Stereoisomers and isomerism for A-Level Chemistry

Stereoisomers are molecules that share the same molecular formula and the same structural formula, but their atoms are arranged differently in three-dimensional space. There are two kinds you need to know at A-Level: E/Z isomers (which involve a C=C double bond) and optical isomers (which involve a chiral carbon with four different groups attached).

This guide walks through both types, how to name them, how to spot them on a structure, and where students lose marks in AQA Chemistry papers. By the end you should be able to identify any isomer type in seconds and write a model definition that picks up full marks.

Isomers are compounds with the same molecular formula but different arrangements of atoms. There are two broad families: Structural isomers (different structural formulas, where atoms are connected in a different order) and stereoisomers (same structural formula, but a different arrangement in space).

At A-Level, AQA focuses on three structural types (chain, position, functional group) and two stereo types (E/Z and optical). Knowing which family a pair of isomers belongs to is often the first mark in a longer question, so do not skip the classification step.

E/Z isomerism occurs when a molecule has a C=C double bond with two different groups attached to each carbon of the double bond. Because the double bond cannot rotate, the groups are locked in position, giving two distinct arrangements.

In the Z isomer (from German zusammen, meaning together), the two higher-priority groups sit on the same side of the double bond. In the E isomer (from entgegen, meaning opposite), they sit on opposite sides. Priority is decided using the Cahn-Ingold-Prelog (CIP) rules, where the atom with the higher atomic number wins.

Step 1: Look at each carbon of the C=C double bond and identify the two groups attached to it.

Step 2: For each carbon, decide which of its two groups has higher CIP priority. The atom with the higher atomic number wins (so Br beats Cl, Cl beats F, and any halogen beats carbon or hydrogen).

Step 3: If the two higher-priority groups are on the same side of the double bond, the molecule is Z. If they are on opposite sides, it is E.

Optical isomers arise when a carbon atom has four different groups bonded to it. This carbon is called a chiral centre (or asymmetric carbon). The molecule and its mirror image are not superimposable, so they exist as two distinct enantiomers.

Enantiomers have identical physical and chemical properties in most respects. They differ in one specific way: They rotate the plane of plane-polarised light in opposite directions. One enantiomer rotates it clockwise (+, or dextrorotatory), the other anticlockwise (-, or laevorotatory). A 50:50 mix of the two is called a racemic mixture and shows no net rotation.

One famous example is thalidomide, prescribed in the late 1950s as a racemic mixture. One enantiomer treated morning sickness; the other caused severe birth defects. Modern drug synthesis therefore aims to produce a single enantiomer wherever possible.

Ibuprofen is another well-known case. Only the S-enantiomer is pharmacologically active, but it is sold as a racemic mixture because the body converts the inactive R-form into the S-form. AQA often uses pharmaceutical examples in synoptic essay-style questions.

Consider 2-bromobutane, CH3CHBrCH2CH3. Which carbon is the chiral centre?

Step 1: Label the four carbons in the chain. Carbon 1 is CH3, carbon 2 is CHBr, carbon 3 is CH2, carbon 4 is CH3.

Step 2: Look at each carbon's four attached groups. Carbon 2 has: H, Br, CH3 (carbon 1), and CH2CH3 (carbons 3 and 4). All four groups are different, so carbon 2 is the chiral centre.

Step 3: Confirm no other carbon qualifies. Carbon 3 has two hydrogens, so it is not chiral. Carbons 1 and 4 have three hydrogens each, so they are not chiral either. 2-bromobutane therefore exists as two enantiomers.

AQA examiner reports flag the same mistakes year after year. Most of them are wording slips rather than chemistry mistakes, which makes them frustrating but easy to fix once you know what examiners want to see.

AQA expects 3D structures using wedge and dash notation. A solid wedge means the bond comes out of the page towards you. A dashed wedge means the bond goes back into the page. The other two bonds are drawn as normal lines in the plane of the page.

To draw the second enantiomer, swap any two groups around the chiral carbon. The easiest swap is the H and one other group. The result should be the mirror image of the first structure. Always draw the mirror line if a question asks you to show the relationship between the two.