Introduction to Organic Chemistry

What organic chemistry is

• Organic chemistry is the chemistry of compounds containing the element carbon
• By long-standing convention, a handful of carbon-containing substances are treated as inorganic and not studied as part of organic chemistry: carbon monoxide (CO), carbon dioxide (CO₂) and the metal carbonates (e.g. CaCO₃)
• Carbon is uniquely able to form four covalent bonds and to chain with itself, which is why organic compounds span tens of millions of known molecules — from methane to DNA

Hydrocarbons

• A hydrocarbon is an organic compound that contains only hydrogen and carbon atoms — no other element
• Methane (CH₄), ethane (C₂H₆), petrol (a mixture of C₅–C₁₀ alkanes) and natural rubber are all hydrocarbons
• An alcohol such as ethanol (C₂H₅OH) is an organic compound but not a hydrocarbon because it contains an oxygen atom

Five ways to write the same molecule

• The same organic molecule can be drawn in several different forms; each form throws away or keeps different amounts of detail. Using ethanol as the running example:

Homologous series

• A homologous series is a family of organic compounds that all share the same functional group and a common general formula
• Members of a homologous series:
  • Share the same general formula
  • Carry the same functional group, so they react in the same chemical way
  • Show a steady gradation in physical properties (boiling point, density, viscosity) as the chain length grows
  • Differ from one neighbour to the next by a single CH₂ unit

Functional groups

• A functional group is a particular arrangement of atoms within a molecule that gives it its characteristic chemistry
• Two molecules in the same homologous series react in the same way precisely because they share the same functional group

Isomers

• Isomers are different compounds that share the same molecular formula but whose atoms are joined together differently — they therefore have different structural (and often different displayed) formulae
• Isomers can have similar physical and chemical properties if they are in the same homologous series, or quite different properties if their atoms are arranged into different functional groups
• A clean example: the molecular formula C₃H₆ describes two unrelated compounds
  • Propene: a straight-chain alkene with a C=C double bond — CH₂=CH−CH₃
  • Cyclopropane: a closed three-carbon ring of single bonds — no C=C double bond, no functional group
• Propene reacts with bromine water and decolourises it (because of its C=C); cyclopropane does not. Same molecular formula, very different chemistry