Chiral vs. Achiral vs. Meso Compounds

Chiral compounds are optically active molecules that are non-superimposable on their mirror images. Achiral compounds are optically inactive molecules that superimpose on their mirror images. However, meso compounds are optically inactive compounds with multiple chiral centers.

Chirality in chemistry is a property of certain compounds to rotate the planes of polarized light when subjected. Chiral compounds are able to do so which corresponds to the asymmetrical nature. Achiral compounds are relatively more symmetrical. One can say these conventions to be like:

‘Chiral ↔ Asymmetrical’

‘Achiral ↔ Symmetrical’

Meso compounds are also chiral, like chiral compounds themselves but meso compounds have a relatively greater number of chiral centers than chiral ones. Meso compounds are basically achiral compounds with more than one point of chirality (chiral centers).

Difference Between Chiral, Achiral, and Meso compounds

Chiral compounds Achiral compounds Meso compounds
Compounds having central carbon atom tetrahedrally bonded to four different groups Compounds having central carbon atom tetrahedrally bonded to at least two same groups Compounds having two or more carbon atoms tetrahedrally bonded to four different groups
These compounds are optically active These compounds are optically inactive These compounds are optically inactive
Chiral compounds have non-superimposable mirror images Achiral compounds have superimposable mirror images Meso compounds have superimposable mirror images
There is no plane of symmetry in chiral compounds Achiral compounds have planes of symmetries Meso compounds have planes of symmetries but these plains are limited inside of the molecules

Chiral center

When an atom in a molecule is tetrahedrally bonded to four different substituents, it is known as a chiral atom or chiral center. If the chiral center is a carbon atom, it must be sp3 hybridized. Mirror images of chiral molecules are not superimposable on each other, yet they are asymmetrical.

Chiral molecules do not have a plane of symmetry. They have all single bonds around them.

For example, starred central carbon is a chiral center, containing different groups among it and being asymmetrical.

Chiral molecules example

Stereo-center (Origin of stereochemistry)

Stereocenters are the origin of the whole concept of stereochemistry. When an atom is bonded to four different groups and the exchange of any two groups produces a stereoisomer, the central atom of that molecule is known as a stereocenter. Stereocenters may have single or double bonds around them. Molecules with stereocenters sometimes may or may not have a plane of symmetry.

  • Their stereo central atom may be sp3 or sp2 hybridized.

For example, starred central carbon atoms are stereocenters as they are chiral.

Stereocenters in Chirality

Swapping of two groups over a stereocenter produces new isomers known as stereoisomers.

Chiral compounds

The term chiral is derived from the Greek word “Cheir” meaning ‘hand’ and this property of handedness is called chirality. Chirality is a necessary concept for the optical activity of molecules. A carbon atom that is bonded to four different groups is called chiral carbon.

The chiral center is the origin of chirality. A tetrahedrally coordinated atom bonded to four different groups is having chirality.

Diagramatic representation:

Chiral compounds and chiral center in a molecule

  • Chirality cannot be localized around one atom or group of atoms. It is the property of the molecule as a whole.
  • Chiral molecules are asymmetric meaning there is no symmetry at all.
  • There is no plane or angle for symmetry to establish.
  • Chiral molecules cannot be superimposed on their mirror image.

For example,

  • A human hand does not possess a plane of symmetry. Its mirror image is another hand but the opposite hand e.g. mirror image of a right hand will only be a left hand. The left and right hands are not identical because they can not be superimposed on each other.

Similarly,

  • Gloves
  • Shoes, etc.

Chiral compounds can rotate plane-polarized light either in a clockwise direction or in an anticlockwise direction.

Chiral vs Achiral compounds

In the above example, the molecules have the same molecular formulae, although they have different rotation patterns for plane polarized light. An ‘S-1-chloroethan-1-ol’ rotates the plane of polarized light in the left or anticlockwise direction, while ‘R-1-chloroethan-1-ol’ rotates the plane in right or clockwise direction.

Non superimposable mirror images of a chiral compound

The above image shows the non-superimposable mirror images of a compound, proving it to be a chiral compound.

Achiral compounds

Molecules having identical mirror images which are superimposable are called achiral molecules. Being superimposable means being able to replace or fix onto each other without any change in chemical or physical properties especially the rotation of plane polarized light.

