Optical Activity

Core Concepts

In this article you will be able to understand what is optical activity, after reading this article you will be able to differentiate the different types of optical activity and its properties within different molecules.

Optical Activity in Chemistry

A compound’s capacity to rotate the plane of polarized light is known as optical activity. This characteristic results from the interaction of polarized light’s electromagnetic radiation with the asymmetric electric fields produced by chiral molecules’ electrons. The varying concentrations of molecules undoubtedly affect the observed rotation by exerting their influence.

Diagram demonstrating the ability of optically active (chiral) substances to rotate light

There are two types of optical activity, which depend on the ability to rotate the plane of polarization of plane-polarized light; those are active and inactive.

Optically Active vs. Optically Inactive

Chiral molecules will always be optically active, except when they are in a racemic mixture. Chiral molecules must have at least one chiral center, which is a carbon with four different substitutents.

Consequently, pure samples of chiral molecules, will always be optically active. Specific forms of chiral molecules are called stereoisomers, which are a kind of isomer in which molecules differ in their atoms’ three-dimensional spatial orientations despite sharing the same chemical formula and bonding order.

Meso Compounds

The term “meso compound” refers to an achiral compound with chiral centers. Despite having two or more stereocenters, a meso compound is optically inactive and has a superimposable internal plane of symmetry that renders it superimposable on its mirror image.

A meso compound needs at least two substituted stereocenters that are exact duplicates. Also, the compound is divided in two by an internal symmetry plane. The internal mirror allows these two sides to reflect one another. Reflected stereocenters should “cancel out” their stereochemistry. This indicates that when a compound is divided into two symmetrical sides by an internal plane, the stereochemistry of the left and right sides should be in opposition to one another, rendering the molecule optically inactive.

Polarimeter

Scientists use a polarimeter to calculate the angle of rotation caused by polarized light passing through an optically active material.

Plane polarized light, which is uni-directional, will rotate to the left or the right as it passes through certain chemical substances because these substances are optically active. The measure of how much the light is turned is the angle of rotation. The direction and magnitude of the observed rotation reveal the chiral properties of the sample, including the relative concentration of enantiomers present in the sample.

The observed rotation will depend on different factors, including; the concentration of the solution, its density, the type of solvent, the length of the sample tube, temperature and the wave length of the light from the polarimeter.

Dextrorotatory vs. Levorotatory

When discussing how various chemical compounds rotate plane-polarized light, the terms dextrorotatory and levorotatory are helpful. Dextrorotatory refers to the rotation of plane-polarized light to the right side, whereas levorotatory refers to the rotation of plane-polarized light to the left. This is the main distinction between the two terms. Furthermore, we give the names “dextrorotation” and “levorotation” to this process of mild rotation. Additionally, we refer to the rotation of plane-polarized light in a clockwise direction as dextrorotatory and in the opposite direction as levorotatory.

Dextrorotatory Optical Activity

Direction: When seen along the direction of light propagation, dextrorotatory substances rotate plane-polarized light in a clockwise direction. An angle signifying this rotation is often positive, such as +10 degrees.
Symbol: Moreover, dextrorotation is frequently denoted by a plus sign (+) before the angle of rotation in chemical and scientific notation. For instance, a value of +10 denotes that the substance rotates light in the clockwise direction.
Chirality: One enantiomer, or mirror image, of a chiral molecule exhibits dextrorotatory activity.

Levorotatory Optical Activity

Direction: When seen along the direction of light propagation, levorotatory compounds rotate plane-polarized light in a counterclockwise direction. Usually, scientists indicate this rotation with a negative angle, such as -10 degrees.
Symbol: Levorotation is frequently denoted with a minus sign (-) before the angle of rotation when using the symbol used in chemical and scientific notation. For instance, -10 degrees means that the substance rotates light in the opposite direction.
Chirality: Levorotatory behavior is connected to chirality when a chiral molecule has a distinct mirror-image (enantiomer) from its dextrorotatory counterpart. To put it another way, if one enantiomer of a chemical is dextrorotatory, the other enantiomer will be levorotatory.
Enantiomers: Additionally, the optically active isomers of the dextrorotatory and levorotatory enantiomers are one another. They differ in optical activity despite having identical chemical properties because to their unique three-dimensional atomic configurations.

