Formulae of organic compounds (Edexcel AS Chemistry): Revision Note

Formulae of Organic Compounds

• Organic chemistry is the chemistry of carbon compounds

• Carbon forms a vast number of compounds because it can form strong covalent bonds with itself

• This enables it to form long chains of carbon atoms, and hence an almost infinite variety of carbon compounds are known

• Hydrocarbons are compounds that are made up of carbon and hydrogen atoms ONLY

The diagram shows examples of hydrocarbons

Ethanol is NOT a hydrocarbon as the molecule also contains an oxygen atom and is not solely made up of carbon and hydrogen

• Hydrocarbons can be placed into two categories

  • Saturated molecules containing single carbon-carbon bonds only

  • Unsaturated molecules containing double or triple carbon-carbon bonds

• The general formula is a formula that represents a homologous series of compounds using letters and numbers

  • Eg. the general formula of alkanes is C_nH_2n+2

  • A homologous series is a group of organic compounds that have the same functional group, the same general formula and the same chemical properties

• The structural formula is a formula that shows how the atoms are bonded to each carbon atom in a molecule

• The displayed formula is a 2D representation of an organic molecule showing all its atoms (by their symbols) and their bonds (by single, double or triple bonds)

• The skeletal formula is a simplified displayed formula with all the carbon and hydrogen (C-H) bonds removed

Overview of the Formulae of Organic Compounds Table

• The molecular formula shows the number and type of each atom in a molecule

  • Eg. the molecular formula of ethanoic acid is C₂H₄O₂

• The empirical formula shows the simplest whole number ratio of the elements present in one molecule of the compound

  • Eg. the empirical formula of ethanoic acid is CH₂O