While most of us are familiar with molecules in the form of water, glucose, and ethanol, many people do not know about meso compounds. Meso compounds are a special class of molecular structures that have special properties.
These compounds are composed of one chiral center and two identical mirrored halves. These halves are also called enantiomers due to their mirrored nature.
The difference between these enantiomers is how they interact with light. When light hits a meso compound, it will either absorb or transmit the light depending on the structure. This gives them distinct colors that can be observed with a microscope.
Identifying whether a molecule is chiral or a meso compound can be tricky at times. This article will go into detail about how to distinguish each structure as either chiral, achiral, or a meso compound.
All structures that have a mirror-image version of themselves are classified as achiral. This includes molecules, ions, and coordination complexes.
Molecules are made up of one or more atoms connected by chemical bonds. If a molecule could be rotated such that it looked the same, then it is an achiral molecule.
Ions are charged atoms or molecules. An ion can be considered achiral if it could be placed into an environment where it would not be influenced by external forces, thus sitting in an asymmetric position.
Coordination compounds are made of a central atom bonded to ligands that come from the outside. If all of the ligands on the outside are symmetric, then the compound is considered achiral.
A chiral structure is a molecule or compound that is not superimposable on its mirror image. In other words, one of the enantiomers of the molecule cannot be rotated onto the other enantiomer to produce an identical molecule.
This is due to how the atoms are arranged in the molecule. Molecules that are not superimposable on their mirror images are called chiral molecules or compounds.
Chiral molecules can have two different configurations of its atoms, called enantiomers. These are labeled as left and right, with regard to which direction the molecule can be turned to superpose it on its mirror image.
The word chiral comes from the Greek word for hand, because it refers to how difficult it is to superpose a molecule on its mirror image when holding it in your hand.
There are two types of chiral compounds: achiral and chiral.
A meso compound is a molecule that has a chiral center, but the molecule is shaped in a way that it cannot be superimposed onto its mirror image.
For example, consider n-pentane, C5H12, which has one carbon atom and five hydrogen atoms. If one of the hydrogen atoms was replaced by a carbon atom, the shape of the molecule would be altered.
Since it cannot be superimposed onto its mirror image, n-pentane is chiral, but it is not an individual enantiomer or diastereomer; it is classified as a meso compound.
Different types of chirality
Chirality is a structural feature that occurs in many compounds. There are three types of chirality that can be recognized: Achiral, Chiral, and Meso Compounds.
Achiral compounds are not chiral; they do not contain any asymmetric structures. These compounds can be rotated into symmetric structures without a change in structure. Water is an example of an achiral compound.
Chiral compounds contain at least one asymmetric structure which can be rotated into a symmetric structure without a change in structure. An example of this is glucose which contains an asymmetric carbon atom.
Meso compounds contain both chiral and achiral components which cannot be separated. Cellulose is an example of this – it contains glucose which is a meso compound, you cannot separate the chirality of one from the other.
R and S configurations
The last two categories of chemical compounds are chiral and meso. Chiral compounds are molecules that are not superimposable on its mirror image.
That is, it is not possible to rotate it 180 degrees and have it match the original molecule. This is due to a special type of symmetry called enantiomerism.
Enantiomers are opposite versions of a molecule, and they can only be present in equal amounts in pure liquid form. If you mix the two enantiomers together, they will dissolve each other.
Chiral compounds can be either a R or S configuration of a molecule depending on how its atom groups are oriented. For example, imagine there is a molecule that has three carbon atoms and one hydrogen atom attached to each carbon atom. There are two possible configurations for this compound: RC HC or SC HC.
In some cases, diastereomers can be defined as being improper. This happens when the stereochemistry of one molecule is reversed with respect to another molecule.
For example, a right-handed propeller twisting to the left is an improper stereoisomer. It would be considered a diastereomer of a left-handed propeller twisting to the right.
Because these are not actually possible compounds, this designation is only given when the molecules have different chiralities. The difference in chirality between the two molecules determines whether one is called an improper diastereomer or not.
Proper diastereomers do not exist, so every possible diastereomer pair is either achiral or has at least one chiral element. Identify each of the above structures as either achiral, chiral, or a meso compound.
A chiral center is a position in a molecule where there is a distinct left and right side. A chiral center can be either an atom or a group of atoms.
Atoms are either hydrogen or carbon, so those are the only two possibilities for a chiral center. If a molecule contains more than one chiral center, it is called a meso compound.
Meso compounds contain an even number of chiral centers, so there is no middle atom on which to build a mirror image. Because of this, meso compounds are not enantiomers- they are diastereomers.
The rest of the molecule can be configured in many ways, making each configuration unique. This makes it hard to identify whether or not the substance is enantiomer or diastereomer specific.
Examples of chirality in everyday life
Chirality is a fundamental concept in chemistry, and one that students should be familiar with long before entering the field of research. As mentioned earlier, many new researchers in the field are unaware of this concept, even though it is quite simple to recognize when explained.
Even students taking chemistry courses should be able to identify chiral structures as either achiral, chiral, or meso compounds. Some examples of chirality in everyday life include: your left and right hands, optical mirrors (such as those found in bathrooms), and wine glasses.
Wine glasses are a interesting case study for chirality. When pouring the same type of wine into both a right-handed and left-handed glass, the two wines will not mix. This is because the molecules within the wine are structured differently, making one side of the glass more resistant to the other side’s flow.