Specific Rotation Formula

Measuring its rotation, referencing a 100mm thick layer, and dividing by the relative density at the measurement temperature, as described in the monograph, determines the specific optical rotation of a liquid substance.

A 1mm slab for solids or a 100mm route length for liquids creates the rotation, measuring a sample’s optical activity. The particular rotation is the name of this measure. In contrast to solids, liquids typically rotate the light significantly less. The concentration of the active ingredient in a solution of a solid will certainly have an influence, as will, to a lesser extent, temperature and solvent.

optical activity- specific rotation formula

Enantiomeric Excess

For chiral compounds, enantiomeric excess (ee) is a unit of purity. It illustrates how much more of one enantiomer is present in a sample than of the other. Moreover, a single, absolutely pure enantiomer has an ee of 100%, compared to a racemic mixture’s ee of 0%. An ee of 40% (70% 30%) is the result of a sample containing 70% of one enantiomer and 30% of the other.

You can determine the ee by the difference in the percentages of the two enantiomers:

(1) $\begin{gather*} {\text{Enantiomer 1: } 60\% \text{ Enantiomer 2: } 40\%} \\ {\% ee = \lvert \%\text{Enantiomer 1} - \%\text{Enantiomer 2} \rvert = \lvert 60\% - 40\% \rvert = 20\%} \end{gather*}$

Racemic Mixtures

Furthermore, a racemic mixture, also known as a racemate, is a combination of two enantiomers, or substances that are mirror images of one another, in equal amounts. Each enantiomer spins the plane of polarization of plane-polarized light by a distinctive angle; however, the racemic mixture is optically inert because the rotatory effects of each component precisely cancel out one another.

The name originates from racemic acid, the first chemical of its kind to undergo thorough investigation. Moreover, equal parts of dextrorotatory and levorotatory tartaric acids combine to form racemic acid, more precisely known as racemic tartaric acid.

Optical Activity Practice Problems

Problem 1

What is optical activity, and how does it relate to chiral compounds? Provide an example of a chiral molecule.

Problem 2

Calculate the specific rotation of a compound if a 1 cm thick sample solution in a polarimeter turns the plane-polarized light by 25 degrees and the concentration of the compound is 2 g/mL.

Problem 3

You have a sample of an unknown chiral compound. When you pass plane-polarized light through it, it rotates the light clockwise. What can you infer about the enantiomeric nature of the compound, and how can you determine if it is dextrorotatory or levorotatory?

Problem 4

You are given a mixture of two enantiomers. The specific rotation of the mixture is found to be +10 degrees, and the specific rotation of one enantiomer is +15 degrees. Calculate the enantiomeric excess (ee) of the mixture.

Problem 5

Is it possible for a compound to be optically inactive even if it contains chiral centers? Explain with an example and provide conditions under which this can occur.

Optical Activity Practice Problems Solutions

Solution 1

Optical activity refers to the ability of chiral compounds, which are compounds that lack superimposable mirror images, to rotate the plane of polarized light.

Solution 2

In this case, $\alpha = \frac{25^\circ}{2 \, \text{g/mL} \cdot \frac{1 \, \text{cm}}{10}} = +125 \, \frac{\text{degrees}}{\text{g} \cdot \text{cm}^3 \cdot \text{dm}}$

Solution 3

If the compound rotates plane-polarized light clockwise, it is dextrorotatory. To determine if it is the dextrorotatory or levorotatory enantiomer, you would need to compare its rotation to that of the pure enantiomers.

Solution 4

In this case, $ee = \frac{(+10^\circ) - (+15^\circ)}{+15^\circ} = -\frac{5}{15} = -\frac{1}{3} \text{ or } -33.33\%$

Solution 5

Yes, it is possible for a compound to be optically inactive despite having chiral centers. This can occur when there is an equal mixture of both enantiomers present, such as in a racemic mixture. A racemic mixture has an ee of 0%, causing the optical activities of the enantiomers to cancel each other out.