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Isomerism

Chemistry: AQA A Level, topic 5.1.3

Structural Isomerism

Structural isomers are molecules which have the same molecular formula but a different arrangement of atoms in space.

Chain isomers

Chain isomers

  • Chain isomers are molecules that have the same molecular formula but one is linear and one is branched. For example:
  • Butane is a linear molecule with the molecular formula C4H10.
  • Methylpropane is a branched molecule with the molecular formula C4H10.
Position isomers

Position isomers

  • Position isomers are molecules that have the same molecular formula but have their functional group in different places. For example:
  • Butan-1-ol has a hydroxyl group attached to carbon-1.
  • Butan-2-ol has a hydroxyl group attached to carbon-2.
Functional group isomers

Functional group isomers

  • Functional group isomers are isomers that have had their structures changed so that they have different functional groups. For example:
  • Propanoic acid.
  • Methyl ethanoate.

Stereoisomerism

Stereoisomers are molecules that have the same molecular formula, same functional groups and same functional group positions but different arrangements of atoms in space.

Alkene stereoisomerism

Alkene stereoisomerism

  • Stereoisomerism can be seen in alkenes.
  • It is possible in alkenes because of the presence of double bonds:
  • Unlike single bonds, double bonds are rigid and you cannot rotate around them.
  • So alkenes can display stereoisomerism at the double bond.
An example

An example

  • The simplest way to understand what this means is with an example.
  • Above are the two different alkenes, (E)-but-2-ene and (Z)-but-2-ene.
    • The molecules are the same in every way, except for the atoms' position in space.
What do E and Z mean?

What do E and Z mean?

  • E and Z come from the German words 'entgegen' and 'zusammen' for 'opposite' and 'together'.
  • An E-alkene has the larger groups on opposite sides.
  • A Z-alkene has the larger groups on the same side.
  • A good way to remember this is that Z alkenes have large groups on the 'zame zide'.

Cahn-Ingold Prelog Rules

The Cahn-Ingold Prelog rules help us assign E/Z isomerism to more complicated molecules.

The Cahn-Ingold Prelog rules

The Cahn-Ingold Prelog rules

  • They help to distinguish between E and Z isomers for more complicated cases.
  • The rules work by numbering the four directly connecting atoms in order of their atomic mass number. From this, we give them a 'priority'.
  • The highest atomic number gets priority 1.
  • The second highest atomic number gets priority 2, and so on.
E vs Z

E vs Z

  • E:
  • Priority 1 and 2 are on different sides of the double bond.
    • I.e. The two atoms with the highest atomic mass number are on different sides.
  • Z:
  • Priority 1 and 2 are on the same side of the double bond.
    • I.e. The two atoms with the highest atomic mass number are on the same side.
An example: E

An example: E

  • The shown but-2-ene is an E geometry because the two methyl groups are on opposite sides of the double bond.
An example: Z

An example: Z

  • The shown but-2-ene is a Z geometry because the two methyl groups are on the same side of the double bond.

Test yourself on Isomerism

  • What do position isomers have?

  • Which of the following is NOT a chain isomer of pentane?

  • What do stereoisomers NOT have in common?

  • E-alkene isomers are identified by which of the following?

  • Which of the groups on the double bond below has the highest priority?

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