Introduction
The chapter on Enantiomers comes from the module of Molecule and is classified under the subject of Chemistry. It deals with the compound elements with formulas. Stereoisomers are molecules with the same geometrical arrangement and have the same connectivity. Enantiomers are one of the two groups of stereoisomers. The first group is diastereomers. They are compounds that are not mirrored and are completely symmetrical.
Stereochemistry:
The word ‘stereo’ means “three-dimensionality”. Stereochemistry is the study of how a molecule is affected due to the orientation of its atoms in space. An important part of stereochemistry is stereoisomerism, which deals with chemical compounds having the same molecular formula and different 3D spatial arrangement. Stereoisomers can be broadly classified into enantiomers and diastereomers.
Optically Active:
The ability of a molecule to rotate plane-polarized light is known as optical activity and the molecules that are capable of doing so are said to be optically active.
Racemic Mixture:
An equimolar mixture of dextrorotatory and levorotatory compounds, i.e., 50% Dextro and 50% Levo is called a racemic mixture.
Chiral Centre:
A carbon atom, which is directly attached to four distinct groups, is called a chiral carbon or a chiral centre
Superimposable:
It is the ability to place one object over another, in such a way that the objects are visible clearly, which is known as superimposable.
Technically, enantiomers are a pair that have non-superimposable – meaning it shows differences when it is mirrored – molecules. Enantiomers have the same chemicals but are different in appearance in terms of symmetry.
Enantiomers are explained by the example of hands: the left hand and the right hand are not symmetrical when placed on top of each other from the back of the hands, but are mirror images when the palms are connected to each other.
In the same way, enantiomers are a pair of molecular compounds called enantiomorphs that are not symmetrical when placed on top of each other or overlapped but are symmetrical when placed side by side.
Properties of Enantiomers
Enantiomers are famous for their distinctive characteristic, which is the reflective symmetry of the molecules it contains. It has different spatial configurations and can be mirrored. That means that if you flip the image of an enantiomer, you will get a mirrored view. Each mirrored molecule is called an enantiomorph.
Enantiomers have at least one chiral centre. This centre is the factor on which the geometry of the enantiomorph differs.
Enantiomers have identical physical properties.
Enantiomers have identical chemical properties.
Enantiomers have an identical attachment to atoms.
They bend light in opposite directions.
Enantiomers are unique molecules with unique spatial configurations.
They cannot be rotated or tilted in any way so that they are the same.
FAQs on Enantiomers
1. What are enantiomers in Chemistry?
Enantiomers are a type of stereoisomer. Specifically, they are pairs of molecules that are non-superimposable mirror images of each other. A common analogy is your left and right hands; they are mirror images but cannot be perfectly overlapped. For a molecule to have an enantiomer, it must be chiral, which means it lacks an internal plane of symmetry.
2. What is the main difference between enantiomers and diastereomers?
Both enantiomers and diastereomers are types of stereoisomers, but they differ in a crucial way. Enantiomers are non-superimposable mirror images of each other. In contrast, diastereomers are stereoisomers that are not mirror images of each other. A molecule can have only one enantiomer, but it can have multiple diastereomers.
3. What is a chiral centre and why is it important for enantiomers?
A chiral centre (or chiral carbon) is a carbon atom that is attached to four different atoms or groups. The presence of at least one chiral centre is often the reason a molecule is chiral and can exist as a pair of enantiomers. This tetrahedral arrangement around the chiral carbon allows for two distinct spatial configurations that are mirror images of each other.
4. How are enantiomers named using the R and S system?
The R/S system, or Cahn-Ingold-Prelog (CIP) rules, provides an absolute configuration for a chiral centre. The steps are:
- Assign priority (1 to 4) to the four groups attached to the chiral carbon based on atomic number.
- Orient the molecule so the lowest priority group (4) points away from you.
- Trace the path from priority 1 → 2 → 3.
- If the path is clockwise, the configuration is R (Rectus).
- If the path is counter-clockwise, the configuration is S (Sinister).
5. Why do enantiomers have identical physical properties but different biological activities?
Enantiomers have identical physical properties like melting point, boiling point, and solubility because the intermolecular forces between identical molecules are the same. However, their biological activities differ because biological systems, such as enzymes and receptors, are themselves chiral. An enzyme can distinguish between two enantiomers just like your right hand can only fit properly into a right-handed glove. For example, the (S)-enantiomer of Ibuprofen is an active painkiller, while the (R)-enantiomer is largely inactive.
6. What is a racemic mixture and why is it optically inactive?
A racemic mixture contains an equal amount (a 50:50 ratio) of two enantiomers. It is considered optically inactive because the two enantiomers rotate plane-polarised light in equal but opposite directions. The dextrorotatory (+) rotation of one enantiomer is perfectly cancelled out by the levorotatory (-) rotation of the other, resulting in a net optical rotation of zero. This phenomenon is called external compensation.
7. How is the D/L notation different from the R/S notation for classifying enantiomers?
The D/L and R/S notations are two different systems for describing the configuration of enantiomers.
- The D/L system is a relative system that compares a molecule's configuration to that of a standard compound, glyceraldehyde. It is commonly used for carbohydrates and amino acids.
- The R/S system (Cahn-Ingold-Prelog) is an absolute system that assigns configuration based on a set of priority rules for the groups attached to the chiral centre. It provides an unambiguous description of the 3D arrangement at each chiral centre.
8. How do enantiomers affect plane-polarised light?
Enantiomers are optically active, meaning they can rotate the plane of plane-polarised light. One enantiomer of a pair will rotate the light in a clockwise direction, and it is called dextrorotatory (+). Its mirror image counterpart will rotate the light by the exact same magnitude but in a counter-clockwise direction, and is called levorotatory (-).
9. Can a molecule have chiral centres but still be achiral overall?
Yes, a molecule can have two or more chiral centres and still be achiral. Such a compound is called a meso compound. This occurs when the molecule possesses an internal plane of symmetry that makes it superimposable on its mirror image, despite containing chiral carbons. A classic example is Tartaric acid, which has two chiral centres but one of its stereoisomers (meso-tartaric acid) is achiral and thus optically inactive due to internal compensation.
