Some molecules, despite containing chiral centers, do not rotate plane-polarized light. This behavior is characteristic of meso compounds. Understanding their optical inactivity requires exploring their unique structural features and how they interact with light.
Understanding Optical Activity
Optical activity refers to the ability of certain molecules to rotate the plane of plane-polarized light. Normal light waves oscillate in all directions, but when passed through a polarizing filter, only waves oscillating in a single plane are allowed through, creating plane-polarized light. When this plane-polarized light interacts with a chiral substance, its plane of oscillation is rotated.
The rotation can be either clockwise (dextrorotatory, denoted by ‘d’ or ‘+’) or counterclockwise (levorotatory, denoted by ‘l’ or ‘-‘). The extent and direction of this rotation are unique to each optically active compound. A polarimeter is the instrument used to measure this rotation, quantifying the angle by which the plane of polarized light is rotated after passing through a sample. Optical activity is a property exhibited exclusively by chiral molecules, which are molecules that are non-superimposable on their mirror images.
Defining Meso Compounds
A meso compound is a stereoisomer that possesses two or more chiral centers but is optically inactive. Despite containing chiral centers, a meso compound is considered an achiral molecule. This achirality stems from an internal plane of symmetry or a center of inversion within the molecule.
An internal plane of symmetry effectively divides the molecule into two mirror-image halves. If a mirror were placed through the center of the molecule, one side would perfectly reflect the other. Because a meso compound is superimposable on its mirror image, it does not exhibit the handedness necessary to rotate plane-polarized light. For example, tartaric acid can exist as a meso compound, where an internal plane of symmetry makes it achiral despite having two chiral centers.
The Internal Compensation
The optical inactivity of meso compounds is a direct consequence of “internal compensation.” The internal plane of symmetry causes the optical rotation generated by one part of the molecule to be precisely canceled by the equal and opposite rotation from another part. Individually, each chiral center would rotate plane-polarized light, but their combined effect within the same molecule negates this rotation.
Consider a meso compound with two chiral centers. Often, one chiral center will have an ‘R’ configuration and the other an ‘S’ configuration. The ‘R’ configured portion of the molecule rotates light in one direction, while the ‘S’ configured portion rotates it in the opposite direction with an equal magnitude. Since these two oppositely rotating effects occur within the same molecule and are perfectly balanced due to the internal symmetry, the net optical rotation observed is zero.
This internal cancellation means that the molecule, as a whole, behaves as if it were achiral, even though it contains chiral centers. The overall molecular structure, not just the presence of individual chiral centers, determines whether a compound is optically active. Thus, despite the potential for chirality at specific points, the complete molecule lacks the overall asymmetry required to interact with plane-polarized light.
Meso Compounds Versus Racemic Mixtures
Meso compounds are often confused with racemic mixtures because both are optically inactive, but their underlying reasons for inactivity differ significantly. A racemic mixture is an equimolar (50:50) mixture of two enantiomers. Enantiomers are non-superimposable mirror images of each other, and each enantiomer individually rotates plane-polarized light in equal but opposite directions. In a racemic mixture, the rotation caused by one enantiomer is externally canceled by the rotation of the other enantiomer in the mixture.
In contrast, a meso compound is a single, achiral molecule due to its internal plane of symmetry. Racemic mixtures can be separated, or “resolved,” into their individual optically active enantiomers using techniques such as chromatography or crystallization. However, a meso compound cannot be separated into optically active components because it is inherently a single, achiral entity.