Calculate the percentage of major enantiomer if enantiomer excess (ee) is 95%.
Answers
Answer:
The (-)-X is in excess. The mixture contains 67 % (-)-X and 33 % (+)-X.
Explanation:
The rotations of the two enantiomers cancel each other, so the rotation of the mixture will be that of the excess enantiomer.
The mixture has a negative sign of rotation, so (-)-X is in excess.
To calculate the enantiomeric excess, you divide the observed specific rotation by the maximum specific rotation of the excess enantiomer.
The formula for enantiomeric excess is
e
e
=
observed specific rotation
maximum specific rotation
×
100
%
=
-5.1
°
-15.2
°
×
100
%
=
34
%
We can calculate the percent of each enantiomer as described in this Socratic question.
If we have a mixture of (+) and (-) isomers and (-) is in excess,
%
(
-
)
=
e
e
2
+
50
%
We have a 34 % enantiomeric excess of (-).
∴
%
(
-
)
=
34
%
2
+
50
%
=
(
17
+
50
)
%
=
67
%
So, the mixture contains 67 % (-)-X and 33 % (+)-X.