A racemic mixture is not optically active. This optical inactivity occurs despite the mixture being composed of individual molecules that are optically active, and it is a direct result of a perfect internal cancellation effect.
Understanding Optical Activity
Optical activity is a physical phenomenon where a substance has the ability to rotate the plane of plane-polarized light. Normal light waves oscillate in all directions, but when passed through a polarizing filter, the resulting light vibrates in a single plane, creating plane-polarized light. When this polarized light is passed through an optically active compound, the plane of vibration is rotated either clockwise or counter-clockwise.
The extent and direction of this rotation are measured using an instrument called a polarimeter. If the compound rotates the light clockwise, it is labeled as dextrorotatory, indicated by a plus sign (+). Conversely, a counter-clockwise rotation signifies a levorotatory compound, marked with a minus sign (-). These directional rotations are unique physical properties of the substance, much like its boiling point or density.
The Concept of Molecular Handedness
The prerequisite for a substance to be optically active is a structural feature known as chirality, or “molecular handedness”. A molecule is chiral if it cannot be perfectly superimposed onto its mirror image, similar to how a left hand cannot be perfectly placed onto a right hand. This geometric property is often due to a central carbon atom bonded to four different groups, which is called a chiral center.
Chiral molecules exist as two non-superimposable mirror images called enantiomers. These “twin” molecules have identical chemical formulas and most physical properties, but they interact differently with other chiral entities, such as biological receptors in the human body. Each individual enantiomer is optically active, meaning it will rotate plane-polarized light in a specific direction.
What Exactly is a Racemic Mixture
A racemic mixture, also referred to as a racemate, is defined as a precise 50:50 mixture of a pair of enantiomers. This means the solution contains equal concentrations of both the right-handed and left-handed forms of the chiral molecule. The presence of both enantiomers in an exact one-to-one ratio is the defining characteristic of this type of mixture.
These mixtures are commonly formed during laboratory synthesis when starting materials lack chirality. The chemical reaction can proceed with equal probability from either side of the molecule, a process known as racemization, resulting in the equal production of both the (+) and (-) forms.
Why Racemic Mixtures are Optically Inactive
The optical inactivity of a racemic mixture is the result of a phenomenon known as external compensation. While the individual molecules within the mixture are indeed optically active, their effects on polarized light cancel each other out precisely. One enantiomer rotates the plane of light clockwise (dextrorotatory) by a certain angle.
The other enantiomer, present in the exact same quantity, rotates the plane of light counter-clockwise (levorotatory) by an angle of equal magnitude. Because the two opposing rotations are perfectly balanced, the net rotation of the plane-polarized light passing through the entire mixture is zero. This cancellation means a polarimeter detects no rotation, leading to the classification of the racemic mixture as optically inactive.