ENANTIOMERS: CHARACTERISTICS, PROPERTIES AND EXAMPLES - CHEMISTRY - 2025

The enantiomers are those pairs (inorganic) organic compounds consisting of two mirror images are not superimposable one upon the other. When the opposite occurs - for example, in the case of a ball, golf club, or fork - they are said to be achiral objects.

The term chirality was coined by William Thomson (Lord Kelvin), who defined that an object is chiral if it cannot be superimposed with its mirror image. For example, the hands are chiral elements, since the reflection of the left hand, even if it turns around, will never coincide with the original.

One way to demonstrate this is by placing the right hand over the left, finding that the only fingers that overlap are the middle. In fact, the word chiral derives from the Greek word cheir, which means "hand."

In the case of the fork in the upper image, if its reflection were turned over, it would fit perfectly under the original, which translates as an achiral object.

Asymmetric carbon

What geometric shape must a set of atoms have to be considered chiral? The answer is tetrahedral; that is, for an organic compound the carbon atom must have a tetrahedral arrangement around it. However, while this applies to most compounds, this is not always the case.

For this hypothetical compound CW _{4 to} be chiral, all substituents must be different. If it were not this way, the reflection of the tetrahedron could overlap after some rotations.

Thus, compound C (ABCD) is chiral. When this occurs, the carbon atom attached to four different substituents is known as an asymmetric carbon (or stereogenic carbon). When this carbon is "looked at" in the mirror, its reflection and this make up the enantiomeric pair.

Illustrated in the upper image are three enantiomeric pairs of compound C (ABCD). Considering only the first pair, its reflection is not superimposable, because when flipping only the letters A and D coincide, but not C and B.

How are the other pairs of enantiomers related to each other? The compound and its image of the first enantiomeric pair are diasteromers of the other pairs.

In other words, the diasteromers are stereoisomers of the same compound, but without being the product of their own reflection; that is, they are not your mirror image.

A practical way to assimilate this concept is through the use of models, some of these as simple as those assembled with an anime ball, some toothpicks and some masses of plasticine to represent the atoms or groups.

Nomenclature

Changing the place of two letters produces another enantiomer, but if three letters are moved, the operation returns to the original compound with a different spatial orientation.

In this way, changing two letters gives rise to two new enantiomers and, at the same time, to two new diastereomers of the initial pair.

However, how do you differentiate these enantiomers from each other? This is when the absolute RS configuration arises.

The researchers who implemented it were Cahn, Sir Christopher Ingold and Vladimir Prelog. For this reason it is known as the Cahn-Ingold-Prelog Notation System (RS).

Sequence rules or priorities

How to apply this absolute setting? First, the term "absolute configuration" refers to the exact spatial arrangement of the substituents on the asymmetric carbon. Thus, each spatial arrangement has its own R or S configuration.

The top image illustrates two absolute configurations for a pair of enantiomers. To designate one of the two as R or S, the sequencing or priority rules must be followed:

1- The substituent with the highest atomic number is the one with the highest priority.

2- The molecule is oriented so that the lowest priority atom or group points behind the plane.

3- The arrows of the links are drawn and a circle is drawn in a descending direction of priority. If this direction is the same clockwise, the setting is R; if it is counterclockwise, then the setting is S.

In the case of the image, the red sphere marked with the number 1 corresponds to the substituent with the highest priority, and so on.

The white sphere, the one with number 4, almost always corresponds to the hydrogen atom. In other words, hydrogen is the lowest priority substituent and is counted last.

Absolute configuration example

In the compound in the upper image (amino acid l-serine), the asymmetric carbon has the following substituents: CH ₂ OH, H, COOH and NH ₂.

Applying the previous rules for this compound, the substituent with the highest priority is NH ₂, followed by COOH and, lastly, CH ₂ OH. The fourth substituent is understood to be H.

The COOH group has priority over CH ₂ OH, because the carbon forms three bonds with the oxygen atoms (O, O, O), while the other forms only one with the OH (H, H, O).

Characteristics of the enantiomers

The enantiomers lack elements of symmetry. These elements can be either the plane or the center of symmetry.

When these are present in the molecular structure, it is very likely that the compound is achiral and therefore cannot form enantiomers.

Properties

A pair of enantiomers exhibit the same physical properties, such as boiling point, melting point, or vapor pressure.

However, a property that differentiates them is the ability to rotate polarized light, or what is the same: each enantiomer has its own optical activities.

The enantiomers that rotate the polarized light clockwise acquire the (+) configuration, while those that rotate it counter-clockwise acquire the (-) configuration.

These rotations are independent of the spatial arrangement of the substituents on the asymmetric carbon. Consequently, a compound of R or S configuration can be (+) and (-).

Additionally, if the concentrations of both enantiomers (+) and (-) are equal, the polarized light does not deviate from its path and the mixture is optically inactive. When this happens, the mixture is called a racemic mixture.

In turn, spatial arrangements govern the reactivity of these compounds against stereospecific substrates. An example of this stereospecificity occurs in the case of enzymes, which can only act on a certain enantiomer, but not on its mirror image.

Examples

Of many possible enantiomers, the following three compounds are exemplified:

Thalidomide

Which of the two molecules has the S configuration? The one of the left. The order of priority is as follows: first the nitrogen atom, second the carbonyl group (C = O), and third the methylene group (–CH ₂ -).

When going through the groups, use the clockwise direction (R); however, since the hydrogen points out of the plane, the configuration seen from the rear angle actually corresponds to the S, while in the case of the molecule on the right, the hydrogen (the one with the lowest priority) points back once of the plane.

Salbutamol and limonene

Which of the two molecules is the R enantiomer: the top or bottom? In both molecules the asymmetric carbon is linked to the OH group.

Establishing the order of priorities for the molecule below that gives as follows: first the OH, second the aromatic ring and third the group CH ₂ -NH-C (CH ₃) ₃.

Going through the groups, a circle is drawn in a clockwise direction; therefore, it is the R enantiomer. Thus, the bottom molecule is the R enantiomer, and the top one the S.

In the case of the compound (R) - (+) - limonene and (S) - (-) - limonene, the differences are in their sources and odors. The R enantiomer is characterized by having an orange smell, while the S enantiomer has a lemon smell.

References

1. TW Graham Solomons, Craigh B. Fryhle. Organic Chemistry. (Tenth Edition, p 188-301) Wiley Plus.
2. Francis A. Carey. Organic Chemistry. In Stereochemistry. (Sixth ed., Pp. 288-301). Mc Graw Hill.
3. Zeevveez. (August 1, 2010). Fork Mirror Reflection.: Retrieved on April 17, 2018, from: flickr.com
4. GP Moss. Basic terminology of stereochemistry (IUPAC Recommendations 1996) Pure and Applied Chemistry, Volume 68, Issue 12, Pages 2193–2222, ISSN (Online) 1365-3075, ISSN (Print) 0033-4545, DOI: doi.org
5. Molecule of the Week Archive. (September 1, 2014). Thalidomide. Retrieved on April 17, 2018, from: acs.org
6. Jordi picart. (July 29, 2011). Assignment of the R and S configurations to a chiral center.. Retrieved on April 17, 2018, from: commons.wikimedia.org