Diagramatic representation:

Non superimposable achiral compounds

  • Achiral compounds are having molecules with at least two similar groups attached to the central atom.
  • Achirality makes molecules able to become symmetrical, so they have a plane of symmetry.
  • They are optically inactive.
  • Achiral compounds make superimposable mirror images.
  • A molecule with all different four groups attached to the central atom is also an achiral molecule.

For example,

Superimposable mirror images of achiral molecule

Superimposable mirror images seem to be superimposable when we rotate one of them to 180°.

Meso compounds

Meso compounds are achiral compounds with more than one stereo-centers that are chiral. Meso Compounds are optically inactive compounds because of the cancellation of optical activity of one stereocenter by any other one.

Meso compounds must have two chiral centers. Usually, they have one chiral center with an ‘R’ configuration and the other one with an ‘S’ configuration.

Diagramatic representation:

Meso compounds representation

Mesocompounds are having (R-S, S-R, S-S, R-R) type of configurations in case. when there are only two chiral stereocenters.

  • Mesocompounds have an internal plane of symmetry.
  • They are superimposed on their mirror image.
  • Because of internal symmetry, one half of the meso compound is a mirror image of the other half.
  • Meso-compounds can be separated by the resolution process.

For example,

1. Butane-2,3-diol can perfectly show the meso characteristics of a meso compound.

S and R configuration of a meso compound

. Mirror images of meso compounds

Mirror images of ‘Butan-2,3-diol’ clear the perception of the internal plane of symmetry which can be seen in the right side mirror image.

2. Another example of a meso compound is 1,2-dimethyl cyclohexane.

Meso compound with plane of symmetry

1,2-dimethyl cyclohexane has two chiral centers but is still an optically inactive compound. One of the chiral centers rotates plane-polarized light in a clockwise direction (R) while the other in the anticlockwise direction (S). The net rotation is thus zero and it has a plane of symmetry as well.

Related Resources

Factors Affecting optical activity (Chiral centers)

Enantiomers

Enantiomers are non-superimposable mirror images of a single molecule in different orientations.

For example, 2-hydroxypropionic acid (lactic acid) is having a stereocenter with the ability to form enantiomers.

2,4-dihydroxypropanoic acid as enantiomers

They have the same physical properties i.e boiling point, melting point, density, but have different stereochemical properties. They have a difference in rotation of plane-polarized light. ‘R configured’ compound rotates the plane of polarized light to the right and ‘S configured’ one to the left.

Diastereomers

Diastereomers are non mirrored, not superimposable images of a compound. They have a different angle of rotation. They have different physical and chemical properties. These compounds are easy to separate by separation techniques e.g. fractional distillation.

For example,

Diastereomers stereocenter for chirality

[(1S,2R)-2-chlorocyclohexan-1-ol] and [(1S,2S)-2-chlorocyclohexan-1-ol] are two diastereomers that have chiral centers for optical rotation but that depends on whether a compound is ‘R’ configured or ‘S’.

Racemic mixture

A sample having an equal amount of two enantiomers of a chiral compound is called a racemic mixture (equimolar mixture of a pair of enantiomers). A racemic mixture is optically inactive as if one half rotates the plane-polarized light in a clockwise (R) direction, the other half will rotate the plane towards the anticlockwise (S) direction. There is no net rotation of plane-polarized light in a racemic mixture.

For example,

Racemic mixture of alpha chloropropionic acid

A mixture of 2-chloropropionic acid in R and S configurations makes a racemic mixture, (±)-α-chloropropionic acid, which is neither an R nor S configured because it is quite optically inactive or neutral.

Key takeawayS

  • Chiral compounds have optical activity and no plane of symmetry.
  • Achiral compounds have no optical activity but are symmetric.
  • Meso compounds are optically inactive too, but they have an internal plane of symmetry.

Chiral vs Achiral vs Meso compounds

Concepts Berg

What is the difference between achiral and meso?

Achiral molecules have no chiral center while meso compounds have two or more chiral centers. Furthermore, achiral compounds are symmetrical while meso compounds are internally symmetrical only.

Is meso chiral or achiral?

Meso compounds are achiral molecules and have multiple chiral centers. They have superimposable mirror images but are optically inactive.

What is the difference between chiral and achiral?

