Optical Isomerism, Chiral Centres & Racemic Mixtures (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching A-Level Chemistry, and this video is about optical isomerism, chiral centres and racemic mixtures.

A chiral centre is what creates enantiomers that rotate polarised light in opposite directions, and when equal amounts of both enantiomers occur together in a racemic mixture, those opposite rotations cancel out.

Stereoisomers with a chiral centre exist as enantiomers, non-superimposable mirror images that rotate plane-polarised light in opposite directions. Identifying chiral centres is how you spot which carbon in a molecule is responsible for that behaviour. Racemic mixtures are what happens when equal amounts of both enantiomers occur together and the opposite rotations cancel out, which is why racemic mixtures matter so much when chiral drugs are made.

We'll start with what optical isomerism is and where it comes from, then look at how to identify chiral centres in a structure, and finish with racemic mixtures and why they matter in drug synthesis.

Stereoisomers share the same structural formula but differ in how their atoms or groups of atoms are arranged in space. There are two types: E/Z, isomerism and optical isomerism. A carbon bonded to four different atoms or groups of atoms is called a chiral carbon, or chiral centre. Compounds with a chiral centre exist as two enantiomers or a pair of optical isomers which are non-superimposable mirror images of each other. In the same way your left and right hands are mirror images that can't be perfectly aligned on top of each other. Enantiomers have identical physical and chemical properties, except in how they affect plane-polarised light. One rotates it clockwise, and the other rotates it anticlockwise by the same amount, which is why they're called optically active. This rotation is the property that identifying chiral centres and racemic mixtures both build on.

Identifying a chiral centre means checking whether a carbon is bonded to four different atoms or groups of atoms. If you're only given a molecular formula, drawing it out as a condensed or displayed formula lets you see every bond and group clearly. In 3D drawings, chiral centres are marked with an asterisk, and stereochemical convention uses a solid line for a bond in the plane of the paper, a dashed line for a bond receding behind it, and a solid wedge for a bond coming out of it. Spotting these centres is what tells you whether a molecule can exist as the enantiomers we just covered.

A racemic mixture, or racemate, contains equal amounts of both enantiomers. Because the two enantiomers rotate plane-polarised light by the same amount in opposite directions, those rotations cancel out, so a racemic mixture is optically inactive. In the pharmaceutical industry, it's much easier to produce a racemic mixture than to isolate a single enantiomer, so most chiral drugs, including ibuprofen, are sold as racemic mixtures even though only half the drug is pharmacologically active. Occasionally only one enantiomer is produced, because the other could be harmful. Thalidomide is the clearest example of why that matters: introduced as a sedative and morning sickness treatment, it was later linked to severe birth defects caused by the other enantiomer.

When describing the optical activity of enantiomers you must describe the light as plane-polarised, not just light. And you must say that the plane-polarised light is rotated, not turned or bent and you must say that this happens in opposite directions.

Enantiomers are commonly distinguished using the symbols (plus) and (minus) to show the direction of rotation. You may also come across the D/L and R/S naming systems, which describe the arrangement of atoms in space, but you don't need to know these naming systems for the exam.

Chiral centres create enantiomers that rotate plane-polarised light in opposite directions. You spot a chiral centre by checking for a carbon bonded to four different groups. A racemic mixture contains equal amounts of both enantiomers, so those rotations cancel out and it's optically inactive. That matters in drug synthesis. Most chiral drugs, like ibuprofen, are sold as racemic mixtures, while thalidomide shows why separating enantiomers can sometimes be essential.