A question on enantiomeric excess

[biochemist]

Why does a racemic mixture of 98:2 enantiomers consist of one enantiomer in 96% over the other? I'm still not clear about this after reading textbooks. And how about the other one? does it have a negative percentage value of enantiomeric excess?

 

[HazZy]

ee = %(+) - %(-) assuming the (+) enantiomer is predominant.

also, there is no excess of the (-) enantiomer because there is only an excess of the (+) enantiomer (see the assumption above).

just think of it as if the lesser enantiomer shades out an equal percentage of the predominant enantiomer. so if you have 98%(+) and 2%(-), 2%(-) will shade out 2%(+) and you're left with 96%(+) pure enantiomer. i don't know if that makes sense, but i hope it helps .

 

[ravenprp]

The reason for this is due to the concept of enantiomeric excess. Enantiomeric excess (EE) is a measure of the difference in the amount of two enantiomers present in a mixture. It is calculated using the formula: (moles of desired enantiomer - moles of undesired enantiomer) / total moles in the mixture.

In the case of a racemic mixture, the moles of each enantiomer are equal, resulting in an EE of 0%. However, when one enantiomer is present in a higher amount (98%) compared to the other (2%), the EE is calculated as follows: (0.98 - 0.02) / 1 = 0.96 or 96%. This means that the desired enantiomer is present in excess of 96%, while the undesired enantiomer is present in the remaining 4%.

As for the other enantiomer, its EE would be calculated as: (0.02 - 0.98) / 1 = -0.96 or -96%. This indicates that the undesired enantiomer is present in excess of 96%, while the desired enantiomer is present in the remaining 4%.

It is important to note that the EE of one enantiomer is always the inverse of the other. In the case of a racemic mixture, both enantiomers have an EE of 0%, while in a mixture with a higher amount of one enantiomer, the EE of that enantiomer will be a positive value and the EE of the other enantiomer will be a negative value.

I hope this helps to clarify the concept of enantiomeric excess for you. It is a crucial concept in understanding the composition of chiral mixtures and how to analyze and separate them.