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LearnYCH11WCH14 · Unit 4: Rates, Equilibria and Further Organic ChemistryCarbonyls, carboxylic acids and chiralityOptical isomerism and chirality
Carbonyls, carboxylic acids and chirality · Subtopic 3 of 4

Optical Isomerism and Chirality

A chiral centre is a carbon with four different groups attached. Molecules with one chiral centre exist as two enantiomers: non-superimposable mirror images. Enantiomers have identical physical properties except that they rotate plane-polarised light by equal amounts in opposite directions. A racemic mixture (50 : 50) is optically inactive. Reactions through planar intermediates (carbonyl addition, SN1 carbocations) give racemic products; SN2 gives a single enantiomer with inversion. Drugs are often made as single enantiomers because only one fits the biological target (thalidomide is the classic warning).

Cambridge 9701 Paper 4Edexcel YCH11 WCH14AQA 7405 Paper 2≈ 40 minRevision 1 · updated 2026-10-30
Section 1 of 14

Learning objectives

  • Identify chiral centres.
  • Draw enantiomers in 3D.
  • Explain optical activity and racemic mixtures.
  • Link stereochemistry to mechanisms and drugs.
Pause and think

Your left and right hands are mirror images. Why can't a left glove fit a right hand?

Section 2 of 14

Core idea

Start hereLeft-handed molecules

One form of carvone smells of spearmint, its mirror image of caraway. Your nose can tell left from right.

The big idea

A carbon with four different groups is chiral. Its two mirror-image forms rotate polarised light in opposite directions and can act very differently in the body.

1. Spotting chiral centres

Worked exampleLactic acid

Identify the chiral centre in 2-hydroxypropanoic acid, CH3CH(OH)COOH\mathrm{CH_3CH(OH)COOH}.

Goal
Locate C*.
Strategy
List groups on each carbon.
  1. C1 (COOH): only three groups\text{C1 (COOH)}:\ \text{only three groups}
    C=O double bond.
  2. C2: H, OH, CH3, COOH\text{C2}:\ \mathrm{H,\ OH,\ CH_3,\ COOH}
    Four different groups.
AnswerC2 is chiral.
✓ CheckCH₃ carbon has three H ✓.
Mechanism and optical activity

Planar intermediates give racemic products.

Exam tip3D drawings Draw two wedge-dash tetrahedra as mirror images either side of a dashed line; groups on the wedges must swap sides.

Summary

  • Four different groups → chiral.
  • Enantiomers rotate light oppositely.
  • Racemic = 50 : 50, inactive.
Key words
Enantiomers
Non-superimposable mirror-image molecules.
Racemic mixture
Equal amounts of two enantiomers.
Plane-polarised light
Light oscillating in one plane only.
HistoryPasteur's crystals. In 1848, Louis Pasteur separated mirror-image crystals of sodium ammonium tartrate with tweezers, discovering molecular chirality.
Fun factMint vs caraway. The two enantiomers of carvone smell different: one of spearmint, the other of caraway seeds. Your nose is chiral.
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