Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

11.10Organic Chemistry

Structural isomers — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 11 parts Lesson 10 of 10 1 quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read
Lesson summary

11.10 Structural isomers — Notes — IGCSE Chemistry notes: Define structural isomers and identify and name isomers of butane, butene and propanol from their structural formulae.Structural isomers — compounds with the same molecular formula but different structural formulae.Same at…

  • Same atoms
  • Different compounds
  • Double bond
  • Functional group
  • Displayed formula
  • Methylpropane

Section 1 of 11Definition

Structural isomers
  • Structural isomers compounds with the same molecular formula but different structural formulae.
Same atoms
  • Same atoms isomers contain exactly the same number of each type of atom, and only the way the atoms are joined differs.
Different compounds
  • Different compounds they can have different boiling points and other physical properties.

Section 2 of 11Ways atoms can be rearranged

Ways atoms can be rearranged
Chain
  • Chain a branched instead of a straight carbon chain.
Double bond
  • Double bond a moved C=C double bond.
Functional group
  • Functional group a moved functional group such as –OH.
Pentane, C5H12, has three isomers.

Section 3 of 11Formula vocabulary

Molecular formula
  • Molecular formula the number of each type of atom, such as C4H10.
Structural formula
  • Structural formula shows how atoms are arranged, such as CH3CH2CH2CH3.
Displayed formula
  • Displayed formula shows every atom and every bond.

Section 4 of 11Isomers of C4H10

Butane
  • CH3CH2CH2CH3, a straight chain of 4 carbons.
Methylpropane
  • CH3CH(CH3)CH3, a 3-carbon chain with a CH3 branch on the middle carbon.
Boiling point
  • Boiling point the branched isomer has a lower boiling point.
A branch is named with a prefix such as methyl- before the main chain name.

Section 5 of 11Isomers of C4H8

But-1-ene
  • CH2=CHCH2CH3, with the double bond after carbon 1.
But-2-ene
  • CH3CH=CHCH3, with the double bond after carbon 2.
Methylpropene
  • CH2=C(CH3)CH3, a branched chain.
Numbering
  • Numbering the number in the name shows the position of the C=C bond.

Section 6 of 11Isomers of C3H7OH

Propan-1-ol
  • CH3CH2CH2OH, with –OH on an end carbon.
Propan-2-ol
  • CH3CH(OH)CH3, with –OH on the middle carbon.
Molecular formula
  • Molecular formula both are C3H8O.

Section 7 of 11Finding isomers

1
Write the molecular formula.
2
Draw the straight-chain version.
3
Move the double bond or functional group.
4
Try branching the chain.
5
Check that each structure is genuinely different.
6
Name each isomer, numbering from the end that gives the lowest number.
7Same molecule
  • Same molecule bending a chain on paper does not create a new isomer.
8Numbering examples
  • CH3CH2CH=CH2 is but-1-ene, not but-3-ene, and HOCH2CH2CH3 is propan-1-ol, not propan-3-ol.

Section 8 of 11Common pairs of isomers

Molecular formulaIsomer 1Isomer 2What differs
C4H10ButaneMethylpropaneStraight or branched chain
C4H8But-1-eneBut-2-enePosition of the C=C bond
C4H8But-1-eneMethylpropeneStraight or branched chain
C3H8OPropan-1-olPropan-2-olPosition of the –OH group

Section 9 of 11More from the specification

More from the specification
Characteristic functional groups
  • Characteristic functional groups homologous series share specific atomic groupings such as carbonyl or carboxyl groups dictating chemical behavior.
Hydrogen
  • A gas that gives a squeaky pop with a lighted splint.
Hydroxyl group
  • The –OH functional group of alcohols.
Saturated
  • Saturated a compound with only single carbon–carbon bonds, such as alkanes with general formula CnH2n+2.
Unsaturated
  • Unsaturated a compound with at least one C=C double bond, such as alkenes with general formula CnH2n.
Homologous series
  • Homologous series compounds with the same functional group and general formula, such as alcohols (–OH) and carboxylic acids (–COOH).

Section 10 of 11Key terms

TermMeaning
Structural isomersCompounds with the same molecular formula but different structural formulae.
Molecular formulaThe number of each type of atom in a molecule.
Structural formulaShows how the atoms are arranged.
Branched chainA carbon chain with a side group attached.
Position isomerIsomers that differ in the position of a double bond or functional group.

Section 11 of 11Exam technique

1
Saying isomers have different molecular formulae — they have the same molecular formula.
2
Counting a bent drawing of the same chain as a new isomer.
3
Numbering from the wrong end — the group must get the lowest number.
4
Saying isomers have identical physical properties — they can differ.

Last updated: