Back to the on-screen lesson ·

Chiral centers

A stereocentre is an sp3 carbon bonded to four different groups, compared as whole groups; a molecule with n of them has at most 2 to the power n stereoisomers.

Paper packet. Every task here also exists on screen, where it is checked automatically; answers written on paper are not assessed by Nydus. When you are back at a device, enter your answers there.

1. What you will learn

You will count a molecule's stereocentres, decide whether a carbon is one, explain why a carbon is not, mark stereocentres in a chain, find one in a drug, and give the most stereoisomers possible.

2. What you already have

You met R and S as names for the arrangement at a stereocentre. This lesson is about finding stereocentres in the first place.

3. Words for this lesson

A chiral object is not superimposable on its mirror image, like a hand. A stereocentre (chiral centre) is an sp3 carbon bonded to four different groups. An achiral object is superimposable on its mirror image.

4. Four different groups

Hold a model of butan-2-ol's carbon 2: H, OH, CH3 and CH2CH3 point to the corners of a tetrahedron. Its mirror image has the same four groups, arranged the other way round, and no rotation makes the two coincide: carbon 2 is a stereocentre.

Change the ethyl to a second methyl and you have propan-2-ol, whose CH carbon carries two identical CH3 groups. Its mirror image can be turned to fit: not a stereocentre.

The test: list the four groups, comparing each whole group — follow the branch outwards until it differs. CH3 and CH2CH3 differ one carbon out; two CH2CH3 groups never differ.

Another way: steps

To find stereocentres:

  1. Skip sp2 and sp carbons.
  2. Skip CH3 and CH2 carbons: identical hydrogens.
  3. For each remaining carbon, list its four groups.
  4. All four different: a stereocentre.

5. Counting, and why it matters

A molecule can have several stereocentres. 2,3-dichloropentane has two, at carbons 2 and 3. Each can independently be R or S, so a molecule with n stereocentres has at most 2 to the power n stereoisomers; the next lessons show when symmetry makes some of them the same.

The question is not academic. Many drugs have a stereocentre, and the two mirror forms act differently in the body, because the body's own molecules are chiral: one form of ibuprofen relieves pain, and the body slowly converts the other.

6. Try to make them coincide

Carbon 2 of butan-2-ol with its four different groups, H, OH, CH₃ and C₂H₅, at the corners of a tetrahedron, and on the other side of a mirror its mirror image. Each group in one sits opposite the same group in the other. Turn either molecule any way you like: when two groups line up with their partners, the other two are swapped. The two cannot be made to coincide, so carbon 2 is a stereocentre.
Carbon 2 of butan-2-ol with its four different groups, H, OH, CH₃ and C₂H₅, at the corners of a tetrahedron, and on the other side of a mirror its mirror image. Each group in one sits opposite the same group in the other. Turn either molecule any way you like: when two groups line up with their partners, the other two are swapped. The two cannot be made to coincide, so carbon 2 is a stereocentre.

The figure shows carbon 2 of butan-2-ol with its four groups, H, OH, CH3 and C2H5, and its reflection in a mirror. Turn the two molecules and try to lay one on the other. You can always line up two groups — say C2H5 up and H towards you — but then the OH and CH3 are on opposite sides in the two molecules. No rotation fixes that, because the only way to swap two groups is to break bonds. That is what a stereocentre is: a carbon with four different groups, whose mirror image is a different molecule. Replace the C2H5 by a second CH3 and the two mirror images can be turned to match, because swapping two identical groups changes nothing.

7. Where this goes wrong

Any carbon with four bonds is chiral. The four groups must differ.

Compare only the first atom of each group. Compare whole groups.

An sp2 carbon can be a stereocentre. It has only three groups.

A molecule with a stereocentre always has exactly two stereoisomers. With n centres, up to 2 to the power n.

8. Lactic acid, CH3CH(OH)COOH

  1. The CH carbon carries H, OH, CH3 and COOH.

    List the four groups.

  2. All four differ, so it is a stereocentre.

    Four different groups.

  3. Lactic acid therefore has two mirror-image forms, found in sour milk and in muscle.

    Chiral molecule.

9. Your turn: 2-methylbutane, CH3CH(CH3)CH2CH3

  1. What four groups does the CH carbon carry?

    H, CH3, CH3 and CH2CH3.

  2. Is it a stereocentre?

  3. Your turn: work this step out. Its working is at the end of the packet.

    No: two of its groups are identical methyls.

10. Guided practice

How many stereocentres does 3-methylpentane, CH3CH2CH(CH3)CH2CH3, have?

Answer:

11. Guided practice

In butan-2-ol, CH3CH(OH)CH2CH3, is carbon 2 — the carbon carrying the OH — a stereocentre?

12. Practice

Match each carbon to whether it is a stereocentre, and why.

not a stereocentre: two identical groupsnot a stereocentre: sp2, only three groupsa stereocentre
the CH carbon of propan-2-ol, (CH3)2CHOH
the CH carbon of 3-methylpentane
the C=O carbon of propanone
carbon 2 of 2-chlorobutane

13. Practice

In 2,3-dichloropentane, CH3CHClCHClCH2CH3, the carbons are numbered 1 to 5 from the left. Mark every stereocentre.

This task has no paper form; do it on a device.

14. Somewhere new

Ibuprofen is written Ar–CH(CH3)–COOH, where Ar is a substituted benzene ring. Which atom is its stereocentre?

15. Lesson test

Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.

16. Test question

At most, how many stereoisomers can 2-methylbutane, CH3CH(CH3)CH2CH3, have?

The molecule can have at most a stereoisomers.

17. What you can do now

You can find a molecule's stereocentres. Tell someone why propan-2-ol is not chiral. Next: assigning R or S to a stereocentre.

Working for the steps left to you

9. Your turn: 2-methylbutane, CH3CH(CH3)CH2CH3, step 3