An achiral molecule is a chemical compound that is identical to its mirror image, meaning it can be perfectly placed on top of its reflection in a way that all parts align. This property is significant in chemistry because it distinguishes these molecules from chiral molecules, which have a “handedness” and cannot be superimposed on their mirror images. Understanding achiral molecules helps explain various chemical behaviors and their roles in different scientific fields.
The Concept of Chirality
The term “chirality” originates from the Greek word “cheir,” meaning “hand,” which provides a useful analogy for understanding this concept. Just as your left hand is a mirror image of your right hand but cannot be perfectly overlaid onto it, chiral molecules possess a similar non-superimposable mirror image relationship.
In contrast, an achiral molecule is one that can be perfectly superimposed on its mirror image. The ability to superimpose on its mirror image means that an achiral molecule possesses certain elements of symmetry within its structure. This fundamental difference in symmetry is what distinguishes achiral molecules from their chiral counterparts.
Identifying Achiral Molecules Through Symmetry
Identifying achiral molecules often involves looking for specific symmetry elements within their structure. One primary element is a “plane of symmetry,” also known as a mirror plane. This is an imaginary plane that divides a molecule into two halves, where one half is a perfect mirror reflection of the other. If a molecule can be bisected by such a plane, it is considered achiral because its mirror image would be identical and superimposable.
Another symmetry element that indicates achirality is a “center of inversion,” symbolized by ‘i’. This is a single point within the molecule through which, if every atom is inverted to an equal distance on the opposite side, it maps onto an identical atom. For example, if an atom is at position (x, y, z), an identical atom would be found at (-x, -y, -z) relative to the center of inversion. The presence of either a plane of symmetry or a center of inversion means a molecule is achiral.
Common Examples of Achiral Molecules
Many common substances we encounter are achiral molecules. Water (H₂O) is a simple example; it has a plane of symmetry that cuts through the oxygen atom and bisects the angle between the two hydrogen atoms. Methane (CH₄), with its tetrahedral shape, also exhibits achirality because it possesses multiple planes of symmetry.
Carbon dioxide (CO₂) is another example, as its linear structure allows for a plane of symmetry passing through the carbon atom and both oxygen atoms. Ethanol (CH₃CH₂OH) is also an achiral molecule. While it has a slightly more complex structure than water or methane, its carbon atoms are not bonded to four different groups, preventing the formation of a chiral center and allowing for superimposability with its mirror image. Even molecules with multiple potential chiral centers can be achiral if an internal plane of symmetry exists, as seen in meso compounds like meso-tartaric acid.