Enantiomers: Nomenclature, Characteristics, Properties and Examples

The enantiomers are those pairs of organic (and inorganic) compounds that consist of two mirror images that can not overlap one over the other. When the opposite occurs-for example, in the case of a ball, a golf club or a 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 can not overlap with its mirror image. For example, the hands are chiral elements, because the reflection of the left hand, although it turns, will never coincide with the original.

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

For the case of the fork of the upper image, if its reflection were to turn, it would fit perfectly under the original, which translates as an achiral object.

Asymmetric carbon

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

So that this hypothetical CW compound ₄ be chiral, all substituents must be different. If it were not in this way, the reflection of the tetrahedron could overlap after some rotations.

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

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

What relation do the other pairs of enantiomers have to each other? The compound and its image of the first enantiomeric pair are diastereomers of the other pairs. In other words, the diastereomers are stereoisomers of the same compound, but without being the product of their own reflection; that is, they are not its mirror image.

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

Nomenclature

The change of place of two letters produces another enantiomer, but if three letters are moved, the operation returns to the original compound with 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 to differentiate these enantiomers from each other? This is where the absolute R-S configuration arises. The researchers who implemented it were Cahn, Sir Christopher Ingold and Vladimir Prelog. For this reason it is known as the notation system (R-S) of Cahn-Ingold-Prelog.

Rules of sequences or priorities

How to apply this absolute configuration? First, the term â???? absolute configurationâ???? it alludes to the exact spatial arrangement of the substituents in asymmetric carbon. Thus, each spatial arrangement has its own R or S configuration.

The upper image illustrates two absolute configurations for a pair of enantiomers. To designate one of the two as R or S, the rules of sequences or priorities 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 atom or group of least priority points behind the plane.

3- Draw the arrows of the links and draw a circle in descending direction of priority. If this direction is the same clockwise, the configuration is R; if it is counter-clockwise, then the configuration 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, that of number 4, almost always corresponds to the hydrogen atom. In other words: hydrogen is the lower priority substituent and counts last.

Example of absolute configuration

In the composite of the upper image (amino acid l-serine), the asymmetric carbon has the following substituents: CH ₂ OH, H, COOH and NH ₂ . Applying the above rules for this compound, the substituent with the highest priority is NH ₂ , followed by the COOH and, finally, the CH ₂ OH. The fourth substituent is understood to be the H.

The COOH group has priority over CH ₂ OH, because carbon forms three bonds with oxygen atoms (O, O, O), while the other forms only one with 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, can not form enantiomers.

Properties

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

However, one 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 towards the opposite acquire the configuration (-).

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

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

In turn, the 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 specular image.

Examples

Of many possible enantiomers, we have as examples the following three compounds:

Thalidomide

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

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

Salbutamol and limonene

Which of the two molecules is the R enantiomer: the one above or the one below? In both molecules the asymmetric carbon is linked to the OH group. Establishing the order of priorities for the lower molecule that gives thus: first the OH, second the aromatic ring and third the CH group 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 molecule below is the R enantiomer, and the top one is the S.

For 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 odor of oranges, while the S-enantiomer has a smell of lemons.

References

1. T.W. Graham Solomons, Craigh B. Fryhle. Organic Chemistry (Tenth Edition, p 188-301) Wiley Plus.
2. Francis A. Carey. Organic Chemistry In Stereochemistry . (Sixth edition., P. 288-301). Mc Graw Hill.
3. Zeevveez (August 1, 2010). Fork Mirror Reflection. [Figure]: Retrieved on April 17, 2018, from: flickr.com
4. G. P. 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 in a chiral center. [Figure]. Retrieved on April 17, 2018, from: commons.wikimedia.org