Definition and Properties of Racemate
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.
FAQs on Racemic Mixture or Racemate
1. What is a racemic mixture or racemate?
A racemic mixture, also known as a racemate, is an equimolar mixture containing equal amounts (a 50:50 ratio) of two enantiomers. Enantiomers are stereoisomers that are non-superimposable mirror images of each other. Although the individual enantiomers are optically active, the racemic mixture itself is optically inactive because the rotational effects of the two enantiomers cancel each other out.
2. What are enantiomers and how do they relate to a racemic mixture?
Enantiomers are a pair of molecules that are mirror images of each other but cannot be superimposed. Think of them like your left and right hands. They have identical chemical formulas and physical properties (like boiling point and solubility), except for their ability to rotate plane-polarised light. A racemic mixture is formed when these two enantiomers are combined in equal proportions, resulting in a single sample that does not exhibit net optical activity.
3. What is the main property that makes a racemic mixture optically inactive?
A racemic mixture is optically inactive due to a phenomenon called external compensation. This occurs because the mixture contains:
- One enantiomer (dextrorotatory or '+') that rotates plane-polarised light in a clockwise direction.
- An equal amount of the other enantiomer (levorotatory or '–') that rotates the light by the exact same degree but in the opposite (counter-clockwise) direction.
The equal and opposite rotations perfectly cancel each other, resulting in zero net rotation of the light.
4. Can you provide a common example of a racemic mixture from the CBSE syllabus?
A classic example of a racemic mixture is (±)-lactic acid. This mixture contains 50% (+)-lactic acid (dextrorotatory) and 50% (–)-lactic acid (levorotatory). Another common example is (±)-2-butanol, which is an equimolar mixture of (R)-2-butanol and (S)-2-butanol. The prefix (±)- or dl- is used to denote a racemic mixture.
5. How are racemic mixtures represented in chemical nomenclature?
In chemical nomenclature, a racemic mixture is typically indicated by specific prefixes placed before the name of the compound. The most common representations are:
- The (±)- prefix, for example, (±)-tartaric acid.
- The dl- prefix, for example, dl-alanine.
- The descriptor rac- or racemate, for example, rac-ibuprofen.
These symbols signify that the sample contains an equal mixture of both the dextrorotatory and levorotatory enantiomers.
6. If a racemic mixture is optically inactive, how is it different from an achiral compound?
This is a crucial distinction. An achiral compound is optically inactive because its individual molecules are symmetrical and superimposable on their mirror images; they lack a chiral centre. In contrast, a racemic mixture is optically inactive due to external compensation. The mixture itself is composed of chiral molecules, but the presence of equal amounts of the right-handed and left-handed enantiomers causes their individual optical activities to cancel out.
7. Why are the physical properties of a racemic mixture, like melting point, often different from its pure enantiomers?
While individual enantiomers have identical physical properties (except optical rotation), a racemic mixture can have different properties like melting point, boiling point, and solubility. This is because the way chiral molecules pack into a crystal lattice can differ. When two different enantiomers (R and S) are crystallised together, the packing arrangement in the solid state can be less efficient or organised differently than the packing of a pure enantiomer (all R or all S). This difference in the crystal lattice structure leads to different physical properties for the racemate.
8. What is the importance of racemic mixtures in the pharmaceutical industry?
The concept is vital in pharmaceuticals because biological systems, including our bodies, are chiral. Often, only one enantiomer of a drug is therapeutically effective, while the other may be inactive or cause harmful side effects. For example, the drug Ibuprofen is often sold as a racemic mixture, but only the S-(+)-enantiomer has anti-inflammatory properties. Understanding racemates is essential for synthesising safer, more effective drugs by isolating the active enantiomer, a process known as chiral resolution.
9. What is racemization and how does it occur?
Racemization is the process of converting an optically active substance (containing only one pure enantiomer) into an optically inactive racemic mixture. This typically happens in a reaction where the chiral centre of a molecule is temporarily destroyed and then reformed. For instance, reactions involving the formation of a planar intermediate like a carbocation or a carbanion can lead to an attack from either side with equal probability, resulting in a 50:50 mixture of both enantiomers.
10. How can a racemic mixture be separated into its individual enantiomers?
Separating a racemic mixture into its pure enantiomers is a process called resolution. Since enantiomers have nearly identical physical properties, it cannot be done by simple methods like distillation or fractional crystallisation. Common methods for resolution include:
- Chemical Method: Reacting the racemic mixture with a pure chiral agent to form a pair of diastereomers. Diastereomers have different physical properties and can be separated.
- Chromatographic Method: Using a technique called chiral chromatography, where the stationary phase in the column is itself chiral and interacts differently with each enantiomer, causing them to separate.
