Organic Chemistry: Optical Isomerism

For a quick recap, molecules having the same molecular formula can have different arrangement of the atoms, giving rise to different forms called isomers. Isomerism can be broadly classified into 2 types:

• constitutional isomerism, in which atoms are linked together in different ways
• stereoisomerism, in which atoms have the same connectivity but different arrangements in space

Optical isomerism is one type of stereoisomerism and is commonly also known as enantiomerism. Stereoisomers have the same structure and functional groups but differ in the way their atoms are arranged in space (orientated differently with respect to each other). The other type of stereoisomerism is known as geometric isomerism (also known as cis-trans isomerism).

In this post, we will focus our attention on optical isomerism in which many A-Level H2 Chemistry students are having a hard time to understand them from their JC H2 Chemistry tutors and teachers. Recently, i conducted a 2 hours complimentary revision workshop on Isomerism in Organic Chemistry and we have a full house attendance. Many of them are recommended by my students in our JC2 H2 Chemistry Tuition Classes.

We will split the discussion into 4 different sections:

1. Concept of Optical Isomerism
2. Physical Properties of Optical Isomers
3. Racemic Mixture
4. Chemical Properties of Enantiomers

1. Concept of Optical Isomerism

The non-superimposability of mirror images is the definitive criterion for a molecule to exhibit enantiomerism i.e. the molecule is chiral. If the mirror image is non-superimposable, the mirror image is the enantiomer of the compound in question. If the mirror image is superimposable on itself, the compound is then said to be achiral and does not exhibit enantiomerism.

First of all, when the carbon atom of a covalent molecule has four different groups attached to it and has a tetrahedral molecular shape about it, the carbon atom is known as a chiral centre. A chiral centre is usually denoted with an asterisk (*) on the structure (required if you are taking A-Level H2 Chemistry Exam syllabus code 9729 in Singapore).

Let’s consider the mirror image of an organic molecule.

The molecule on the RHS also has a chiral centre attached to the same four groups that are present in the molecule on the LHS. However, the two molecules differ in the spatial arrangement of these groups around the chiral centre.

Because of this different arrangement in space, the two molecules are not superimposable on the each other. They are known as enantiomers or optical isomers. Each of these molecules is also said to be chiral.

For A-Level H2 Chemistry Examinations (as well as H1 Chemistry , H3 Chemistry and equivalents), you will need to draw a pair of enantiomers showing the three-dimensional orientation about their chiral centres.

Note that students are required to use the dashed and wedged bonds with dotted mirror plane to illustrate the 3D orientation about the chiral centre.

• A bond coming out of the plane of the paper is indicated by wedged bonds
• A bond going into the plane of the paper is indicated by dashed bonds
• Bonds lying on the plane of the paper are represented by solid lines

Last, but not least, you need to know that enantiomers rotate plane-polarised light in opposite directions and are said to exhibit optical activity.

2. Physical Properties of Enantiomers

Enantiomers have identical physical properties (such as melting points, boiling points, solubilities, conductivities, etc) except towards plane-polarised light.

Each enantiomer rotates plane-polarised light by the same angle, but in opposite directions. Do note that the direction on which a particular enantiomer rotates plane-polarised light cannot be determined directly from the structure, but must be determined experimentally.

The enantiomer that rotates the plane-polarised light in a clockwise direction is said to be dextrorotatory, and we use the identifier (+) prefixed to its name. The enantiomer that rotates the plane-polarised light in an anti-clockwise direction is said to be laevorotatory, and we use the identifier (-) prefixed to its name. [For A-Level H2 Chemistry students in Singapore, please note that dextrorotatory (+) and laevorotatory (-) is Not-In-Syllabus. You just need to understand that the pair of enantiomers rotate plane-polarised light at the same angle but in opposite directions].

3. Racemic Mixture

A racemic mixture (also known as racemate) is one whereby there are equal amounts of both enantiomers, and the equal and opposite rotation of plane-polarised light cancels out, resulting in optical inactivity. In contrast to the two enantiomers, which have identical physical properties except for the direction of rotation of plane-polarised light, a racemic mixture sometimes has different properties from either form of the pure enantiomers. Examples could be different in melting points, boiling points and solubilities.

4. Chemical Properties of Enantiomers

Enantiomers have the same chemical properties except in the way they interact with other chiral molecules. This has a lot of biological relevance, particularly in drug action.

The human body is essentially a chiral structure having many chiral drug targets, such as enzymes, receptors and ion channels. The effect is that the enantiomers can have very distinct pharmacodynamic and pharmacokinetics properties that may be significant enough to translate into different biological effects.

For example. the action of adrenaline differs depending on which enantiomer is being used.

• (-)-adrenaline is a natural hormone with a stimulating effect
• (+)-adrenaline cannot fit into the enzyme active site and is mildly toxic

If we use a drug in the form of a racemic mixture, we are actually serving a mixture of drugs, which could be dangerous. As such, racemic mixture can also be considered as compounds that contain 50% impurity.

Two examples where racemic mixture could be dangerous when used in drugs:

1. Ibuprofen is known to be a pain reliever. One of its enantiomers is more potent than the other by about three-fold.
2. One of the enantiomers of thalidomide cures morning sickness (which is the intended therapeutic effect) while the other is a potent teratogen and causes birth defects. Based on actual historical events, thalidomide was unfortunately being sold as a racemic mixture, and as a result, a lot of severely deformed children were born in Europe.

The biological impact of enantiomers also highlights the importance of enantiomeric purity in the medicinal industry, which has to be achieved either through synthesis of only one particular enantiomer or through the separation of mixtures of enantiomers.

I hope you find the content easy for your understanding and if you have any questions, leave me a comment below. Feel free to share this blog post with your friends.

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PS: Under related articles below, there are several blog post discussions and questions related to Isomerism in Organic Chemistry. You can also do a keyword search using the search box at the top right hand corner.

Related Articles:

• Organic Chemistry: Optical Activity & Plane-Polarised Light
• Organic Chemistry: Special (Unusual) Cases of Chirality and Achirality
• Organic Chemistry: Meso Compounds and Maximum Possible Numbers of Enantiomers
• Organic Chemistry: Total Number of Stereoisomers
• Organic Chemistry: Isomerism in Organic Compounds

Sean Chua

Winners Education Centre Pte. Ltd.

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