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3.2.7Additional lessons

Enantiomers (HL) — IB Chemistry HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 64 of 172

📚 IB Chemistry HL Hub / Topical Revision
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Part 1 of 4Chirality

Chiral centre
  • Chiral centre A carbon atom bonded to four different atoms or groups, which can be arranged in two non-superimposable ways.
Enantiomers
  • Enantiomers The two forms are mirror images of each other, like a left and right hand, and cannot be placed on top of each other.
Example
  • Example Butan-2-ol has four different groups on carbon 2: H, OH, CH₃ and CH₂CH₃, so it has two enantiomers.
Not chiral
  • Not chiral Propan-2-ol has two identical methyl groups on the central carbon, so it is not chiral.
Number of isomers
  • Number of isomers A molecule with n chiral centres can have up to 2ⁿ stereoisomers.

Part 2 of 4Why It Matters

Why It Matters
Drugs
  • Drugs Often only one enantiomer of a drug is active, and the other may be inactive or harmful.
Thalidomide
  • Thalidomide One enantiomer was effective against morning sickness while the other was linked to birth defects.
Synthesis
  • Synthesis Ordinary synthesis from achiral starting materials gives a racemic mixture, and separation is difficult.
Biology
  • Biology Natural amino acids are almost all of one handedness, so enzymes recognise only one enantiomer.

Part 3 of 4Worked Example

Question Decide whether 2-chlorobutane, CH₃CHClCH₂CH₃, is chiral.

  1. Identify the groups on carbon 2: H, Cl, CH₃ and CH₂CH₃.
  2. All four groups are different from each other.
  3. Carbon 2 is therefore a chiral centre.
  4. The molecule has two enantiomers, which rotate plane-polarised light in opposite directions.

Answer Yes, chiral at C2

Part 4 of 4Exam Checklist & Tips

3.2.7 – Key Points for the Exam

  • Chiral centre A carbon atom attached to four different groups.
  • Enantiomers Non-superimposable mirror-image stereoisomers.
  • Optical activity The ability to rotate the plane of plane-polarised light.
  • Racemic mixture An equimolar mixture of two enantiomers that shows no optical rotation.

Common Mistakes

  • Treating a carbon atom with two identical groups as chiral.
  • Saying enantiomers have different boiling points.
  • Confusing the direction (+ or −) of rotation with the R or S label.
  • Assuming a racemic mixture is a single compound that is optically active.

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