Isomerism: structural isomerism and stereoisomerism

AS Level Chemistry · An introduction to AS Level organic chemistry. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

Isomers have the same molecular formula but a different arrangement of atoms. Structural isomers have their atoms joined in a different order. The difference can be in the carbon chain, in the position of the functional group, or in the functional group itself.

Stereoisomers have the same structural formula but a different arrangement in space. There are two kinds. Geometrical (cis/trans) isomers exist because the π bond stops rotation about a C=C bond. Optical isomers exist when a molecule has a chiral centre: a carbon atom joined to four different atoms or groups. The two optical isomers are mirror images that cannot be superimposed.

Rules to remember

Common mistake

Students often say an alkene has cis/trans isomers just because it has a C=C bond. If either carbon of the double bond carries two identical groups, as in but-1-ene, there are no cis/trans isomers.

Worked example

How many structural isomers have the molecular formula C3H6Cl2, and how many isomers are there when stereoisomers are included?

  1. Both Cl atoms on one carbon: 1,1-dichloropropane and 2,2-dichloropropane.
  2. Cl atoms on different carbons: 1,2-dichloropropane and 1,3-dichloropropane. That gives 4 structural isomers.
  3. In 1,2-dichloropropane, CH3CHClCH2Cl, carbon 2 is joined to H, Cl, CH3 and CH2Cl: four different groups, so it is a chiral centre with two optical isomers.
  4. No other isomer has a chiral centre, so the total is 3 + 2 = 5.

Answer: 4 structural isomers; 5 isomers including stereoisomers.

Practice questions

Try each one, then open the answer.

1. Which compound shows both optical and geometrical isomerism?

  1. A
    4-methylhex-2-ene
  2. B
    but-2-ene
  3. C
    butan-2-ol
  4. D
    2-methylbut-2-ene
Show answer

Answer: A. 4-methylhex-2-ene, CH3CH=CHCH(CH3)CH2CH3, has different groups on each C=C carbon (cis/trans) and C4 carries H, CH3, C2H5 and the alkenyl group (chiral). But-2-ene is only geometrical, butan-2-ol only optical, and 2-methylbut-2-ene shows neither.

2. Which alcohol contains a chiral centre?

  1. A
    propan-2-ol
  2. B
    2-methylpropan-2-ol
  3. C
    butan-1-ol
  4. D
    butan-2-ol
Show answer

Answer: D. A chiral carbon carries four different groups. C2 of butan-2-ol is bonded to H, OH, CH3 and C2H5. In propan-2-ol the two methyl groups are identical, so there is no chiral centre.

3. Butane and 2-methylpropane are isomers. Which type of isomerism do they show?

  1. A
    positional isomerism
  2. B
    chain isomerism
  3. C
    functional group isomerism
  4. D
    geometrical isomerism
Show answer

Answer: B. Both are C4H10 alkanes; they differ only in the arrangement of the carbon skeleton (straight versus branched), which is chain isomerism.

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