Structural Isomerism

Isomerism is central to organic chemistry because it explains why two compounds with exactly the same atoms can behave completely differently. This lesson covers OCR A-Level Chemistry A (H432) specification 4.1.1 (e): structural isomerism — chain, position, and functional group isomerism.

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1. Defining Isomers

Key Definition — Isomers: Compounds with the same molecular formula but a different arrangement of atoms.

There are two broad classes of isomers:

graph TD
  A[Isomers] --> B[Structural isomers<br/>different atom connectivity]
  A --> C[Stereoisomers<br/>same connectivity,<br/>different 3D arrangement]
  B --> D[Chain isomerism]
  B --> E[Position isomerism]
  B --> F[Functional group isomerism]
  C --> G[E/Z isomerism<br/>around C=C]
  C --> H[Optical isomerism<br/>A2 only]

At AS level (and this topic), we deal with structural isomerism. Stereoisomerism (E/Z) is covered in Lesson 8.

Key Definition — Structural isomers: Compounds with the same molecular formula but different structural formulae — i.e., their atoms are joined together in a different order.

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2. Chain Isomerism

Chain isomers differ in the arrangement of their carbon skeleton. One may be a straight chain, the other branched, or the branches may be in different positions.

2.1 Example — Butane (C₄H₁₀)

Butane has two chain isomers:

Isomer	Structure	B.p. / °C
Butane	CH₃–CH₂–CH₂–CH₃	−0.5
Methylpropane (isobutane)	(CH₃)₃CH	−12

2.2 Example — Pentane (C₅H₁₂)

Pentane has three chain isomers:

1. Pentane: CH₃CH₂CH₂CH₂CH₃
2. 2-methylbutane: (CH₃)₂CHCH₂CH₃
3. 2,2-dimethylpropane: (CH₃)₄C

The more branched the molecule, the more compact its shape, and the lower its boiling point, because London forces act over less contact area.

2.3 Example — Hexane (C₆H₁₄)

Hexane has five chain isomers:

• Hexane
• 2-methylpentane
• 3-methylpentane
• 2,2-dimethylbutane
• 2,3-dimethylbutane

The number of chain isomers grows rapidly with carbon count.

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3. Position Isomerism

Position isomers have the same carbon skeleton and the same functional group, but the functional group is attached at a different position along the chain.

3.1 Example — C₃H₇Cl

Two position isomers:

• 1-chloropropane: CH₃CH₂CH₂Cl
• 2-chloropropane: CH₃CHClCH₃

3.2 Example — C₄H₉OH

Four structural isomers in total, three of which are position isomers of one another:

• Butan-1-ol: CH₃CH₂CH₂CH₂OH
• Butan-2-ol: CH₃CH₂CH(OH)CH₃
• 2-methylpropan-2-ol: (CH₃)₃COH (a chain isomer of butan-1-ol/butan-2-ol)
• 2-methylpropan-1-ol: (CH₃)₂CHCH₂OH (another chain isomer)

3.3 Example — Pentene

• Pent-1-ene: CH₂=CHCH₂CH₂CH₃
• Pent-2-ene: CH₃CH=CHCH₂CH₃

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4. Functional Group Isomerism

Functional group isomers have the same molecular formula but contain different functional groups. They therefore belong to different homologous series and have very different chemical and physical properties.

4.1 Common Examples

Molecular formula	Functional group isomers
C₂H₆O	Ethanol (CH₃CH₂OH) and methoxymethane (CH₃OCH₃)
C₃H₆O	Propanal (CH₃CH₂CHO) and propanone (CH₃COCH₃)
C₃H₆O₂	Propanoic acid (CH₃CH₂COOH) and methyl methanoate (HCOOCH₃)
C₃H₆	Propene (CH₂=CHCH₃) and cyclopropane

4.2 Worked Example — C₃H₆O