How to Identify a Chiral Centre (Chiral Carbon) Step by Step

The test

A carbon is a chiral centre (also called a stereocentre or asymmetric carbon) if it is sp³ and has four different groups attached. Such a molecule has a mirror image that can't be superimposed on it: a pair of enantiomers.

Step by step

Skip carbons that can't be chiral: CH₃, CH₂, and any carbon in a C=C, C≡C or C=O. They can't have four different groups.
For each remaining carbon, list its four groups. Remember the hydrogen that isn't drawn in skeletal structures.
Compare the groups by walking outwards. Two groups that start the same (–CH₂…) may differ further along. Keep going until you find a difference or reach the end.
If all four differ, mark it with an asterisk (*).

Examples

butan-2-ol: C2 has H, OH, CH₃ and CH₂CH₃, so it is a chiral centre

propan-2-ol: C2 has two identical CH₃ groups, so it is not chiral

3-methylhexane: C3 has H, CH₃, ethyl and propyl, so it is chiral

3-methylpentane: C3 has two ethyl groups, so it is not chiral

alanine: the α-carbon has H, CH₃, NH₂ and COOH

2-methylcyclohexanone: C2 is chiral, because going round the ring one way meets C=O first

Rings

For a ring carbon, the two ring "groups" are the two paths round the ring. Walk both ways from the carbon. If the paths meet different atoms or branches at the same distance, they are different groups. In methylcyclohexane the two paths are identical, so C1 is not chiral. In 2-methylcyclohexanone one path reaches the C=O immediately, so C2 is chiral.

How many stereoisomers?

A molecule with n chiral centres has at most 2ⁿ stereoisomers: 2 for one centre, 4 for two, 8 for three. It can have fewer if the molecule has an internal mirror plane, because then one of the forms is a meso compound. Double bonds that can be E or Z add to the count too.

Common mistakes

Forgetting the hidden H in skeletal structures.
Calling a carbon chiral because the groups look different near the centre but are the same further out (or the reverse).
Marking carbons in C=C or C=O. They're sp², so they can't be stereocentres.

Quick answers

What is a chiral centre?

A chiral centre is an atom, usually an sp³ carbon, with four different groups attached. A molecule with one chiral centre has two non-superimposable mirror-image forms called enantiomers.

How do you know if a carbon is chiral?

Check that it has four single bonds to four different groups. Compare groups by walking outwards along each one until you find a difference. CH₃, CH₂ and carbons in double bonds are never chiral centres.

How many stereoisomers does a molecule with 3 chiral centres have?

At most 2³ = 8. It has fewer if one or more of the forms is a meso compound.

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How to identify chiral centres (stereocentres)