Topic 4 · page 8 of 17

Structural isomerism

Structural isomers have the same molecular formula but different structural formulae. There are three ways the atoms can be arranged differently, and naming a drawing is the way to check that two drawings are really different molecules.

10 min read4.10

Read first: Naming organic compounds

Same atoms, different structures 4.10

The atoms are joined together in a different order. Isomers are different compounds with different names and different properties. For C₄H₁₀ there are two:

Butane Methylpropane
Structural formula CH₃CH₂CH₂CH₃ CH₃CH(CH₃)CH₃
Boiling temperature −1 °C about −12 °C

Both are C₄H₁₀, but their structures differ and so do their boiling temperatures. A molecular formula on its own does not identify a compound.

Three kinds of structural isomerism 4.10

Three kinds of structural isomer in skeletal formulae. Chain isomers: butane and methylpropane. Position isomers: 1-chloropropane and 2-chloropropane. Functional group isomers: propene and cyclopropane.
The three kinds of structural isomerism.
Kind What differs Example
Chain the carbon skeleton is arranged differently, with more or less branching butane and methylpropane (C₄H₁₀)
Position the same functional group is on a different carbon 1-chloropropane and 2-chloropropane (C₃H₇Cl)
Functional group a different functional group, so a different homologous series propene and cyclopropane (C₃H₆)

For this topic, most questions are about chain isomers of alkanes. Position isomers appear when a halogen is substituted onto a chain (Further substitution), and functional group isomers when the general formula is shared by two series, such as CₙH₂ₙ for alkenes and cycloalkanes.

MethodFinding isomers from a molecular formula

  1. Work out the molecular formula, and the number of carbons, and decide if the compound is an alkane, a cycloalkane or contains a functional group.
  2. Draw the longest possible chain first.
  3. Make the chain one carbon shorter and attach the spare carbon as a branch on each possible carbon. Remove drawings that give the same name.
  4. Shorten again and repeat, until no new structure is possible.
  5. Name every structure. Two drawings with the same name are the same molecule.

Check yourself

  1. Define structural isomers.
  2. State, with a reason, whether CH₃CH₂CH₂CH₂CH₃ and CH₃CH(CH₃)CH₂CH₃ are isomers.
  3. Classify each pair as chain, position or functional group isomers: (a) butane and methylpropane; (b) 1-chloropropane and 2-chloropropane; (c) propene and cyclopropane.
  4. A student draws a four-carbon chain with a methyl on carbon 1, and another with a methyl on carbon 4. Explain why these are not two isomers.
  5. Two compounds both have the molecular formula C₃H₇Br. Give their names and structural formulae.
  6. The boiling temperatures of butane and methylpropane are different. Explain why this does not contradict their having the same molecular formula.

Answers cover these until you have tried the questions

  1. Structural isomers are compounds with the same molecular formula but different structural formulae.
  2. Yes. The first is pentane, C₅H₁₂, and the second is 2-methylbutane, which is also C₅H₁₂. They have the same molecular formula and different structural formulae, so they are structural isomers (chain isomers).
  3. (a) Chain isomers. (b) Position isomers. (c) Functional group isomers.
  4. A methyl on carbon 1 of a four-carbon chain just makes the chain longer: it is a five-carbon chain (pentane). A methyl on carbon 4 does the same. Both drawings are pentane, so they are the same molecule.
  5. 1-bromopropane, CH₃CH₂CH₂Br, and 2-bromopropane, CH₃CHBrCH₃. These are position isomers.
  6. A molecular formula only gives the number of each kind of atom. The two compounds have the same formula but different structures, so they are different compounds with different properties. The branched molecule is more compact, so it has less contact with its neighbours and weaker London forces between molecules, so it boils at a lower temperature.