A racemic mixture is a mixture of two different enantiomers, present in almost equal amounts. In a racemic mixture Dextro (D) and Levo (L) enantiomers are present in 1:1 amounts. A racemate cannot rotate plane-polarized light as both enantiomers (Dextro and Levo) cancel out each other’s effect.
Tartaric acid was the first racemate discovered by Louis Pasteur (1822-1895).
Nomenclature of Racemic Mixture
When both D and L enantiomers are present in a racemic mixture, then:
- A (±) this sign is used before the IUPAC name of that compound.
- The prefix ‘rac’ or ‘racem’ is used before the IUPAC name.
- A (DL) is written before the name of the compound instead of the (±) sign.
- SR or RS is added before the names of right and left handed optically active compounds.
When an unequal amount of enantiomers is present in a mixture, it is not a racemic mixture and the following points are followed:
- A (+/-) this sign is used.
- A (D/L) sign is used.
- They are optically inactive mixtures because they cannot rotate the plane of plane-polarized light.
- They may have different physical properties from either of the two enantiomers.
- In racemic mixtures, two enantiomers cannot be separated by ordinary separation methods. Therefore a special set of techniques is applied for the separation of these enantiomers from a racemate.
Separation of a racemate
It is very difficult to separate the two enantiomers in a racemic mixture because both the enantiomers have similar physical properties like melting point and boiling points, etc. But, we can separate the two enantiomers by using the method of crystallization, by using an enzyme, or by the conversion of enantiomers into diastereomers. These three techniques are collectively known as resolution.
Resolution of a Racemate
Any process used to separate two enantiomers present in a racemic mixture is known as resolution. Following are some methods to separate a racemate into its components.
Mechanical separation of crystals
In this method, crystals of different enantiomers having different shapes can be mechanically separated from one another. The first successful resolution of a racemate was done by Louis Pasteur using this method.
Usage of enzymes
Enzymes are stereospecific in their work. If enzymes were added to a racemic mixture, they act only on a single enantiomer. The remaining unreacted enantiomer can easily be separated from the racemate by using ordinary separating methods like distillation, recrystallization, etc.
Conversion of enantiomers to diastereomers
It is the most widely used method for resolution in which enantiomers are converted into their respective diastereoisomers. Diastereomers are those isomers that have different physical properties and can easily be separated by using ordinary separating methods. After separation, diastereomers can be converted to respective enantiomers by reacting with a specific reagent.
Examples of Racemic mixtures
SN1reaction usually yields a pure racemic mixture because of 50% inversion and 50% retention in the reaction mechanism of an optically active organic compound. The reason behind this racemate formation is the intermediate sp2 hybridized carbonation. This reaction mechanism (SN1) is favored by polar aprotic solvents.
The planar nature of an sp2 hybridized reaction intermediate gives nucleophiles a probability of attacking from both sides of the carbocation making the resultant mixture, a racemate. An example of 2-phenylbutan-2-ylium being converted to (±) 2-phenylbutan-2-ol by SN1 is given below.
How do you identify a racemic mixture?
It is a mixture that consists of an equal amount of both enantiomers which is optically inactive. It is usually measured by a UV-Vis spectrophotometer.
What is the difference between enantiomers and racemic mixture?
Enantiomers are nonsuperimposable mirror images that are optically active. They are optical isomers. Whereas, a racemic mixture is the two or more enantiomers that combine to form an optically inactive racemate.
What’s a Racemic Mixture?
it is an optically inactive mixture of two enantiomers present in an almost equal amount that cancel out the effect of each other.
Why is it difficult to resolve a racemic mixture?
It is a mixture of two enantiomers and enantiomers have the same physical properties which are why it is very difficult to separate them.
A racemic mixture is optically inactive. Why?
Because a racemate contains an equal amount of S and R enantiomers, they cancel out each other’s effect. So, as a whole, it is optically inactive.
What is a racemic modification?
The manual mixing of two different enantiomers to make a racemate is called a racemic modification.
Does the SN1 reaction always give complete racemization?
Yes, in SN1 reaction the attacking nucleophile can attack the carbocation from both sides. So, there is a 50-50% chance of retention or inversion of configuration. Therefore, the product is always a pure recemic mixture in SN1 reactions.
What do you call a mixture of two Enantiomers in a ratio that’s not 1:1?
A mixture of non-equimolar concentrations of enantiomers is not a racemic mixture.
Is it true that a person dies, if he/she eats a racemic mixture of amino acids, and why?
Some specific enantiomers of optically active compounds are lethal to the human body because they block the active sites of enzymes that are otherwise used for the same compounds with different optical nature. This blocks the enzymes of the body and eventually leads to death.
- Handbook of Biochemical Kinetics By Daniel L. Purich (University of Florida), R. Donald Allison (University of Florida)
- Racemic mixture (britannica.com)
- What’s a Racemic Mixture? (masterorganicchemistry.com)
- Racemic mixture (Wikipedia)