Optical isomerism

In a nutshell

Optical isomerism is a type of stereoisomerism: the isomers have the same structural formula, but their atoms are arranged differently in space.

It arises when a molecule has a chiral centre, a carbon atom bonded to four different groups. Such a molecule exists as two non-superimposable mirror images called enantiomers.

The two enantiomers are almost identical. The one thing that tells them apart: they rotate plane polarised light equally in opposite directions. This matters because most molecules in living things are chiral, so the body can treat the two enantiomers of a drug very differently.

This subtopic is limited to molecules with a single chiral centre.

Assumed knowledge: Introduction to organic chemistry (structural isomerism and E-Z isomerism), Alkenes.

Core content

Stereoisomerism and chirality

Stereoisomers have the same structural formula but a different arrangement of their atoms in space.

Get this definition exact. Saying stereoisomers have the "same molecular formula" is not enough, and saying they have "different structures" is wrong.

There are two kinds of stereoisomerism:

• E-Z (geometrical) isomerism, from restricted rotation about a C=C double bond (covered in alkenes).
• Optical isomerism, which arises from chirality, and is the subject of this note.

A molecule is chiral if it is not identical to its own mirror image. In the molecules you meet here, that happens because of a single chiral centre.

Chiral centres: spotting the asymmetric carbon

A chiral centre (an asymmetric carbon atom) is a carbon atom bonded to four different atoms or groups.

The four groups point to the corners of a tetrahedron around the carbon.

To test a molecule, look at every carbon in turn: if one carbon has four different groups, the molecule is chiral and shows optical isomerism.

• Butan-2-ol, CH₃CH(OH)CH₂CH₃: carbon 2 carries OH, H, CH₃ and C₂H₅, four different groups, so it is a chiral centre.
• Propan-2-ol, (CH₃)₂CHOH: the middle carbon carries two CH₃ groups, so it is not a chiral centre and the molecule shows no optical isomerism.

Enantiomers: non-superimposable mirror images

A molecule with one chiral centre exists as two optical isomers (enantiomers).

Enantiomers are non-superimposable mirror images of each other: you cannot lay one on top of the other so that every group lines up.

Enantiomers have identical physical properties (same melting point, boiling point and density) and identical chemical properties when they react with non-chiral reagents.

They differ in only two ways:

• their effect on plane polarised light (below), and
• how they react with other chiral molecules, which is how the body can tell them apart.

Still don't get it? · non-superimposable mirror images

Hold up your two hands, palms towards you. They are mirror images: your right hand reflected in a mirror looks exactly like your left hand.

Now try to lay your right hand exactly on top of your left, both palms down, so that thumb covers thumb and every finger lines up. You cannot. One has the thumb on the left, the other has it on the right. They match in a mirror but never when stacked. That is what non-superimposable means.

A chiral carbon is the same story in three dimensions. Swap two of the four groups and you get the mirror-image arrangement; no amount of turning the molecule in space will make it sit exactly on top of the original.

In the exam, say the two enantiomers are non-superimposable mirror images. "Mirror images" on its own can be read as identical, so the word non-superimposable is the one that secures the mark.

Drawing enantiomers

To show a pair of enantiomers, draw the chiral carbon in 3D using wedge-and-dash bonds (a bold wedge for a bond coming towards you, a dashed line for one going away), then draw the second isomer as its mirror image.

A reliable method: keep the two vertical bonds pointing to the same groups in both drawings, and swap the two horizontal groups between them, as if there were a mirror down the middle (see Figure 2). That guarantees you have drawn a genuine mirror image rather than the same molecule twice.

Effect on plane polarised light

Ordinary light vibrates in all planes at right angles to its direction of travel. Plane polarised light vibrates in one plane only.

A single enantiomer rotates the plane of plane polarised light. Because of this, the isomers are described as optically active.

• One enantiomer rotates the plane clockwise (to the right). It is dextrorotatory, labelled (+).
• The other rotates the plane anticlockwise (to the left) by an equal angle. It is laevorotatory, labelled (−).

The exam wording to learn: the two enantiomers rotate the plane of plane polarised light equally in opposite directions.

Every word is worth checking:

• it is rotated, not "bent", "refracted" or "reflected";
• it is rotated in opposite directions, not just "different" directions;
• it is the plane of the light that is rotated, not the molecules themselves, and the molecules do not polarise the light.

Racemic mixtures (racemates) and why they are optically inactive

A racemic mixture (racemate) is a mixture containing equal amounts of the two enantiomers.

A racemate is optically inactive: it does not rotate the plane of plane polarised light.

The reason: each enantiomer rotates the plane by the same angle but in the opposite direction, so in an equal mixture the two rotations exactly cancel.

Still don't get it? · why a racemate shows no rotation

Picture a tug-of-war with two teams pulling a rope in opposite directions. If one team pulls with exactly the force of the other, the flag in the middle does not move at all. Not because nobody is pulling, but because the two pulls cancel.

A racemate is the same. The (+) enantiomer twists the plane of light, say, 5 degrees clockwise. The (−) enantiomer twists it 5 degrees anticlockwise. There are equal numbers of each molecule, so for every clockwise twist there is a matching anticlockwise twist, and the net result is zero.

So in the exam: a racemate is an equal mixture of the two enantiomers, and it is optically inactive because the rotations of the two enantiomers cancel out.

How a racemate forms

A racemate forms when a reaction builds a new chiral centre from a trigonal planar reactant or intermediate that can be attacked equally from either side.

