What is the difference between Achiral and Chiral Molecules?

A chiral molecule is one that has a carbon atom bonded to four different groups, resulting in non-superimposable mirror images, while an achiral molecule lacks this asymmetry, typically having at least two identical groups attached to the carbon atom or possessing a plane of symmetry.

Understanding these concepts is crucial not only in academic studies but also in real-world applications such as drug design, biochemistry, and material science. In this blog, we will explore what makes a molecule chiral or achiral and why this distinction matters

Achiral and Chiral Molecules

When four different groups are attached to the carbon atom, then the molecule becomes “asymmetric”. The carbon atom is called a Chiral centre, and the molecule is called a “Chiral molecule”. If a molecule has ≥ 2 chiral centres, in that case molecule should not have any plane of symmetry to show Chirality.

“Achiral” is the reverse of Chiral, i.e. four different groups are not attached to the carbon atom.

• Chiral molecules are like left and right hands – they are mirror images.
• Right and left hands are non-superimposable on their mirror images
• Chiral compounds are optically active, which means they rotate plane-polarised light to the left or to the right depending on their configuration.

What is Chirality?

The term chirality comes from the Greek word “cheir,” meaning hand. Just like your left and right hands are mirror images of each other but cannot be superimposed onto each other, chiral molecules have a similar relationship with their mirror images. In simple terms:

• Chiral molecules are non-superimposable on their mirror images.
• Achiral molecules can be superimposed onto their mirror images.

This non-superimposability leads to the concept of chirality in molecules, where the molecule exhibits handedness, much like how your left and right hands are different even though they look similar.

The Basics of Chiral Molecules:

A molecule is considered chiral if it has at least one chiral centre (also called a stereocenter), which is typically a carbon atom bonded to four different groups. This arrangement results in two non-superimposable mirror images, also known as enantiomers. These enantiomers have identical physical properties, such as boiling point and melting point, but they differ in their ability to rotate plane-polarised light and interact with other chiral substances.

Key Characteristics of Chiral Molecules:

• Asymmetry: A chiral molecule lacks symmetry, and thus its mirror image cannot be superimposed on itself.
• Stereochemistry: Chiral molecules have a specific three-dimensional arrangement that distinguishes their enantiomers.
• Optical Activity: Chiral molecules can rotate plane-polarised light, with each enantiomer rotating light in opposite directions (clockwise or counterclockwise).

Examples of Chiral Molecules:

• Lactic Acid: Found in sour milk, the two enantiomers of lactic acid have different biological effects.
• Amino Acids: Most amino acids (except glycine) are chiral and are important in protein synthesis.
• Thalidomide: A notorious example where one enantiomer was effective as a sedative, while the other caused birth defects.

The Basics of Achiral Molecules:

An achiral molecule, on the other hand, has a plane of symmetry and can be superimposed onto its mirror image. Achirality is typically associated with molecules that either:

• Have a symmetrical structure, where one half is identical to the other, or
• Do not have a chiral centre (e.g., molecules where no carbon is bonded to four different groups).

Key Characteristics of Achiral Molecules:

• Symmetry: Achiral molecules exhibit symmetry, meaning their mirror images are identical and can be superimposed.
• No Optical Activity: Achiral molecules do not rotate plane-polarized light since their mirror images are identical.
• Simpler Structures: Achiral molecules often feature repetitive, symmetrical structures that lead to superimposability.

Examples of Achiral Molecules:

• Methane (CH₄): The central carbon is bonded to four identical hydrogen atoms, making the molecule symmetrical.
• Benzene (C₆H₆): A highly symmetrical molecule with alternating double bonds in a hexagonal ring.
• Carbon Dioxide (CO₂): A linear molecule with no chiral centers and symmetrical properties.

Chiral vs. Achiral Molecules: Applications

Chirality plays a huge role in drug development. The two enantiomers of a chiral molecule can have vastly different effects in the body. One enantiomer might be therapeutic, while the other could be harmful. A famous example of this is thalidomide, where one enantiomer was an effective sedative, while the other caused severe birth defects. The study of chirality ensures that only the desired enantiomer is used in drugs, reducing the potential for harmful side effects.

