Explain chiral centres and enantiomers
Identify a tetrahedral carbon bonded to four different groups and connect it to a pair of optical isomers.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Use the published functional-group scope and nomenclature limits; draw formula and mechanism evidence with correct connectivity, electron movement and stereochemistry. Written practice is self-checked
Before you begin
- Use covalent structures, valency, molecular formulas, lone pairs, and single, double and triple bonds.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How many different groups must be attached to a chiral tetrahedral carbon?
What you will learn
- Define a chiral centre
- Explain non-superimposable mirror images
- Use enantiomer terminology
Four different groups create a chiral carbon centre
A chiral centre in these organic examples is a tetrahedral carbon atom bonded to four different atoms or groups. If any two attached groups are identical, that carbon is not chiral.
The four different attachments allow two mirror-image spatial arrangements that cannot be superimposed by rotation. These are optical isomers called enantiomers.
Check the full attached groups
A group is compared by its complete connectivity, not only the first atom. For example, –CH₂CH₃ differs from –CH₃ even though both begin with carbon.
A molecule may contain more than one chiral centre. This syllabus does not require meso-compound or diastereoisomer nomenclature; keep claims within the stated scope.
Pause and explain
What is a pair of non-superimposable mirror-image optical isomers called?
Worked example
Is the central carbon in CH₃CH(OH)CH₂CH₃ a chiral centre? Explain.
Show the worked solution
- It is bonded to H, OH, CH₃ and CH₂CH₃.
- These four groups are different.
- The carbon is tetrahedral and chiral, giving a pair of enantiomers.
Answer Yes. The central carbon is bonded to four different groups: H, OH, CH₃ and CH₂CH₃, so it is a chiral centre.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Test the central carbon in CH₃CH(Br)CH₂CH₃ for chirality.
Give me a hint
List all four groups directly attached to the second carbon.
Compare my reasoning
- The groups are H, Br, CH₃ and CH₂CH₃.
- All four are different.
- The carbon is a chiral centre.
It is chiral because it is bonded to four different groups.
Look for these in your work
- All groups listed
- Duplicates checked
- Conclusion justified
Explain why the middle carbon in CH₃CH₂CH₂CH₃ is not a chiral centre.
Give me a hint
Inspect the groups attached to each candidate tetrahedral carbon.
Compare my reasoning
- The internal carbon has two H atoms or repeated chain environments.
- It does not have four different attachments.
- It is not a chiral centre.
It is not chiral because it lacks four different attached groups.
Look for these in your work
- Candidate examined
- Four-group test applied
- Conclusion stated
A carbon is attached to –H, –OH, –CH₃ and –CH₂CH₃. What stereoisomer pair can arise?
Give me a hint
Apply the four-different-groups test and describe the mirror images.
Compare my reasoning
- All four substituents differ.
- The tetrahedral carbon is chiral.
- Two non-superimposable mirror-image enantiomers can form.
A pair of optical isomers called enantiomers.
Look for these in your work
- Distinct groups
- Chiral centre
- Enantiomer result
Common mistakes
- Counting a carbon with two identical methyl groups as chiral
- Calling mirror images enantiomers without checking superimposability
- Comparing attached groups only by their first atom
What can you explain now?
How many different groups must be attached to a chiral tetrahedral carbon?
Compare with the explanation
Four
All four attachments must be different.
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Make it stick
What makes two enantiomers different from a pair of structural isomers?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.