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3.2.5Additional lessons

Structural Isomers — IB Chemistry HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 62 of 172

📚 IB Chemistry HL Hub / Topical Revision
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Part 1 of 5Three Types

Chain isomers
  • Chain isomers The carbon skeleton is arranged differently, as in butane and 2-methylpropane, both C₄H₁₀.
Position isomers
  • Position isomers The functional group sits on a different carbon of the same skeleton, as in propan-1-ol and propan-2-ol.
Functional group isomers
  • Functional group isomers The atoms form different functional groups, as in ethanol and methoxymethane, both C₂H₆O.
Definition
  • Definition Structural isomers have the same molecular formula but different arrangements of atoms bonded together.
Properties
  • Properties Isomers differ in physical properties and may differ in chemical properties if the functional group differs.
Molecular formulaIsomers
C₄H₁₀Butane; 2-methylpropane
C₃H₈OPropan-1-ol; propan-2-ol; methoxyethane
C₅H₁₂Pentane; 2-methylbutane; 2,2-dimethylpropane

Part 2 of 5Finding All Isomers

1
Start by drawing the longest straight carbon chain for the formula.
2
Shorten the chain by one carbon and attach it as a methyl branch at each different position.
3
Continue shortening the chain and using ethyl or two methyl branches, avoiding repeats that are the same molecule rotated.
4
Insert the functional group at each distinct position on every skeleton to find position isomers.
5
Consider different functional groups with the same atoms, such as an alcohol and an ether.

Part 3 of 5Effects on Properties

Effects on Properties
Branching
  • Branching More branching lowers the boiling point because compact molecules have less surface contact for dispersion forces.
Hydrogen bonding
  • Hydrogen bonding Alcohols have higher boiling points than ether isomers because they form hydrogen bonds.
Reactivity
  • Reactivity Propan-1-ol can be oxidised to an aldehyde and acid, but propan-2-ol gives a ketone.
Naming
  • Naming Each isomer has its own systematic name, so the name identifies the structure.

Part 4 of 5Worked Example

Question Draw and name all the isomers of C₅H₁₂.

  1. The straight chain of five carbons is pentane.
  2. A four-carbon chain with one methyl branch on carbon 2 is 2-methylbutane.
  3. A three-carbon chain with two methyl groups on carbon 2 is 2,2-dimethylpropane.
  4. A methyl on carbon 3 of a pentane chain is the same as 2-methylbutane, so no further isomers exist.

Answer Three isomers: pentane, 2-methylbutane, 2,2-dimethylpropane

Part 5 of 5Exam Checklist & Tips

3.2.5 – Key Points for the Exam

  • Structural isomers Compounds with the same molecular formula but different connectivity of atoms.
  • Chain isomerism Isomerism caused by different branching of the carbon skeleton.
  • Position isomerism Isomerism caused by different location of a functional group.
  • Functional group isomerism Isomerism in which the isomers have different functional groups.

Common Mistakes

  • Counting a molecule twice because it is drawn bent or rotated.
  • Believing that isomers must have the same boiling point.
  • Naming 2-ethylpropane, which would have a longer chain and is actually 2-methylbutane.
  • Confusing structural isomers with stereoisomers.

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