (-40). Plugging these values into the ee formula, we can calculate the enantiomer excess:
ee = (-40 - (-20)) / (-40 + (-20)) x 100
= (-40 + 20) / (-40 - 20) x 100
= (-20) / (-60) x 100
= 1/3 x 100
= 33.33%
Therefore, the ee of this sample is 33.33%.