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Organic chemistry (Topics 6, 17 and 18)›Chirality (Topic 17A)

Chirality (Topic 17A)

Optical isomerism and the formation of racemic mixtures.

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Optical isomerism is a type of stereoisomerism. Stereoisomers are molecules which have the same structural formula but different arrangements in space.

A chiral carbon atom that bonds to four different atoms or groups has no plane of symmetry. This is known as a chiral centre.

A diagram showing two molecular structures of a carbon atom (C) bonded to different halogens (Cl, Br) and hydrogen (H) and fluorine (F) atoms. The left structure features Cl at the top, with H and F on the left and Br on the right. The right structure mirrors the left but has Br on the left and H and F on the right.

A chiral carbon atom can exist as optical isomers.

Optical isomers are based on the tetrahedral geometry.

In most cases, they are drawn with two bonds on the plane of the paper (lines), one bond emerging (bold wedge) and one bond receding (dashed wedge).

A diagram illustrating a carbon atom (C) with various types of chemical bonds. It shows two solid line bonds connecting to atoms C, Z, and W, indicating they are in the same plane. There is one dashed bond representing atom X, which is receding behind the surface, and one wedge bond indicating atom Y, which is emerging towards the observer.

Question walkthrough

Isomerism

Predicting possible isomers of a product

Enantiomers, otherwise known as optical isomers, are non-superimposable mirror images of each other.

Optical isomers have similar chemical and physical properties, but they rotate plane polarised light in different directions.

One enantiomer will rotate the plane of plane-polarised light clockwise and the other enantiomer rotates it by the same amount but anticlockwise.

A diagram illustrating the process of polarizing light. It shows unpolarised light entering a polariser, resulting in polarized light. The diagram also depicts two enantiomers, with one enantiomer rotating light anticlockwise and the other rotating light clockwise.

A racemic mixture contains a 50/50 mixture of two enantiomers. This is sometimes described as a racemate

A racemic mixture contains equal amounts of two optical isomers (enantiomers). These will both rotate the plane of plane-polarised light by the same amount, but in opposite directions.

Three circular diagrams illustrating optical activity of enantiomers: the first shows a negative enantiomer with an anticlockwise rotation, the second shows a positive enantiomer with a clockwise rotation, and the third shows a racemate with no rotation.

This means that overall, there will be no optical rotation as the effects of both enantiomers effectively cancel each other out.

In an SN2 reaction, inversion of stereochemistry occurs because the nucleophile attacks the electrophilic carbon from the opposite side of the leaving group.

This ‘ backside attack ’ pushes the groups around the carbon into the opposite configuration, flipping the stereochemistry like an umbrella turning inside out.

This means that the product will have opposite optical activity compared to the reactants.

A diagram illustrating the nucleophilic substitution reaction of bromoethane. It shows the nucleophile (hydroxide ion) attacking the carbon atom of bromoethane, leading to the formation of a transition state where the C-OH bond is forming and the C-Br bond is breaking. The final products are ethanol and a bromide ion.

In SN1, any optical activity will be lost completely as the reaction proceeds via a planar intermediate. The product will be a racemic mixture of two stereoisomers.

A racemic mixture may form when a reactant, or intermediate in a reaction mechanism, has a trigonal planar group.

A diagram illustrating a chemical reaction with a central carbon atom bonded to a hydroxyl group (OH), a methyl group (CH3), and a carbonyl group (C=O). Two nucleophiles, NC- and CN-, are shown approaching the carbon from opposite sides, indicating an equal chance of attack from either side.

Planar groups can be approached from both sides by an attacking species.

During the nucleophilic addition of HCN to aldehydes or ketones, when the planar carbonyl group is approached by the nucleophile, there is an equal probability that the cyanide attacks from the front or back face of the carbonyl.

This results in the formation of a racemate.