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Introduction of Optical Activity

Differentiating and Identifying enantiomer is inherently complex, because their chemical and physical properties are very identical. Fortunately, just about two hundred year old discovery by the French physicist Jean-Baptiste Biot has made this task so easy. This discovery disclosed that the right- and left-handed enantiomers of a chiral compound perturb plane-polarized light in opposite ways. This perturbation is distinct to chiral molecules and has been known as optical activity.

Plane-polarized light is formed by passing ordinary light via a polarizing device, which may be as simple as a lens taken from polarizing sun-glasses. This type of devices transmit selectively only that component of a light beam having magnetic and electrical field vectors oscillating in a single plane. The plane of polarization can be defined from an instrument called a polarimeter, displayed in the picture below.

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By a fixed polarizer next to the light source, Monochromatic light is polarized. A sample cell holder is placed in line with the light beam, which is followed by a movable polarizer (the analyzer) and an eyepiece from which the intensity of light can be observed. An electronic light detector obtains the place of the human eye in modern instruments. The light intensity at the detector is at a maximum when the second (movable) polarizer is set parallel to the first polarizer (α = 0º) in the nonexistence of a sample. All the light will be blocked from reaching the detector if the analyzer is turned 90º to the plane of initial polarization.

Chemists make use of polarimeters to examine the affect of compounds (in the sample cell) on plane polarized light. Samples that are composed only of achiral molecules (example water or hexane), have no effect on the polarized light beam. Though, if a single enantiomer is examined, the plane of polarization is rotated in either a counter-clockwise (negative) direction or clockwise (positive) direction, and the analyzer must be turned a suitable matching angle, α, if full light intensity is to reach the detector. The sample has rotated the polarization plane clockwise by +90º, and the analyzer has been turned this amount to permit maximum light transmission, in the above example.
In direction the observed rotations (α) of enantiomers are opposite. One enantiomer will rotate polarized light in a clockwise direction, termed as dextrorotatory or (+) and its mirror-image partner in a counter-clockwise manner, termed levorotatory or (-). The prefixes levo and dextro and come from the Latin laevus, for left and dexter, meaning right, and are abbreviated as l and d correspondingly. If equivalent quantities of each enantiomer are examined, using similar sample cell, then the magnitude of the rotations will be identical, with one being positive and the other negative. To be absolutely sure whether an observed rotation is positive or negative it is frequently essential to make a second measurement by using a different amount or concentration of the sample. In the above example, for an instance, α might be -90º or +270º rather than +90º. By 10% if the sample concentration is reduced, then the positive rotation would change to +81º (or +243º) while the negative rotation would change to -81º and the correct α would be identified unambiguously.
Because it is not always possible to get or use samples of exactly similar in size, the observed rotation is generally corrected to compensate for variations in sample quantity and cell length. So it is general practice to convert the observed rotation, α, to a specific rotation, [α], by the formula given below:

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Optically active are the Compounds that rotate the plane of polarized light. Each enantiomer of a stereoisomeric pair is optically active and has an identical but opposite-in-sign specific rotation. Specific rotations are useful in that they are experimentally determined constants that identify and characterize pure enantiomers. For an instance, the carvone and lactic acid enantiomers that are discussed before have the following specific rotations.

Carvone from caraway:   [α]D = +62.5º     this isomer may be considered as (+)-carvone or d-           carvone

Carvone from spearmint:   [α]D = -62.5º   this isomer may be considered as (-)-carvone or l-carvone

Lactic acid from muscle tissue:   [α]D = +2.5º this isomer may be considered as (+)-lactic acid or d-lactic acid

Lactic acid from sour milk:   [α]D = -2.5º this isomer may be considered as (-)-lactic acid or l-lactic acid

A 50:50 mixture of enantiomers has no observable optical activity. These types of mixtures are called racemates or racemic modifications and are entitled as (±). When chiral compounds are created from achiral compounds, products are racemic unless a single enantiomer of a chiral co-reactant or catalyst is involved in the reaction. The addition of HBr to any cis- or trans-2-butene is an instance of racemic product formation (the chiral center is red in color in the equation).

CH3CH=CHCH3 + HBr     (±) CH3CH2CHBrCH3

Chiral organic compounds isolated from living organisms are generally optically active, which is indicating that one of the enantiomers predominates (usually it is the only isomer present). This is a effect of the action of chiral catalysts we describe enzymes and reflects the inherently chiral nature of life itself. Chiral synthetic compounds, alternatively, are usually racemates, unless they have been prepared from enantiomerically pure starting materials.

The condition of a chiral substance may be altered in two ways that are:
 1. The process Resolution is a racemate may be separated into its component enantiomers.

 2. The process racemization is a pure enantiomer may be transformed into its racemate.

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