Enantiomers

What is an Enantiomer?

Enantiomers are a pair of molecules that exist in two forms that are mirror images of one another but cannot be superimposed one upon the other.

Enantiomers are in every other respect chemically identical. A pair of enantiomers is distinguished by the direction in which when dissolved in solution they rotate polarized light, either dextro (d or +) or levo (l or -) rotatory; hence the term optical isomers. When two enantiomers are present in equal proportions they are collectively referred to as a racemic mixture, a mixture that does not rotate polarized light because the optical activity of each enantiomer is cancelled by the other.

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Introduction to Chirality

Enantiomers and Diastereomers

Enantiomers vs Chiral

Chiral recognition can be described as the discrimination between the two enantiomers of a chiral molecule. Because the physical properties that are typically used to separate molecular species are identical in the case of enantiomers, it is difficult to separate the two species. It is only through the interactions with a discriminating secondary species that physical differences can be observed.

  • The structural basis of enantiomerism is called chirality.
  • Enantiomers are a pair of molecules that exist in two forms that are mirror images of one another but cannot be superimposed one upon the other.
  • Enantiomers are in every other respect chemically identical. A pair of enantiomers is distinguished by the direction in which when dissolved in solution they rotate polarized light either dextro (d or +) or levo (l or -) rotatory, hence the term optical isomers.
  • When two enantiomers are present in equal proportions they are collectively referred to as a racemic mixture that does not rotate polarized light because the optical activity of each enantiomer is cancelled by the other.

Structure of Enantiomers

Consider the most common origin of chirality is when a tetrahedrally coordinated atom is bound to four different substituents as shown below.

Chiral Molecule

  • Enantiomers were introduced as stereoisomers that are non-superimposable mirror images of one another.
  • Any molecule that is not superimposable on its mirror image and so exists as a pair of enantiomers is said to be chiral and to exhibit chirality. Conversely, any molecule that is superimposable on its mirror image is achiral.
  • Indeed, whenever a molecule contains a single atom which is tetrahedrally bound to four different substituents, then two enantiomers are possible.
  • It is however important that the four substituents are different to one another as if any two of them are identical then the structure would become superimposable on its mirror image and so achiral. The atom connected to four different atoms is best referred to as a stereogenic centre or simply a stereocenter.
  • A widely used although somewhat misleading alternative name for a stereocenter is a localized around the central atom, whereas chirality is a property of the molecule as a whole that cannot be localized around one atom or a group of atoms.
  • The presence of a stereocenter is not a requirement for a molecule to exhibit chirality; it is simply the most common cause of chirality.

Properties of Enantiomers

  • Enantiomers generally have identical physical properties such as melting point, boiling point, infrared absorptions and NMR spectra.
  • It is important to realize however, that whilst the melting point etc of one enantiomer will be identical to that of the other enantiomer, the melting point of a mixture of the two enantiomers may be different.
  • This is because the intermolecular interactions between opposite enantiomers that are between the R and S enantiomers may be -different to those between like enantiomers that are between two molecules both of R or both of S stereochemistry.
  • The one class of physical techniques that can distinguish between the two enantiomers of a compound are chiroptical techniques, the most common of which is optical rotation.
  • The chiroptical properties of a molecule are determined not just by the bond lengths and angles but also by the sign and magnitude of the torsional angles, the sign of the torsional angles being the one difference between enantiomers.

Frequently Asked Questions on Enantiomers

Are all enantiomers chiral?

Chiral describes an atom that has four separate groups attached to it, while enantiomers describe the two stereoisomer relations. Enantiomers also have chiral centers in the molecules, but not all molecular stereoisomers are mutually enantiomers.

Are enantiomers chemically different?

Enantiomers have the same chemical and physical properties aside from their effect on plane-polarized light (they rotate the polarization axis in equal and opposite quantities) and their interactions with other chiral molecules.

Are enantiomers chiral or achiral?

Two pieces of paper, for example, are achiral. In comparison, chiral molecules are non-superimposable mirror images of each other, much like our faces. A Chiral molecule has a mirror image and can not be completely aligned with it- the mirror images can not be superimposed. The photos in the mirror are called enantiomers.

How are enantiomers different?

Enantiomers vary in the stereogenic core as to their structure (R or S). In an achiral environment, the enantiomers have identical chemical and physical properties. Enantiomers rotate plane polarized light direction to equal, but opposite angles, and interact differently with other chiral molecules.

What is the meaning of diastereomers?

Diastereomers are stereoisomers that do not replicate each other’s images, and are not superimposable on each other. The diastereomers can be stereoisomers of two or more stereocenters. Determining whether two molecules are diastereomers is sometimes challenging.

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