Organic Chemistry/Chirality/Optical activity

Optical Activity

Optical activity describes the phenomenon by which chiral molecules are observed to rotate polarized light in either a clockwise or counterclockwise direction. This rotation is a result of the properties inherent in the interaction between light and the individual molecules through which it passes. Material that is either achiral or equal mixtures of each chiral configuration (called a racemic mixture) do not rotate polarized light, but when a majority of a substance has a certain chiral configuration the plane can be rotated in either direction.

What Is Plane Polarized Light?

Polarized light consists of waves of electromagnetic energy in the visible light spectrum where all of the waves are oscillating in the same direction simultaneously. Put simply, imagine a ray of light as a water wave, with crests and peaks. All the peaks of a water wave point in the same direction (up, against gravity) pretty much at the same time. Light is not usually this way - its peaks and troughs are often in the random array, so one ray of light's peaks might point in a direction 90^o opposite of another ray. When all of the rays have their peaks pointing in the same direction - like all the waves in the ocean have peaks pointing up - then those rays of light are said to be polarized to one another.

The blue line represents an electromagnetic wave. The green and red lines are views of the wave from above and to the side. The purple line shows the angle of the wave as seen end-on.

A polaroid filter allows light through only if the light is polarized at the same angle as the filter.

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Why Polarized Light Is Affected

So why do chiral molecules affect only polarized light, and not unpolarized? Well, they do affect unpolarized light, but since the rays have no particular orientation to one another, the effect can not be observed or measured. We observe the polarized light rays being rotated because we knew their orientation before passing through the chiral substance, and so we can measure the degree of change afterwards.

What happens is this; when light passes through matter, e.g. a solution containing either chiral or achiral molecules, the light is actually interacting with each molecule's electron cloud, and these very interactions can result in the rotation of the plane of oscillation for a ray of light. The direction and magnitude of rotation depends on the nature of the electron cloud, so it stands to reason that two identical molecules possessing identical electron clouds will rotate light in the exact same manner. This is why achiral molecules do not exhibit optical activity.

In a chiral solution that is not a racemic mixture, however, the chiral molecules present in greater numbers are configurationally equivalent to each other, and therefore each possesses identical electron clouds to its molecular twins. As such, each interaction between light and one of these 'majority' molecule's electron clouds will result in rotations of identical magnitude and direction. When these billions of billions of interactions are summed together into one cohesive number, they do not cancel one another as racemic and achiral solutions tend to do - rather, the chiral solution as a whole is observed to rotate polarized light in one particular direction due to its molecular properties.

Enantiomers

It is just such specificity that accounts for the optical isomerism of enantiomeric compounds. Enantiomers possess identical chemical structures (i.e. their atoms are the same and connected in the same order), but are mirror images of one another. Therefore, their electron clouds are also identical but actually mirror images of one another and not superimposable. For this reason, enantiomeric pairs rotate light by the same magnitude (number of degrees), but they each rotate plane polarized light in opposite directions. If one chiral version has the property of rotating polarized light to the right (clockwise), it only makes sense that the molecule's chiral mirror image would rotate light to the left (counterclockwise).

Equal amounts of each enantiomer results in no rotation. Mixtures of this type are called racemic mixtures, and they behave much as achiral molecules do.

History

Via a magneto-optic effect, when a beam of polarized light passes through solution, the (-)-form of a molecule rotates the plane of polarization counterclockwise, and the (+)-form rotates it clockwise. It is due to this property that it was discovered and from which it derives the name optical activity. The property was first observed by J.B. Biot in 1815, and gained considerable importance in the sugar industry, analytical chemistry, and pharmaceuticals.

Louis Pasteur deduced in 1848 that the handedness of molecular structure is responsible for optical activity. He sorted the chiral crystals of tartaric acid salts into left-handed and right-handed forms, and discovered that the solutions showed equal and opposite optical activity.

Artificial composite materials displaying the analog of optical activity but in the microwave regime were introduced by J.C. Bose in 1898, and gained considerable attention from the mid-1980s.