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*Revision Materials* 1 Atomic Structure 2 Atoms, molecules and stoichiometry 3 Chemical Bonding 4 States of matter 5 Chemical energetics 6 Electrochemistry 7 Equilibria 8 Reaction kinetics 9 The Periodic Table, chemical periodicity 10 Group 2 11 Group 17 12 Nitrogen and sulfur 13 Organic 14 Hydrocarbons 15 Halogen compounds 16 Hydroxy compounds 17 Carbonyl compounds 18 Carboxylic acids and derivatives 19 Nitrogen compounds 20 Polymerisation 21 Organic synthesis 22 Analytical techniques 23 Chemical energetics 24 Electrochemistry 25 Equilibria 26 Reaction kinetics 27 Group 2 28 Chemistry of transition elements 29 Organic 30 Hydrocarbons 31 Halogen compounds 32 Hydroxy compounds 33 Carboxylic acids and derivatives 34 Nitrogen compounds 35 Polymerisation 36 Organic synthesis 37 Analytical techniques

29 Organic

29.1 Formulas, functional groups and the naming of organic compounds 29.2 Characteristic organic reactions 29.3 Shapes of aromatic organic molecules; σ and π bonds 29.4 Isomerism, optical

Isomerism: Optical

Specification Reference Organic Chemistry, An introduction to A Level organic chemistry 29.4

Quick Notes

  • Enantiomers are isomers that are non-superimposable mirror-images of each other.
  • Chiral centre: Carbon atom bonded to 4 different groups.
  • Optically active: Substance or enantiomer that rotates plane polarised light.
  • Racemic mixture: Equal mix of enantiomers with no overall optical activity.
  • Relevance to drug design: Enantiomers may act differently in the body; chiral catalysts can help make a single enantiomer.

Full Notes

What Are Enantiomers?

Enantiomers are a type of stereoisomer. They have the same structural formula but differ in how their atoms are arranged in 3D space.

Specifically, enantiomers are non-superimposable mirror images of each other.

CIE A-Level Chemistry diagram showing enantiomers as non-superimposable mirror images.

They arise when a molecule has a chiral centre. A chiral centre is a carbon atom bonded to four different groups.

Example: 2-hydroxypropanoic acid (Lactic Acid)

CIE A-Level Chemistry diagram of lactic acid showing chiral carbon.

Optical Activity and Plane Polarised Light

When plane polarised light is passed through a sample of a single enantiomer, the light gets rotated. One enantiomer will rotate the light to the left or right. This can be measured using a polarimeter.

Each enantiomer rotates light to the same extent but in opposite directions.

CIE A-Level Chemistry diagram showing optical activity and rotation of plane polarised light.

A racemic mixture (or racemate) contains equal amounts of both enantiomers:

Chirality in Drug Design and Biological Systems

Chirality is particularly important in medicinal chemistry. Many biological molecules (like enzymes, DNA, and protein receptors) are chiral, which means only one enantiomer of a drug may be biologically active.

CIE A-Level Chemistry diagram showing stereospecific enzyme active site binding only one enantiomer.

If a drug is chiral, only one enantiomer may be effective. The other enantiomer might be:

As a result, racemic mixtures often need to be separated into two pure enantiomers at some stage during synthetic production of drug molecules. Chiral catalysts can be used to ensure only one single, pure enantiomer is formed.

Summary