Themes

What are enantiomers?

The enantiomers are pairs of compounds that are non – superimposable mirror images of each other. These pairs of compounds are a particular type of isomers, that is, they are different compounds that have the same molecular formula.

There are different types of isomers, among which are stereoisomers, in which all the atoms are united in the same order and with the same type of bonds, but have different orientations in space (stereo means space).

Within the stereoisomers, we find the enantiomers, whose main characteristic is to be mirror images of each other.

Enantiomers are very common in nature. In fact, almost all of the organic compounds present in the cells of all living things are one of two possible enantiomers.

For example, all amino acids that are part of natural proteins correspond to the L enantiomer of the respective amino acid (the other isomer is identified by the letter D).

On the other hand, the active principles of the vast majority of drugs also exist as pairs of enantiomers, of which only one is effective. The difficulty of separating enantiomers from each other makes those drugs that only contain the useful isomer very expensive.

Enantiomers and chirality

The enantiomers are made up of chiral molecules. Chirality is the property of not being superimposable with its mirror image. The word chiral comes from the Greek term, kheir which means hand, recalling the fact that the hands are also mirror images of each other, and cannot be superimposed.

The molecule on the left is chiral, since when you compare it to its mirror image, you can see that they are not superimposable. In other words, there is no way to rotate it or rotate it in such a way that all the atoms coincide with each other.

In view of the above, it can be deduced that for each chiral molecule, there must be another chiral molecule that is its non-superimposable mirror image, that is, its enantiomer. In other words, whenever a molecule is said to be chiral, it is known to be one of two possible enantiomers.

Chiral centers

Many chiral compounds possess one or more asymmetric centers that are responsible for the chirality of the molecule. These are called chiral centers and in many organic compounds they consist of carbon atoms that are bonded to 4 different atoms or groups of atoms.

The particular way in which these four groups are distributed around the asymmetric carbon determines to which of the two enantiomers a chiral molecule corresponds. The presence of a single chiral center ensures that the molecule is chiral, but if there is more than one, it may or may not be chiral.

Most of its physical and chemical properties are identical

Like the right hand and the left hand, enantiomers only come in pairs. These compounds are practically identical to each other. In fact, most of its physical and chemical properties such as melting point, boiling point , vapor pressure and solubility in some solvents, among others, are identical.

Optical activity

All chiral compounds have a unique property that distinguishes them from those that are not: they have the ability to rotate the plane of polarized light. This property is called optical activity , and it is one of the few properties that distinguishes a chiral compound from its enantiomer.

The latter is due to the fact that the plane of polarized light can be rotated in one of two directions, either clockwise (called clockwise and represented by the + symbol) or counter-clockwise (left-handed). , represented by the symbol -).

If a chiral compound rotates the plane of polarized light in one direction, its enantiomer will rotate it in the opposite direction.

  • Example

A D-glucose solution rotates the plane of polarized light clockwise (clockwise, it is clockwise), while an L-glucose solution rotates it counter-clockwise.

Differential reactivity

Another property that allows one enantiomer to be distinguished from the other is its reactivity against other chiral compounds.

Differential reactivity can be compared to how a glove only fits one hand, but not the other, or how a right shoe fits the right foot, but not the left.

An important consequence of differential reactivity is the different effects that the two enantiomers of some drugs can cause. These differences can be harmless, but they can also be very dangerous.

  • Examples

    • Of the two enantiomers of aspartame (which is an artificial sweetener), one is sweet while the other is tasteless.
    • Only the S enantiomer of omeprazole is effective as a gastric protector while the other has no effect.
    • D-penicillamine is a rheumatoid arthritis drug, while its enantiomer, L-penicillamine is a dangerous poison.

Differential absorption

The enantiomers also differ in the way they are absorbed into resins or solids that are also chiral. A mixture of enantiomers can be separated by passing through a chiral separation column, since one of the two enantiomers will be absorbed more strongly than the other.

Nomenclature of the enantiomers

There are several methods to identify one or another enantiomer, but the most widely used is the Cahn-Ingold-Prelog (CIP) system. This consists of the following steps:

  1. A level of hierarchy is assigned to the four groups attached to each chiral center. The priority of the groups is assigned according to the atomic number of the atom directly linked to the chiral center. If there are two equal atoms, we proceed to add the atomic numbers of the atoms that are linked to the first.
  2. The direction in which the three priority groups travel is determined when the lowest priority is pointing backwards.
  3. If the direction of travel is clockwise, it is assigned the R setting , otherwise it is assigned the S setting .

Example

Examples of enantiomers

D-glyceraldehyde and L-glyceraldehyde

Glyceraldehyde is the simplest and smallest chiral carbohydrate that exists, and it is very important for both chemistry and biology.

According to the rules of the Cahn-Ingold-Prelog system, the D isomer corresponds to the (R) isomer and the L isomer corresponds to the (S).

D-Alanine and L-Alanine

Alanine is one of the essential amino acids for the construction of proteins. Like almost all amino acids, it has a chiral carbon so it has two enantiomers:

Of these two, L-alanine is the most common and is present in all living things, while D-alanine is only present in some bacterial cell walls.

D-tartaric and L-tartaric acid

Chirality was discovered by Louis Pasteur in 1848 thanks to tartaric acid. This compound has 2 asymmetric carbons and can exist as two enantiomers plus a third isomer called the meso compound.

The absolute configuration of the two chiral carbons are R for the left-handed enantiomer and S for the right-handed enantiomer.

2-butanol

This alcohol also has a chiral carbon, which makes this compound have two enantiomers.

Bromochloroethane

It is a very simple chiral compound with only two carbons. Chiral carbon has bromine, chlorine, methyl, and hydrogen attached to it.

References

  1. Chirality (Chemistry). (March 18, 2021), on Wikipedia.com
  2. Carey, FA (2008). Organic chemistry . Boston: McGraw-Hill Higher Education.
  3. Smith, M., March, J., & March, J. (2001). March’s advanced organic chemistry: Reactions, mechanisms, and structure . New York: Wiley.
  4. MH Hyun (2012). 8.13 Chromatographic Separations and Analysis: Chiral Crown Ether-Based Chiral Stationary Phases. Editor (s): Erick M. Carreira, Hisashi Yamamoto. Comprehensive Chirality. Elsevier. Pages 263-285. ISBN 9780080951683,
  5. Nguyen, LA, He, H., & Pham-Huy, C. (2006). Chiral drugs: an overview. International journal of biomedical science: IJBS ,  2 (2), 85–100.

Related Articles

Leave a Reply

Your email address will not be published. Required fields are marked *

Back to top button