The example AQA tests is nucleophilic addition of HCN to a carbonyl (the mechanism itself is in aldehydes and ketones, 3.3.8). A cyanide ion adds to the planar C=O group of an aldehyde or an unsymmetrical ketone:

• the carbonyl carbon is trigonal planar, so the nucleophile can attack from above or below the plane;
• attack from each side is equally likely, so equal amounts of the two enantiomers form;
• the product is therefore a racemate, and it is optically inactive.

Not every carbonyl gives a chiral product. Propanone, (CH₃)₂CO, gives (CH₃)₂C(OH)CN: its central carbon has two identical CH₃ groups, so it is not a chiral centre, there are no enantiomers, and no racemate forms.

A racemate also forms in the electrophilic addition of HBr to an unsymmetrical alkene, where a planar carbocation intermediate is attacked by the bromide ion from both sides (Alkenes).

Measuring optical activity: the polarimeter

Optical activity is measured with a polarimeter: plane polarised light is passed through a solution of the compound (for example, a solution of sucrose), and the angle through which the plane is rotated is measured.

• a single enantiomer rotates the plane, and the direction (+ or −) and angle are characteristic of the compound;
• a racemate gives no rotation.

This is the practical skill listed for this subtopic (AT a and k, PS 1.2). It is not one of the numbered required practicals.

Worked examples

Worked example 1: explain why a racemate is optically inactive

"When hydrogen cyanide reacts with ethanal, the organic product is optically inactive. Explain why."

A full answer builds in three stages. Each numbered point is a separate marking point.

1. The carbonyl carbon in ethanal is trigonal planar.
2. The cyanide ion (nucleophile) can therefore attack the carbonyl carbon from either side of the plane, and attack from each side is equally likely.
3. The product, 2-hydroxypropanenitrile, has a chiral centre (a carbon bonded to CH₃, H, OH and CN), so it exists as two enantiomers.
4. Because the two faces are attacked equally, equal amounts of the two enantiomers form: a racemate.
5. The two enantiomers rotate the plane of plane polarised light equally in opposite directions, so in the equal mixture the rotations cancel and the product is optically inactive.

The trap: this question is about the product of the reaction, not the ethanal you started with. An answer that only describes the starting material scores nothing.

Worked example 2: identify the chiral centre and decide on optical activity

"State whether 2-hydroxypropanoic acid (lactic acid), CH₃CH(OH)COOH, shows optical isomerism, and how many optical isomers it has."

1. Check each carbon for four different groups. The central carbon (carbon 2) is bonded to CH₃, H, OH and COOH: four different groups.
2. That carbon is a chiral centre, so the molecule does show optical isomerism.
3. A single chiral centre gives two optical isomers (a pair of enantiomers).

To contrast: propanoic acid, CH₃CH₂COOH, has no carbon bonded to four different groups, so it shows no optical isomerism.

Common exam mistakes

• Describing the optical activity of the starting material (for example the ketone) when the question asks about the product of its reaction. If there is no reference to the product, the marks are lost.
• Assuming every carbonyl gives a racemate. A symmetrical carbonyl such as propanone or pentan-3-one gives a product with two identical groups on the central carbon, so there is no chiral centre and no racemate.
• Defining stereoisomers as having the "same molecular formula" (not enough) or "different structures" (wrong). The mark needs same structural formula, atoms arranged differently in space.
• Saying plane polarised light is "bent", "refracted" or "reflected", or that the enantiomers rotate it in "different" directions. The mark needs rotated, in opposite directions.
• Saying the molecules rotate, or that the molecules "polarise" the light. It is the plane of the light that is rotated by the molecules.
• When asked for the reason a racemate forms, only describing what a racemate is (an equal mixture). The mark needs the planar group attacked equally from both sides.
• Writing "planar molecule" or "planar bond" instead of "planar carbonyl group" when explaining racemate formation.
• Giving the carbonyl carbon a full positive charge rather than a partial charge (δ+).
• Forgetting that optical isomerism is a form of stereoisomerism: for example drawing the optically active isomer when a different stereoisomer (such as an E-Z isomer) was asked for.

Key definitions

• Stereoisomers: molecules with the same structural formula but with the atoms arranged differently in space.
• Chiral centre (asymmetric carbon atom): a carbon atom bonded to four different atoms or groups.
• Optical isomers (enantiomers): non-superimposable mirror images of each other.
• Optical activity: a single enantiomer rotates the plane of plane polarised light; the two enantiomers rotate it equally in opposite directions.
• Racemic mixture (racemate): a mixture containing equal amounts of the two enantiomers, which is optically inactive because the rotations of the two enantiomers cancel.

Specification

• I can state that optical isomerism is a form of stereoisomerism that arises from chirality, in molecules with a single chiral centre.
• I can identify a chiral centre (a carbon bonded to four different groups) in a structure given in 2D or 3D.
• I can state that an asymmetric carbon is chiral and gives rise to two optical isomers (enantiomers), which are non-superimposable mirror images.
• I can state that enantiomers differ in their effect on plane polarised light, rotating it equally in opposite directions.
• I can draw the structural formulae and displayed (3D) formulae of a pair of enantiomers.
• I can state that a racemic mixture (racemate) is an equal mixture of the two enantiomers.
• I can explain how a racemate forms (a planar species attacked equally from both sides) and why it is optically inactive (the equal and opposite rotations cancel).

Related notes

• Introduction to organic chemistry (structural isomerism and E-Z isomerism)
• Alkenes (a planar carbocation attacked from both sides can give a racemate)
• Aldehydes and ketones (3.3.8): nucleophilic addition of HCN, the reaction that builds a chiral centre and a racemate
• Amino acids, proteins and DNA (3.3.13): amino acids are chiral (except glycine)
• Transition metals (3.2.5): optical isomerism in octahedral complexes with bidentate ligands