What makes things optically active

Optical activity is the ability of a chiral molecule to rotate the plane of plane-polairsed light, measured using a polarimeter. A simple polarimeter consists of a light source, polarising lens, sample tube and analysing lens.

When light passes through a sample that can rotate plane polarised light, the light appears to dim because it no longer passes straight through the polarising filters. Two enantiomers are nonsuperimposible mirror images of one another i. Our left hand is a mirror image of our right, yet there is no way our left thumb can be over our right thumb if our palms are facing the same way and placed over one another.

Optical isomers also have no axis of symmetry, which means that there is no line that bisects the compound such that the left half is a mirror image of the right half. Optical isomers have basically the same properties melting points, boiling points, etc. There are drugs, called enantiopure drugs, that have different effects based on whether the drug is a racemic mixture or purely one enantiomer. For example, d-ethambutol treats tuberculosis, while l-ethambutol causes blindness. Optical activity is the interaction of these enantiomers with plane-polarized light.

Optical activity was first observed by the French physicist Jean-Baptiste Biot. He concluded that the change in direction of plane-polarized light when it passed through certain substances was actually a rotation of light, and that it had a molecular basis. His work was supported by the experimentation of Louis Pasteur. The sample may also consists of a mixture of a pair of enantiomers. To do such calculation, the concept of enantiomer excess ee will be needed.

The enantiomeric excess ee tells how much an excess of one enantiomer is in the mixture, and it can be calculated as:.

We will use a series of hypothetic examples in next table for detailed explanation. Sample 3 is for a mixture with equal amount of two enantiomers, and such mixture is called racemic mixture or racemate.

Racemic mixtures do not rotate the plane of polarization of plane-polarized light, that means racemic mixtures are optical inactive and have the observed rotation of zero!

This is because that for every molecule in the mixture that rotate the plane of polarization in one direction, there is an enantiomer molecule that rotate the plan of polarization in the opposite direction with the same angle, and the rotation get cancelled out.

As a net result, no rotation is observed for the overall racemic mixture. This can be shown by the diagram below that helps to understand.

Method II : using diagram, the answer is in blue color, there is Other than optical activity difference, the different enantiomers of a chiral molecule usually show different properties when interacting with other chiral substances. This can be understood by using the analogue example of fitting a hand into the respective glove: right hand only fits into right glove, and it feels weird and uncomfortable if you wear left glove on the right hand.

This is because both right hand and right glove are chiral. A chiral object only fit into a specific chiral environment. In human body, the biological functions are modulated by a lot of enzymes and receptors. Enzymes and receptors are essentially proteins, and proteins are made up of amino acids. Amino acids are examples of naturally exist chiral substances. With the general formula given below, the carbon with amino NH 2 group is the chirality asymmetric center for most amino acids, and only one enantiomer usually S -enantiomer exist in nature.

A few examples of amino acids are given below with the general formula. Objects that possess a similar handedness are said to be chiral literally, "handed". Those that do not are said to be achiral. Gloves are chiral. It is difficult, if not impossible, to place a right-hand glove on your left hand or a left-hand glove on your right hand. Mittens, however, are often achiral. Either mitten can fit on either hand. Feet and shoes are both chiral, but socks are not.

In Jacobus van't Hoff and Joseph Le Bel recognized that a compound that contains a single tetrahedral carbon atom with four different substituents could exist in two forms that were mirror images of each other. Consider the CHFClBr molecule, for example, which contains four different substituents on a tetrahedral carbon atom. The figure below shows one possible arrangement of these substituents and the mirror image of this structure. By convention, solid lines are used to represent bonds that lie in the plane of the paper.

Wedges are used for bonds that come out of the plane of the paper toward the viewer; dashed lines describe bonds that go behind the paper. If we rotate the molecule on the right by around the C H bond we get the structure shown on the right in the figure below. These structures are different because they cannot be superimposed on each other, as shown in the figure below.

CHFClBr is therefore a chiral molecule that exists in the form of a pair of stereoisomers that are mirror images of each other. As a rule, any tetrahedral atom that carries four different substituents is a stereocenter, or a stereogenic atom. However, the only criterion for chirality is the nonsuperimposable nature of the object. A test for achirality is the presence of a mirror plane within the molecule.