Important Differences between Chiral and Achiral


A chiral molecule is a type of molecule that lacks superimposable mirror images, much like a person’s left and right hands. In other words, a chiral molecule cannot be superimposed onto its mirror image. This property arises when a molecule contains an asymmetric carbon atom, which is bonded to four different groups or atoms. Due to their non-superimposable nature, chiral molecules exist in two distinct forms known as enantiomers. These enantiomers have identical physical properties but can interact differently with other chiral molecules, including biological systems. This property is crucial in fields like pharmacology, where the specific shape of a molecule can determine its biological activity and potential as a drug.

Properties of Chiral

  • Asymmetry:

Chiral molecules are asymmetric and cannot be superimposed onto their mirror images.

  • Enantiomers:

Chiral molecules exist in two enantiomeric forms, known as “left-handed” (L) and “right-handed” (D) enantiomers.

  • Identical Physical Properties:

Enantiomers have identical physical properties, such as boiling point, melting point, and solubility.

  • Different Biological Activity:

Enantiomers can interact differently with chiral biological molecules, such as enzymes or receptors, leading to different biological activities.

  • Interaction with Polarized Light:

Enantiomers rotate plane-polarized light in opposite directions, a property known as optical activity.

  • Separation Methods:

Chiral molecules can be separated using techniques like chromatography or crystallization in the presence of a chiral auxiliary.

  • Pharmacological Significance:

In pharmacology, the specific arrangement of atoms in a chiral drug molecule can significantly impact its efficacy, safety, and potential side effects.

  • Environmental Impacts:

Chiral pollutants in the environment may have different toxicities or degradation rates depending on their chirality.


A molecule is considered achiral if it possesses a superimposable mirror image. In other words, an achiral molecule can be aligned with its mirror image in such a way that they perfectly overlap. This means that an achiral molecule lacks handedness or asymmetry. Achiral molecules typically possess a plane of symmetry, a point of inversion, or a center of symmetry. These elements allow the molecule to be divided into two equal halves that are mirror images of each other. Achiral molecules do not exhibit optical activity, which is the ability to rotate plane-polarized light. Many simple, symmetric molecules like methane (CH₄) and ethane (C₂H₆) are examples of achiral compounds.

Properties of Achiral

  • Superimposability:

Achiral molecules can be superimposed onto their mirror images, meaning they have a plane of symmetry, a center of inversion, or a point of symmetry.

  • Lack of Handedness:

Unlike chiral molecules, achiral molecules do not have a distinct left-handed (L) and right-handed (D) form.

  • Absence of Optical Activity:

Achiral molecules do not exhibit optical activity, which means they do not rotate plane-polarized light.

  • Identical Physical Properties:

Achiral molecules have identical physical properties (such as boiling point, melting point, and solubility) to their mirror images.

  • No Enantiomers:

Achiral molecules do not have enantiomers, as they are superimposable with their mirror images.

  • Symmetry Elements:

Achiral molecules possess symmetry elements, like a plane of symmetry, point of inversion, or center of symmetry.

  • Easy Separation:

Achiral molecules can be easily separated from their mirror images using techniques like distillation, crystallization, or chromatography.

  • Common in Simple Structures:

Many simple, symmetric molecules, such as diatomic gases (e.g., O2, N2) and some hydrocarbons (e.g., ethane, CH4), are achiral.

Important Differences between Chiral and Achiral

Basis of Comparison Chiral Achiral
Definition Lacks superimposable mirror image Possesses superimposable mirror image
Handedness Exhibits handedness or asymmetry Lacks handedness or asymmetry
Enantiomers Has non-superimposable mirror image Does not have enantiomers
Optical Activity Exhibits optical activity Does not exhibit optical activity
Symmetry Elements Lacks certain symmetry elements Possesses symmetry elements
Plane of Symmetry Absent Present
Center of Inversion Absent Present
Point of Symmetry Absent Present
Identical Physical Props Enantiomers have identical props Mirror images have identical props
Example Lactic Acid Ethane
Natural Occurrence Common in biological molecules Common in symmetric molecules
Pharmacological Signif. Often crucial in drug design Less critical in drug design
Environmental Impact Can affect interactions in nature Less influential in natural systems
Industrial Applications Important in pharmaceuticals Less crucial in certain industries
Separation Techniques Chiral resolution methods needed Separation is straightforward

Important Similarities between Chiral and Achiral

  • Chemical Composition:

Both chiral and achiral molecules are composed of the same types and numbers of atoms.

  • Bonding Patterns:

They follow the same principles of chemical bonding and atomic interactions.

  • Physical States:

Chiral and achiral molecules can exist in the same physical states (solid, liquid, gas) under similar conditions.

  • Participation in Reactions:

Both types of molecules can participate in chemical reactions and form various compounds.

  • Interaction with Solvents:

Chiral and achiral molecules can dissolve in similar types of solvents based on their polarity and intermolecular forces.

  • Structural Diversity:

They can both exhibit a wide range of molecular structures and conformations.

  • Thermal Properties:

They obey the same principles of thermodynamics and exhibit similar heat capacities, enthalpies, and entropies.

  • Spectroscopic Behavior:

Both types of molecules can be analyzed using various spectroscopic techniques, such as NMR, IR, UV-Vis, etc.

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