Recognise optical isomers and represent a single chiral carbon accurately in three dimensions.
Same connections, different arrangements
Stereoisomers have the same structural formula and atom connectivity but a different spatial arrangement. Optical isomers, called enantiomers, are non-superimposable mirror images. Rotating one molecule in space cannot make every group coincide with its mirror image. Breaking and remaking bonds would change the configuration rather than merely turn the molecule.
AQA limits this organic topic to molecules with a single chiral centre. Look for a tetrahedral carbon bonded to four different groups. A carbonyl carbon is trigonal planar and is not such a centre; a CH₂ carbon has two identical H atoms and cannot qualify. Chirality is a property of the whole arrangement, not an extra functional group.
Compare entire groups, not just the first atom
In CH₃CH(OH)CH₂CH₃, carbon 2 is bonded to H, OH, CH₃ and CH₂CH₃. The two carbon-containing groups differ, so butan-2-ol has a chiral centre. In propan-2-ol, CH₃CH(OH)CH₃, two groups are identical methyl groups, so the central carbon is not chiral.
When two groups begin with carbon, follow their structures outwards until a difference appears. The carbon in CH₃CH(OH)COOH is chiral because CH₃ and COOH are different groups. Glycine, H₂NCH₂COOH, is achiral; alanine, H₂NCH(CH₃)COOH, has a chiral α-carbon.
| Structure | Groups at the proposed centre | Conclusion |
|---|---|---|
| CH₃CHBrCH₂CH₃ | Br, H, methyl, ethyl | Chiral carbon at C2 |
| CH₃CHBrCH₃ | Br, H, methyl, methyl | Not chiral |
| CH₃COCH₂CH₃ | Carbonyl C has three attached groups; planar | Carbonyl C is not chiral |
| HOCH₂CH₂OH | Each carbon carries two H atoms | No chiral carbon |
A mirror pair needs a three-dimensional drawing
Use two ordinary bonds in the page, a solid wedge towards the viewer and a dashed wedge away. Keep all four groups attached to the same central carbon. To obtain the other enantiomer while holding two groups fixed, swap the remaining two groups. Swapping two pairs instead can return the original configuration.
In an examination asking for displayed structures, show the required atoms and bonds rather than replacing everything with an unexplained R. If condensed groups are permitted, make their attachment unambiguous: OH attaches through oxygen, and COOH through its carbonyl carbon.
Diagram placeholder
Butan-2-ol enantiomer pair to add
Labels to include:
- Central C* attached to OH, H, CH₃ and CH₂CH₃
- Two ordinary bonds in the paper
- Solid wedge towards viewer
- Dashed wedge away from viewer
- Mirror plane separating the two drawings
- OH and H interchanged in wedge/dash positions with the carbon groups fixed
Draw two tetrahedral arrangements with unchanged connectivity. Each central carbon has exactly four bonds; their mirror images cannot be superimposed by rotation. A flat cross without wedge/dash information does not show the intended three-dimensional arrangement.
Opposite rotations under the same conditions
A pure enantiomer rotates the plane of plane-polarised light. Its partner produces an equal rotation in the opposite direction when concentration, path length, solvent, temperature and light wavelength are the same. A polarimeter measures the rotation; the molecule does not make unpolarised light become polarised simply by being chiral.
Enantiomers have the same melting and boiling points and the same reactivity in an achiral environment, but can behave differently with chiral reagents, enzymes or receptors. Do not infer the rotation sign from a wedge/dash drawing. R/S configuration and +/− rotation describe different things; assigning R/S is beyond the required scope here.
Quick checks
Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.
Q1. Define enantiomers.Show answer
Stereoisomers that are non-superimposable mirror images. They have the same connectivity but different three-dimensional arrangements.
Q2. Identify the four groups at the chiral carbon in CH₃CH(OH)COOH.Show answer
H, OH, CH₃ and COOH. All four differ, so the central tetrahedral carbon is chiral.
Q3. Why is pentan-3-ol achiral despite having an OH group on a tetrahedral carbon?Show answer
Its central carbon is attached to OH, H and two identical ethyl groups. It does not have four different groups.
Q4. How can you draw the second enantiomer from a correct wedge/dash drawing?Show answer
Keep two groups fixed and interchange the other two. Preserve every bond connection and use wedge/dash bonds to show the resulting spatial arrangement.
Q5. A pure enantiomer rotates light +8.0° under stated conditions. What is expected for its partner under identical conditions?Show answer
−8.0°. The magnitude is equal and the direction opposite; this comparison requires the same concentration, path length and other measurement conditions.
Sources
Sources and examiner guidance (reviewed 2 October 2026)
- AQA 7405 organic chemistry specification — 3.3.7 coverage and required skills.
- Chemrevise: Optical isomerism — Coverage checklist; explanations, data exercises and quick checks on this page are original Finesse material.
- AQA June 2023 Paper 2 mark scheme — Q04.2 pp17–18 and examiner report Q04.2 p3: planar carbocation, attack from either face and product chirality. Single-centre scope follows the specification.
- AQA June 2023 Paper 2 examiner report — Read alongside the question-specific marking guidance; not a universal wording checklist.
Finesse Tuition is not endorsed by AQA or Chemrevise. All explanations and examples here are our own.