What is Enantiomeric Excess (ee) and Optical Purity and how to Calculate Them?
What is Enantiomeric Excess (ee) ?
Enantiomeric excess (ee) is a measurement of the excess amount of one enantiomer over the other enantiomer in a racemic mixture. Enantiomer excess can be calculated by just subtracting the percentage of the minor enantiomer from the percentage of the major enantiomer.
Enantiomeric excess (ee) = % of one enantiomer - % of the other enantiomer
Let us consider, we have a mixture containing 90% of one enantiomer and 10% of the other enantiomer. Then, the enantiomeric excess (ee) will be 80% (90% - 10%). Thus, there is 80% excess of one enantiomer over the other enantiomer in a racemic mixture.
Explanation: 80% excess of the one enantiomer means that the other 20% is a racemic mixture of both enantiomers. Remember that racemic mixtures have a 1:1 ratio of the enantiomers, so within those 20%, there is 10% of one enantiomer and 10% of the other enantiomer. Thus, the total percentage of the one enantiomer is 80% + 10% = 90% and the other-enantiomer makes the 10% of the entire mixture.
A racemic mixture (equal amounts of both enantiomers) has an enantiomeric excess of 0%.
Enantiomeric excess (ee) is numerically equal to Optical Purity in a solution of a chemically (not enantiomerically) pure compound.
The percentage of major and minor enantiomers can be calculated if the enantiomeric excess is known by using the formula given below.
(+)-Tartaric acid has a specific rotation of +12.0°. Calculate the specific rotation of a mixture of 68% (+) tartaric acid and 32% (-) tartaric acid.
Hint: Enantiomer Excess (ee) = 68% - 32% = 36%
Observed specific rotation = [ee/100] x specific rotation of the pure enantiomer
or, Observed specific rotation = (36/100 ) x (+12.0°) = +4.32°
Therefore, the specific rotation of the mixture is +4.32°.
Percentage of Enantiomers from Specific Rotations
Pure adrenaline has a specific rotation of -53. What is the enantiomeric excess of an adrenaline sample that has a specific rotation of -47.7?
Hint: Using the above formula, we can easily calculate the percentage of enantiomer excess as
% ee = (-47.7 / -53) x 100
or, % ee = 90%
Therefore, the enantiomeric excess of the adrenaline sample is 90%.
What is Optical Purity ?
Optical purity is a measure of how much the sample rotates polarized light, which is then used to determine the ratio of enantiomers.
Optical purity can be defined as the observed rotation of a mixture divided by the specific rotation of the pure enantiomer (obtained under identical conditions).
Optical activity of an enantiomeric mixture is +12.6° and the specific rotation of (+) isomer is 30°. Calculate the optical purity.
Hint: Optical Purity = [12.6 x 100] / 30 = 42%
An optically pure substance consists of a single enantiomer and no trace of other enantiomer. If a sample is 50% optically pure, it means that one enantiomer is 50% excess than other enantiomer. And if we have a sample with 90% of the (R) enantiomer and 10% of the (S) enantiomer, the optical purity or enantiomer excess is 80%.
Optical purity of an optically active substance is expressed as the Enantiomeric Excess
Optical Purity = % One Enantiomer – % Other Enantiomer
A sample of a chiral compound contains 60% of the (R) enantiomer and 40% of the (S) enantiomer. Calculate the optical purity or enantiomeric excess of this sample?
Hint: Optical Purity or Enantiomeric Excess = 60% − 40% = 20%
There are three other methods for determining the optical purity, they are
- Enzymatic Method
- Isotope Dilution Method
- Correlative Method
Enantiomeric Excess (ee) and Optical Purity MCQs
Which of the following statements is true regarding enantiomeric excess?
(a) EE is always less than 100%.
(b) EE is always equal to 50%.
(c) EE can be negative.
(d) EE can be zero.
Answer
Options (d) EE can be zero is correct answer.
EE is always less than 100% is somewhat correct because EE is always equal to or less than 100% because it is the difference between the percentage of the major and minor enantiomers in a mixture
A sample of a chiral compound has a specific rotation of +10 degrees, while the pure enantiomer has a specific rotation of +25 degrees. What is the optical purity?
(a) 25%.
(b) 40%.
(c) 50%.
(d) 60%
Answer
Option (b) 40% is correct answer.
What's the relationship between enantiomeric excess (ee) and optical purity for a chemically pure compound?
(a) ee is always higher than optical purity.
(b) Optical purity is always higher than ee.
(c) They are numerically equal.
(d) They have no direct relationship.
Option (c) They are numerically equal is correct answer.Answer
Which of the following methods cannot be use to determine the optical purity?
(a) Enzymatic method.
(b) Isotope dilution method.
(c) Correlative method.
(d) Spectroscopic method.
Answer
Option (d) Spectroscopic method is correct answer.
An optically pure substance consists of:
(a) A racemic mixture.
(b) A mixture of diastereomers.
(c) A single enantiomer without any trace of the other.
(d) Equal amounts of both enantiomers.
Answer
Option (c) A single enantiomer without any trace of the other is correct answer.