How Do Chemists Identify Chiral and Achiral Molecules?

Chemists use several methods to identify whether a molecule is chiral or achiral. The most common techniques include:

1. Analysing Symmetry:
   Looking for symmetry in the molecular structure is one of the first steps. Achiral molecules usually have a plane of symmetry, while chiral molecules do not.
2. Determining Stereocenters:
   A stereocenter (usually a carbon atom) is bonded to four different groups. If a molecule has at least one stereocenter and no symmetry, it is likely chiral.
3. Optical Activity Tests:
   Chiral molecules can rotate plane-polarised light, whereas achiral molecules cannot. This property can be measured using a polarimeter.

Conclusion

The difference between chiral and achiral molecules depends on the symmetry and the ability to superimpose their mirror images on each other. Chiral molecules are non-superimposable on their mirror images and have important implications in fields like pharmacology and biochemistry. On the other hand, achiral molecules exhibit symmetry and can be superimposed onto their mirror images, making them less complex in terms of molecular interaction.

Related:

• Relative Response Factor (RRF) in Pharmaceutical Analysis

Interview Questions on Achiral and Chiral Molecules

What is an enantiomer?

An enantiomer is one of two mirror-image forms of a chiral molecule. They have identical physical properties, except for their ability to rotate plane-polarised light in opposite directions. Enantiomers can also interact differently with other chiral molecules, which is particularly important in biological systems.

• Dextrorotatory (D): Enantiomers that rotate polarised light clockwise.
• Levorotatory (L): Enantiomers that rotate polarized light counterclockwise.

How do you determine if a molecule is chiral?

To determine if a molecule is chiral, you should check for the following:

1. Asymmetry: The molecule should not have any symmetry elements such as a plane or center of symmetry.
2. Stereocenters: Look for atoms (usually carbon) bonded to four different substituents. If at least one chiral centre is present, the molecule is likely chiral.
3. Non-Superimposability: The molecule’s mirror image should not be superimposable on the original structure (similar to left and right hands).

What is optical activity?

Optical activity refers to the ability of a chiral molecule to rotate plane-polarized light. The direction and magnitude of the rotation depend on the specific enantiomer of the chiral molecule. Dextrorotatory (right-handed) molecules rotate light clockwise, while levorotatory (left-handed) molecules rotate light counterclockwise. This property is often used in laboratories to confirm the chirality of a substance.

What are some examples of chiral molecules in nature?

Many naturally occurring molecules are chiral, especially in biological systems. Some notable examples include:

• Amino acids (except glycine): Building blocks of proteins.
• Sugars: Such as glucose and fructose, which are involved in metabolism.
• DNA: The double helix structure of DNA is inherently chiral.
• Caffeine: A stimulant found in coffee and tea that is chiral.

What are achiral molecules commonly made of?

Achiral molecules usually have symmetrical structures. Common examples include:

• Methane (CH₄): A simple molecule where the central carbon is bonded to four identical hydrogen atoms.
• Benzene (C₆H₆): A symmetrical ring structure with alternating double bonds.
• Carbon Dioxide (CO₂): A linear molecule with a symmetrical structure.

How does chirality affect the biological activity of molecules?

Chirality plays a key role in how molecules interact with biological receptors, enzymes, and other molecules. Enzymes, for example, are often selective for a specific enantiomer of a chiral molecule. This means that the biological activity of a molecule can vary greatly depending on whether it is in its “left-handed” or “right-handed” form.

How are chiral molecules represented in chemical structures?

Chiral molecules are typically represented in 3D models using wedge and dash notation or by indicating the stereochemistry of the chiral centres (R/S or D/L configurations). The R or S designation refers to the absolute configuration at the chiral centre, while D or L refers to the molecule’s optical activity.

How does chirality influence the taste and smell of substances?

Chirality can have a significant impact on the taste and smell of a substance. In fact, the same molecule can have a completely different taste or smell depending on its enantiomer. For example, limonene has a lemon scent when it is the (R)-enantiomer and an orange scent when it is the (S)-enantiomer. This is because the shape and structure of the molecule determine how it interacts with receptors in our taste and smell sensors.

Furter reading

• Chiral and achiral molecules