Enantiomeric Excess

Enantiomeric excess, also known as optical activity, is represented by (ee) and is a measurement of purity used for chiral substances. Chiral substances are those which are optically active. It might range from 0 to 100 %. However, 0% does not imply that there is no significant enantiomer present; it simply means that it is 0% pure, which is a 50-50 mix of both enantiomers, resulting in a racemic mixture. Similarly, 100 percent does not imply that the mixture is 100 percent pure or contains 100% of the primary enantiomer.

The formula for determining the enantiomeric excess is relatively simple. We can calculate optical purity by dividing the observed rotation of the mixture by the specific rotation of the pure enantiomer.

Define Enantiomeric Excess 

In order to define enantiomeric excess, let’s look at an example. Consider this situation: you have a box of gloves. The number of left-handed and right-handed gloves in the box is unknown, but one glove type will outnumber the other.

A racemic mixture is an enantiomeric mixture that contains an equal amount of (R)- and (S)-enantiomers. The substance is considered enantiomerically pure if it solely has one enantiomer or the other. An enantiomeric excess occurs due to an excess of either enantiomer (R) or enantiomer (S). An enantiomeric excess occurs when there is an excess of either the (R)-enantiomer or the (S)-enantiomer. Optical purity is another term for enantiomeric excess.

This is because chiral chemicals are “optically active,” which means they rotate plane-polarised light. The enantiomeric excess in an enantiomerically pure sample is 100 percent.

Enantiomeric Excess Examples

The term enantiomeric excess (ee) was first used to define enantiomeric composition and was incorrectly linked to optical purity. Ee and its related dé (diastereomeric excess) have lately been used to quantify stereoselectivity (inadvertently). In chemical isomers, enantiomers are non-superimposable mirror reflections of each other. 

As a result, two enantiomers of the same chemical molecule will have diametrically opposing three-dimensional shapes but identical chemical bonds. It’s imperative to remember that enantiomers are isomers that aren’t identical and can’t be combined. Additionally, these stereoisomers might be considered mirror images of one another.

Enantiomeric excess can be seen in lactic acids like dextro and laevo. Another example of enantiomeric excess is S- and R-methyl chlorophenoxy propionic acid (often abbreviated to MCPP and referred to as mecoprop).The R enantiomer of this chemical is thought to react as a herbicide.

With exception of cis and trans isomers, almost all enantiomer pairs share physical traits such as solubility and melting temperature. They, on the other hand, rotate light in the opposite way (both the enantiomers of a compound must be optically active).

Calculation of Enantiomeric Excess 

The equation for calculating enantiomeric excess is: 

%ee = [(moles of enantiomer – moles of other enantiomer)/total moles of both enantiomers] x 100

The ee can also be calculated using specific rotation, a physical property of a substance that can be looked up in reference books.

%ee = (observed specific rotation/specific rotation of the pure enantiomer) x 100

Enantiomeric excess (abbreviated as ee) is a method of determining how much more of one enantiomer is present in a solution. The enantiomeric excess formula is as follows:

Where R and L are two other ways to differentiate the enantiomers, such as (-) and (+), and both enantiomers are determined in concentration, commonly in mol/Litre, M. It’s sometimes written as (S) instead of (L) or (D) instead of (R), but the enantiomeric excess is the same. 

Conclusion

Enantiomeric excess is an essential concept in organic chemistry because many reactions produce enantiomers, and discovering particular strategies to execute those processes, such that they produce a larger enantiomeric excess, saves time and money.

Enantiomers are non-imposable mirror reflections of each other and are stereoisomers. Stereoisomers are molecules with the same number of atoms connected to the same atoms but in a different order.

Stereospecific reactions need enantiomeric excess or having more of one enantiomer than the other. Various stereoisomers react in different ways in stereospecific reactions, resulting in different products. Though enantiomers have many characteristics, their products in stereospecific reactions might differ.