Chiral molecules have four different groups tetrahedrally bonded with the central atom. They have non-superimposable mirror images and have no plane of symmetry. Chiral compounds are optically active because they turn the plane of polarized light when they come in path.

Achiral molecules have at least two same groups bonded with the central atom. They have superimposable mirror images and a plane of symmetry. Achiral compounds are optically inactive.

How do you know if it is a compound meso?

Meso compounds have two main identifications.

  • Two or more chiral centers
  • Internal plane of symmetry

Are achiral molecules optically active?

Achiral molecules are optically inactive because they have no chiral center and they are symmetrical too.

What is a meso diastereomer?

When compounds have two or more chiral centers and are not mirror images, the compound is said to be a meso diastereomer.

A combination of meso and diastereomer is called meso diastereoisomer.

Diastereomers which are meso compounds are called meso diastereoisomers e.g. Meso-tartaric acid, etc.

What are the similarities between achiral and meso compounds?

Similarities among meso and achiral compounds are:

  • Optically inactivity
  • Having planes of symmetry

What are diastereomers? Give an example.

Diastereomers are non mirrored images to each other. They are not superimposable and have a different angle of rotation. They have different physical and chemical properties and are easy to separate.

For example, 2,3-dihydroxybutanoic acid (Lactic acid) has (RS) and (SS) diastereomers.

What is chiral carbon?

A carbon atom is tetrahedrally bonded, having four different groups, and no symmetry at all.

What defines a stereocenter?

When an atom is bonded to four different groups and the exchange of any two groups across the center produces stereoisomers, this is known as a stereocenter. They either may or may not have a plane of symmetry and maybe sp2 or sp3 hybridized.

How can a compound be achiral as well as meso?

Meso compounds are achiral because they have the plane of symmetry but are optically inactive. This is because of the fact that the optical activity of one center is canceled by the opposite other.

Can meso compounds be called achiral?

Meso compounds can be called achiral compounds because both are optically inactive and have a plane of symmetry.

What are chiral molecules?

Chiral molecules have four different groups tetrahedrally bonded with the central atom. They have non-superimposable mirror images and do not have a plane of symmetry.

Why are meso compounds not considered to be optically active?

Meso compounds are optically inactive because they have two or more chiral centers. One with R configuration and the other with S configuration may cancel each other’s rotation as the rotation of plane-polarized light in a clockwise and anticlockwise direction. This brings us back to square one, and the net rotation is zero. That’s why meso compounds are considered optically inactive.

Are all stereoisomers chiral?

Not all stereoisomers are chiral, some achiral and meso stereoisomers exist as well. Cis/trans stereoisomers may exist as achiral molecules.

Example of the chiral molecule, optically inactive.

Chiral molecules with only one chiral center always have optical activity. Meso compounds and racemic mixtures are examples where the molecule is chiral but optically inactive. The net rotation of plane-polarized light is zero as one of the isomers rotates plane-polarized light in a clockwise direction and the other rotates in the anti-clockwise direction.

What is the difference between a stereocenter and a chiral center?

When an atom in a molecule is tetrahedrally bonded to four different substituents, it is known as a chiral atom or chiral center. Chiral molecules do not have a plane of symmetry.

When an atom is bonded to four different groups and the exchange of any two groups produces stereoisomers, it is known as a stereocenter. Stereocenters may have single or double bonds around them. Their carbon may be sp3 or sp2 hybridized and they may or may not have a plane of symmetry.

Why are chiral compounds optically active?

Chiral molecules are optically active because they can rotate plane-polarized light either in a clockwise or anticlockwise direction.

What are the terms chiral and chiral centers?

The term chiral is derived from the Greek word “cheir” which means hand. While when an atom in a molecule is tetrahedrally bonded to four different substituents at one time, it is known as a chiral atom or chiral center.

Why are atropisomers optically active even though they don’t have a chiral center?

Atropisomers are special cases where the molecules do not have chiral centers but are still optically active. This phenomenon is present in biphenyl and helicenes. In these bulky substitutions, ortho positions prevent rotation of bonds and both rings become mutually perpendicular to each other and the plane of symmetry is no more there. That’s the reason they are optically active.

For example, ortho disubstituted biphenyls, etc.

Are chiral molecules always enantiomers?

Chiral molecules are always enantiomers because enantiomers are non-superimposable mirror images of each other and so does a chiral molecule demand.

What are the differences between stereoisomers and constitutional isomers?

Constitutional isomerism is due to different arrangements of atoms in molecules while stereoisomerism deals with the study of the spatial arrangement of atoms in molecules.

Compounds having the same molecular formula but different structural formulas are called constitutional isomers. They have differences in connectivities. Constitutional isomers have different nomenclature and have significantly different properties.

Compounds having the same connectivities but differ in the arrangement of the same groups in space are termed stereoisomers. They have the same connections. Stereoisomers have the same nomenclature with special prefixes. E.g. cis/Trans, E/Z ,R/S. They have relatively different properties.

What is so important about chiral molecules in regards to biology?

Biology is the study of life. All biochemical processes are catalyzed by enzymes and enzymes are usually containing chiral centers. Enzymes are proteins in nature and proteins are made up of chiral amino acids.

For example,

DNA has a helical structure and rotates plane-polarized light to the right (R).

What is the difference between meso compounds and enantiomers?

Meso compounds are superimposable mirror images. They are optically inactive and have an internal plane of symmetry. Meso compounds are achiral as a whole in nature.

Enantiomers are non-superimposable mirror images. They are optically active and have no plane of symmetry. Enantiomers are chiral.

Are diastereomers always an achiral molecule?

Diastereomers are not always achiral. One diastereomer may be chiral and rotate plane-polarized light while the other may be achiral and not rotate the plane-polarized light at all.

Why 1,3-dichloroallene is chiral?

In the case of allenes, the central carbon atom is sp-hybridized and the terminals are sp2 hybridized. The central atom forms two sp-sp2 sigma bonds. The central atoms have two p-orbitals. These p orbitals form pi bonds and as a result, substituents at the end of molecules are in one plane which is perpendicular to that of the substituent at another end. So, they lack a plane of symmetry and become optically active.

Does every organic chiral molecule show optical activity?

If a molecule is chiral, it must be optically active because it can rotate the plane of polarized light when it is passing through.

How do I check the molecule is chiral and optically active?

Whenever an atom is tetrahedrally bonded to four different groups, it is a chiral molecule.

The optical activity of molecules is measured with the help of a polarimeter.

Are three aromatic rings chiral?

No three aromatic rings (combined e.g. anthracene and phenanthrene) are achiral because they have planes of symmetries. If three aromatic rings are substituted with the different groups, they may become chiral i.e. All unsubstituted aromatic rings are achiral.

Can a chiral molecule have a plane of symmetry?

No chiral molecule can have a plane of symmetry.

What is the difference between enantiomers and diastereomers?

Enantiomers are non-superimposable mirror images of each other. They have similar physical properties i.e. boiling points, melting points, density, etc but they have different stereochemical properties. The difference is in the rotation of plane-polarized light.

Diastereomers are non mirrored images to each other. They are not superimposable. Diastereoisomers have different angles of rotation and they have different physical and chemical properties. They are even easier to separate.

Is optical activity because of chirality?

Chirality is a necessary and sufficient condition for optical activity. A chiral molecule is always optically active.

How is 1,3-dibromo 1 chloropropane a chiral compound?

1,3-dibromo-1-chloropropane is a chiral compound because the first carbon is tetrahedrally bonded to four different groups.

How do you know the number of chiral centers for CH2BrCHBrCH3?

When we draw the structure of the molecule, we can easily locate chiral centers. This molecule has only one chiral center.

How do I find if an atom is chiral or not?

When an atom is tetrahedrally bonded to four different groups then it is chiral and if it doesn’t have four different groups tetrahedrally bonded then it is achiral.

What if the atom has a double bond, can it be chiral?

Double bonded carbon atoms can never be chiral.

What is the number of chiral carbon atoms in 1,2-dimethyl cyclohexane?

1,2-dimethyl cyclohexane has two chiral carbon but is not a chiral compound because of the plane of symmetry.

If a chiral carbon atom is present in a molecule, is that molecule optically active?

Optical activity is the rotation of plane-polarized light either in a clockwise or anticlockwise direction. Chirality is necessary and sufficient condition for optical activity. A chiral molecule is always optically active.

What makes a compound optically active?

Chirality of compound makes a compound optically active.

Do separate enantiomers have the same physicochemical properties as their racemates?

Separate enantiomers have the same physicochemical properties as their racemates. They just have differences in behavior towards plane-polarized light.

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