Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

11.1Organic Chemistry

Formulae, functional groups and terminology — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 10 parts Lesson 1 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

11.1 Formulae, functional groups and terminology — Notes — IGCSE Chemistry notes: Describe the types of organic formula, homologous series, functional groups, saturated and unsaturated compounds and structural isomers.Organic compound — a compound that contains carbon, convention…

  • Hydrocarbon
  • Molecular formula
  • Empirical formula
  • General formula
  • Displayed formula
  • - Structural formula (Supplement)

Part 1 of 8Organic terminology and bonding

Organic compound
  • Organic compound a compound that contains carbon, conventionally excluding carbonates, carbon dioxide and carbon monoxide.
Hydrocarbon
  • Hydrocarbon a compound that contains only hydrogen and carbon atoms.
Carbon always forms 4 bonds, hydrogen 1 and oxygen 2.

Part 2 of 8Types of formula

Types of formula
Molecular formula
  • Molecular formula the actual number and type of each atom in one molecule,
    • such as C2H6O,
    • which can be deduced by counting atoms in a displayed formula.
Empirical formula
  • Empirical formula the simplest whole-number ratio of atoms, so C2H4 has empirical formula CH2 and ethane's is CH3.
General formula
  • General formula applies to every member of a homologous series, such as CnH2n+2 for alkanes.
Displayed formula
  • Displayed formula shows all the atoms and all the bonds in a molecule.
- Structural formula (Supplement)
  • - Structural formula (Supplement) shows enough detail to make the structure clear with most bonds omitted, such as CH3CH2OH or CH3CH2CH3.
FeatureDisplayed formulaStructural formula
Bonds shownAll atoms and all bondsMost bonds omitted
Always drawnEvery bondDouble and triple bonds
GroupingNone; every atom separateIdentical groups bracketed
Side groupsDrawn outWritten in brackets

Part 3 of 8General formulae and functional groups

SeriesFunctional groupGeneral formulaName endingExample
AlkaneC–C single bonds onlyCnH2n+2-aneMethane CH4
AlkeneC=CCnH2n-eneEthene C2H4
Alcohol–OHCnH2n+1OH-olEthanol C2H5OH
Carboxylic acid–COOHCnH2n+1COOH-oic acidEthanoic acid CH3COOH
Ester–COO–None listed-yl -oateEthyl ethanoate CH3COOC2H5
  • Using a general formula
    • the number of carbon atoms gives n, so an alcohol with 5 carbons has H = 2 × 5 + 1 = 11, giving C5H11OH, and an alkene with 10 carbons is C10H20.

Part 4 of 8Homologous series and functional groups

Homologous series and functional groups
Functional group
  • Functional group an atom or group of atoms that determines the chemical properties of a homologous series.
Homologous series
  • Homologous series a family of similar compounds with the same functional group, the same general formula and similar chemical properties.
Successive members differ by a –CH2– unit, which is an increase in Mr of 14.
Physical properties change gradually,
  • and the boiling point rises with chain length because intermolecular forces get stronger,
  • as in ethanol (78 °C) and propanol (97 °C).

Part 5 of 8Saturated and unsaturated compounds

Saturated
  • Saturated all carbon–carbon bonds are single bonds and every carbon has four single bonds, as in the alkanes.
    • (methane, ethane, propane)
Unsaturated
  • Unsaturated one or more carbon–carbon bonds are double bonds, as in the alkenes, detected with bromine water.
    • (ethene, propene, butene)
Alkenes are much more reactive than alkanes because of the C=C bond.
Formula check
  • C5H12 fits CnH2n+2 so it is an alkane, while C5H10 fits CnH2n so it is an alkene.
An alkane with 6 carbons has 2 × 6 + 2 = 14 hydrogens (C6H14), and an alkene with 6 carbons has 12 hydrogens (C6H12).

Part 6 of 8Structural isomers (Supplement)

Structural isomers
  • Structural isomers compounds with the same molecular formula but different structural formulae.
C4H10
  • CH3CH2CH2CH3 and CH3CH(CH3)CH3.
C4H8
  • CH3CH2CH=CH2 and CH3CH=CHCH3.
Propan-1-ol and propan-2-ol are also structural isomers.

Part 7 of 8Series compared

SeriesGeneral formulaSaturated?Test or use
AlkanesCnH2n+2YesBromine water stays orange
AlkenesCnH2nNoBromine water decolourises
AlcoholsCnH2n+1OHYes (single C–C bonds)Used as a fuel and a solvent (ethanol)
Carboxylic acidsCnH2n+1COOHYes (single C–C bonds)Fizzes with carbonate

Part 8 of 8More from the specification

More from the specification
Definitions of Homologous Series
  • A homologous series is a family of organic compounds sharing the same functional group, the same general formula, similar chemical properties,
    • and exhibiting progressive gradation in physical properties.
Saturated versus Unsaturated Carbon Backbones
  • Saturated hydrocarbons (alkanes) contain exclusively single covalent carbon-carbon bonds (C-C),
  • while unsaturated hydrocarbons (alkenes) contain at least one reactive carbon-carbon double bond (C=C).
Molecular, Structural, and Displayed Formulae
  • The molecular formula specifies the actual number of atoms of each element in a molecule,
  • while the displayed formula explicitly illustrates every atom and chemical bond within the molecular architecture.

Key terms

TermMeaning
Hydrocarbona compound containing hydrogen and carbon only.
Homologous seriesa family of compounds with the same functional group and general formula.
Functional groupthe atom or group of atoms that determines the reactions of a series.
Saturatedhaving only single carbon–carbon bonds.
Unsaturatedhaving at least one carbon–carbon double bond.
Structural isomerscompounds with the same molecular formula but different structural formulae.
Displayed formulaa formula showing all atoms and all bonds.

Avoid these mistakes

1
Saying a homologous series has identical properties — chemical properties are similar and physical properties show a trend
2
Saying saturated means full of hydrogen — it means only single carbon–carbon bonds
3
Confusing molecular and structural formulae — a molecular formula only gives the numbers of atoms
4
Saying isomers have different molecular formulae — they have the same molecular formula
5
Mixing up the general formulae of alkanes and alkenes, or drawing a carbon atom with five bonds — carbon always forms four bonds

Last updated: