Stereoisomers and isomerism

Stereoisomers are molecules with the same structural formula, so the same atoms joined in the same order, but with the atoms arranged differently in space. There are two kinds at A-level: E/Z isomerism, caused by the restricted rotation of a C=C double bond, and optical isomerism, caused by a chiral centre, which gives two mirror image molecules called enantiomers.

Types of isomerism

Isomers have the same molecular formula but different arrangements of atoms. They split into two families:

The first three are structural isomers: the atoms are joined in a different order, so the structural formulas differ. The last two are stereoisomers: the structural formula is the same and only the 3D arrangement changes. Pick out structural isomers in this question from the Organic Chemistry course.

What is stereoisomerism?

In a pair of stereoisomers every atom is bonded to the same neighbours, yet the shapes in space differ. You cannot turn one stereoisomer into the other just by rotating the molecule or twisting single bonds; a bond would have to break. To see it, you have to think in three dimensions, which is why building the molecules, in a model kit or the Molecule Builder, helps so much.

E/Z isomerism

A single C-C bond can rotate freely. A C=C double bond cannot, because the second (pi) bond would have to break to turn it. So the groups attached to each carbon of the double bond are locked in place, either on the same side or on opposite sides.

E/Z isomers exist when both conditions are met:

1. There is a C=C double bond.
2. Each carbon of the double bond has two different groups attached.

But-2-ene, CH₃CH=CHCH₃, meets both: each carbon has a CH₃ and an H. But-1-ene, CH₂=CHCH₂CH₃, does not, because the first carbon has two hydrogens, so swapping them makes no difference.

The CIP priority rules

To name an E/Z isomer, use the Cahn-Ingold-Prelog (CIP) rules on each carbon of the double bond separately:

1. Look at the atoms joined directly to the carbon. The one with the higher atomic number has the higher priority. So Br beats Cl, Cl beats O, O beats C, and C beats H.
2. If the two atoms are the same, move one atom further out along each group and compare again.
3. If the two higher priority groups are on the same side, it is the Z isomer (from German zusammen, together). If they are on opposite sides, it is E (entgegen, opposite).

Cis and trans

Cis (same side) and trans (opposite sides) work when the same group is on both carbons, as in but-2-ene, where Z is cis and E is trans. They break down when all the groups differ, which is why the E/Z system is used at A-level. Cis and trans can even give the opposite answer to E and Z, as the next example shows.

Optical isomerism

A carbon atom bonded to four different groups is a chiral centre (often marked with an asterisk, C*). The four groups point to the corners of a tetrahedron, and they can be arranged in two ways that are mirror images of each other. Like your left and right hands, the two cannot be placed on top of each other so that every part matches: they are non-superimposable mirror images, called enantiomers.

Enantiomers have the same physical properties, such as boiling point, and the same chemical reactions with most reagents. They differ in one measurable way: they rotate the plane of plane-polarised light by the same angle in opposite directions. That is why they are called optical isomers. They can also behave very differently in the body, where enzymes and receptors are chiral too.

Racemic mixtures

A racemic mixture (racemate) contains equal amounts of both enantiomers. It is optically inactive: one enantiomer rotates the light one way, the other rotates it back by the same amount, so overall there is no rotation.

Racemates form when a reaction creates a chiral centre at a flat (planar) carbon. In AQA A-level Chemistry, an aldehyde or an unsymmetrical ketone reacting with KCN followed by dilute acid is the example: the C=O carbon is planar, and the cyanide ion is equally likely to attack from above or below, so the two enantiomers form in equal amounts.

Worked examples

Example 1: E or Z?

Name the two isomers of 2-chlorobut-2-ene, CH₃CCl=CHCH₃.

• Left carbon: Cl (atomic number 17) beats CH₃ (carbon, 6), so Cl has priority.
• Right carbon: CH₃ (carbon, 6) beats H (1), so CH₃ has priority.
• Left drawing: Cl is below and the right CH₃ is above, so the priority groups are on opposite sides: E. Yet its two methyl groups are on the same side, so in cis/trans terms it would be called cis.
• Right drawing: Cl and the right CH₃ are both above: Z.

Example 2: which molecules have a chiral centre?

Look at the whole group, not just the first atom: in butan-2-ol, CH₃ and C₂H₅ both start with carbon, but they are different groups, so the carbon is chiral.

Check your understanding.

Common mistakes

• Forgetting the second condition for E/Z. A C=C is not enough: each carbon needs two different groups.
• Ranking groups by size or mass. CIP priority goes by atomic number of the first atom, then the next atoms out.
• Calling a carbon chiral when two groups match. Check all four groups in full.
• Saying enantiomers have different boiling points. They are the same; only the direction they rotate plane-polarised light differs.
• Saying a racemic mixture contains no chiral molecules. It contains equal amounts of two chiral molecules, whose effects cancel.
• Drawing enantiomers that are not mirror images. Draw one, then reflect it in a mirror line: swap left and right, keep up and down.

Practise with a short timed paper

E/Z isomerism is in the first year of A-level Chemistry and optical isomerism in the second. The daily A-level Chemistry: Year 1 and A-level Chemistry: Year 2 papers are short timed papers of 8 questions, with a new one every day.

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