There are organic compounds that have similar chemical formulas but different molecular structures. They are called enantiomers. When enantiomers are present in equal quantities in a mixture, it is called a racemic mixture. In this section, we will discuss what racemate means, the origin of the name, and the feature of these compounds. The description of this mixture shows how structural and molecular structure can vary due to the presence of functional groups in a different orientation. Let us discuss this topic elaborately and understand the meaning of the terms with examples.
What are Enantiomers?
To understand the meaning of racemic mixtures and racemization, we will have to discuss what enantiomers are. These are compounds that have the same chemical formula but different molecular orientations making them a particular set of isomers. They show a different set of physical properties but similar chemical properties in most cases.
To explain this type of isomers, we need to understand how these molecules can affect the plane of polarized light. In fact, the only way to differentiate between two enantiomers is to pass a polarized light beam and check the changes in its plane. Now that we know what enantiomers are, we can easily define racemic compounds and their mixture.
What is a Racemic Mixture?
The functional groups in a molecule of a compound can either be in the right or the left position in the same location. Their orientation might vary in the same carbon position in a molecular structure. The chemical properties are almost the same for both compounds. The mixture of these two compounds is called racemate or racemic mixture.
As per the racemate chemistry, the proportion of two enantiomers in the mixture is equal. The name comes from the first mixture to be recognized by none other than Louis Pasteur. He identified the presence of two types of isomers in racemic acid. It is also called tartaric acid. The term ‘racemic’ comes from the Latin word ‘racemus’ which means a bunch of grapes. The right-oriented molecule is known as tartaric acid.
When Louis Pasteur identified the presence of both the acids in the same mixture in equal proportion, he named it a racemic mixture. This is where the racemic meaning is derived from. Both the compounds are similar in chemical structure. The only difference is the presence of one of two functional groups in two different orientations. To distinguish these isomers, a proper nomenclature has to be followed.
Nomenclature of Racemate Compounds
As we have understood the only difference between two racemic compounds is their capabilities in rotating a polarized light, it is used as the mode of nomenclature to distinguish between these two compounds. In racemate chemistry, when a racemic mixture is not showing any effect or change in a polarized light passing through it, it means that the quantities of both the compounds are equal. When these compounds are separated and then a polarized light is passed, a particular change is noticed. The light either rotates anticlockwise or clockwise. When there is an anticlockwise rotation of the plane-polarized light, it is called dextrorotation. Similarly, vice versa is called levorotation. Hence, for the compounds showing any of the two rotatory effects, the chemical name of the compound comes with a specific prefix.
According to the racemic meaning and nomenclature, a dextrorotatory compound comes with a ‘+’, ‘d’, or ‘D’ prefix. All the meanings are the same. Similarly, a levorotatory compound comes with the prefix ‘-‘, ‘l’, or ‘L’. Let us consider an example here. Dextro-fructose can be represented as D- fructose, +- fructose, or d- fructose. Similarly, an enantiomer with levorotation can be represented as L-fructose, -fructose, or l-fructose.
Hence, these optically active enantiomers can be distinguished and named using their optical activities. The prime feature of two such compounds of similar chemical formula but the different molecular structure is that the degree of rotation occurring in the plane-polarized light will have the same angular value but in opposite directions. This is why the + and – signs are used to signify them. From this explanation, you can now easily identify and define racemic compounds with examples.
Properties of Racemate Mixtures
The definition of racemate mixtures tells us how they can rotate a plane-polarized light. A pair of racemic compounds will rotate the light in opposite directions which means the mixture will be optically inactive. It happens when these compounds are present in equal proportions in the mixture.
These compounds are hard to recognize and distinguish as all their physical properties are similar. You will not be able to identify these compounds unless they are optically active. The differences are generally noticed in the melting points and boiling points. They can also show different solubility.
Many pharmaceutical medicines are present either in racemate form or in pure enantiomer form. This is done to escalate the bioavailability of the medicinal molecules as the biological system of a human body has a lot of such chiral and racemic asymmetries.