3.1.4Physical Chemistry
Structural Isomerism — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 67 of 102
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 5What Are Structural Isomers?
Structural isomers have the same molecular formula but different structural formulae.
Chain isomers
- Chain isomers These have different carbon skeletons, such as butane and methylpropane, both C₄H₁₀.
Position isomers
- Position isomers These have the same carbon skeleton and functional group but at different positions, such as propan-1-ol and propan-2-ol.
Functional group isomers
- Functional group isomers These have different functional groups, such as ethanol (C₂H₅OH) and methoxymethane (CH₃OCH₃), both C₂H₆O.
Same formula
- Same formula Isomers have identical molecular formulae and relative molecular masses but different structures and properties.
Part 2 of 5Examples to Learn
Examples to Learn
Three pentanes
- Three pentanes C₅H₁₂ has exactly three chain isomers: one unbranched, one with a methyl branch and one with two methyl groups.
Position of halogen
- Position of halogen A bromine on the end carbon or the middle carbon of propane gives two different compounds.
Different groups
- Different groups Aldehydes and ketones with the same formula have different functional groups and different reactions.
| Formula | Isomers |
|---|---|
| C₅H₁₂ | Pentane, 2-methylbutane, 2,2-dimethylpropane |
| C₃H₇Br | 1-bromopropane, 2-bromopropane |
| C₃H₆O | Propanal (aldehyde), propanone (ketone) |
| C₂H₆O | Ethanol (alcohol), methoxymethane (ether) |
Part 3 of 5Effect on Properties
1
Isomers have the same relative molecular mass, but their branching changes how closely molecules can pack together.
2
More branching reduces the surface area of contact between molecules, giving weaker London forces.
3
Weaker intermolecular forces mean that a more branched isomer has a lower boiling point than its straight-chain isomer.
4
Pentane boils at about 36 °C, while 2,2-dimethylpropane boils at about 10 °C, showing the effect.
5
Position and functional group isomers can differ much more, because they may have different types of intermolecular force.
Part 4 of 5Worked Example
Question Draw and name all structural isomers of C₄H₉Br.
- Start with the unbranched chainCH₃CH₂CH₂CH₂Br is 1-bromobutane.
- Move the bromine to the second carbon: CH₃CHBrCH₂CH₃ is 2-bromobutane.
- Use the branched skeleton of methylpropane(CH₃)₂CHCH₂Br is 1-bromo-2-methylpropane.
- Place bromine on the central carbon of methylpropane: (CH₃)₃CBr is 2-bromo-2-methylpropane.
Answer Four isomers: 1-bromobutane, 2-bromobutane, 1-bromo-2-methylpropane and 2-bromo-2-methylpropane
Part 5 of 5Exam Checklist & Tips
3.1.4 – Key Points for the Exam
- Structural isomers Compounds with the same molecular formula but different structural formulae.
- Chain isomerism Isomerism arising from different carbon skeletons.
- Position isomerism Isomerism arising from different positions of a functional group on the same skeleton.
- Functional group isomerism Isomerism arising when the atoms form different functional groups.
Common Mistakes
- Drawing the same molecule twice in a different orientation and counting it as two isomers.
- Changing the molecular formula when drawing isomers, for example by adding or losing a hydrogen.
- Saying branched isomers have a higher boiling point; they have a lower one.
- Naming a structure using a shorter chain than the longest one available.
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