3.1.1 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

Worked Example

Answer:

Drawing displayed formulae of molecules

Draw the displayed formula of the following molecules:

Answer

• 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

Worked Example

Answer