Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

3.1.4Organic Chemistry

Structural Isomerism — Edexcel AS Chemistry Revision Notes & Topical Questions

5 min read 4 parts Lesson 17 of 42

📚 Edexcel AS Chemistry Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Word Equation & Conditions: حمض كربوكسيلي + كحول (في وجود حمض الكبريتيك المركز كعامل حفاز ونازع للماء) ⇌ إستر + ماء
Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
0%read

Part 1 of 4What Structural Isomers Are

Definition
  • Definition Structural isomers have the same molecular formula but different arrangements of atoms joined together.
Properties
  • Properties They often have different physical properties, such as boiling points, and may have different chemical properties.
Chain
  • Chain Chain isomers have different carbon skeletons, for example straight-chain and branched forms.
Position
  • Position Positional isomers keep the same skeleton and functional group but place the group at different positions.
Functional group
  • Functional group Functional group isomers have the same molecular formula but different functional groups.

Part 2 of 4Examples to Know

Examples to Know
C₄H₁₀
  • C₄H₁₀ Butane and 2-methylpropane are chain isomers with different carbon skeletons.
C₅H₁₂
  • C₅H₁₂ Pentane, 2-methylbutane and 2,2-dimethylpropane are the three chain isomers of this formula.
C₄H₈
  • C₄H₈ But-1-ene and but-2-ene are positional isomers because the double bond moves along the chain.
C₃H₈O
  • C₃H₈O Propan-1-ol and propan-2-ol are positional isomers, while methoxyethane is a functional group isomer.
Strategy
  • Strategy Work systematically: start with the longest chain, then shorten it and move the groups to each position.
TypeSameDifferentExample
ChainMolecular formulaCarbon skeletonButane / 2-methylpropane
PositionSkeleton and groupGroup positionPropan-1-ol / propan-2-ol
Functional groupMolecular formulaFunctional groupPropan-1-ol / methoxyethane

Part 3 of 4Effect of Branching on Boiling Point

1
Straight-chain molecules lie side by side and have a large surface area in contact with their neighbours.
2
This gives stronger London forces between molecules and therefore higher boiling points.
3
Branched isomers are more compact, so molecules touch over a smaller area.
4
The London forces are weaker, so less energy is needed to separate the molecules.
5
For example, pentane boils at about 36 °C whereas 2,2-dimethylpropane boils at about 10 °C.

Part 4 of 4Exam Checklist & Tips

3.1.4 – Key Points for the Exam

  • Isomers Compounds with the same molecular formula but different structures.
  • Structural isomers Isomers with the same molecular formula but different structural formulae.
  • Chain isomerism Isomerism caused by different arrangements of the carbon skeleton.
  • Positional isomerism Isomerism caused by the functional group occupying different positions.
  • Functional group isomerism Isomerism in which isomers have different functional groups.

Common Mistakes

  • Drawing the same molecule twice in a different orientation and counting it as two isomers.
  • Missing isomers by not working systematically from the longest chain.
  • Thinking that isomers have the same boiling point, when branching generally lowers it.
  • Calling propan-1-ol and methoxyethane positional isomers, when they have different functional groups.

Last updated:

\n