Racemic Mixture in Chemistry and Optical Isomerism

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 structures can vary due to the presence of functional groups in different orientations. 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. 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 a 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 different molecular structures 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 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 solubilities.

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.

Resolution of Racemic Mixture

A racemic mixture, also known as a racemate, is an equimolar mixture of a molecule's enantiomers that may be distinguished by their distinct interactions with chiral compounds or media. The (±)- prefix is used to describe racemic mixes. Rather than a distinct stereoisomer, this 'optical rotation descriptor' applies to the entire solution of a racemic combination. The physical and chemical characteristics of enantiomers are usually the same. As a result, they are difficult to separate. When enantiomers interact with chiral media, however, they can have distinct characteristics. For example, enantiomers of a chemical can interact differently with our body's chiral olfactory receptors, giving each enantiomer a distinct fragrance.

As a result, racemic mixtures may be separated via a procedure known as enantiomeric resolution. The enantiomers react with a chiral resolving agent to form diastereomers in this reaction. To create enantiomerically pure compounds, these diastereomers may be readily separated and reconverted. The contact between chiral resolving agents and enantiomers can be covalent, ionic, or a weak intermolecular connection involving van der Waals forces, depending on the molecular structure. Some racemic mixes, on the other hand, can resolve spontaneously without the need for a chiral resolving agent. The racemic combination of N-trifluoroacetate-amino alcohol in carbon tetrachloride, for example, resolves spontaneously.

Crystallization of Racemic Mixture

Depending on the material, a racemate can crystallize in one of four ways, three of which H. W. B. Roozeboom had identified by 1899:

A Conglomerate (Sometimes Racemic Conglomerate)

A mechanical combination of enantiomerically pure crystals will develop if the molecules of the material have a substantially stronger affinity for the same enantiomer than for the opposite one. A eutectic mixture is made up of enantiomerically pure R and S crystals. As a result, the conglomerate's melting point is always lower than that of the pure enantiomer. When a little quantity of one enantiomer is added to the aggregate, the melting point rises. Conglomerates crystallize in around 10% of racemic chiral substances.

Racemic Compound/True Racemate

The material produces a single crystalline phase in which the two enantiomers are present in an ordered 1:1 ratio in the elementary cell if molecules have a stronger affinity for the opposing enantiomer than for the same enantiomer. The melting point of a racemic substance is lowered by adding a little quantity of one enantiomer. The pure enantiomer, on the other hand, might have a melting point that is greater or lower than the complex. Krypto Racemic compounds (or kryptoracemates) are a kind of racemic compound in which the crystal possesses handedness (is enantiomorphic) despite possessing both enantiomorphs in a 1:1 ratio.

Pseudoracemate (Sometimes a Racemic Solid Solution)

In contrast to the racemic compound and the conglomerate, when there is no significant difference in affinity between the same and opposite enantiomers, the two enantiomers will coexist in an unordered way in the crystal lattice.