Formulae, Functional Groups and Terminology

Structural formula

• Definition: An unambiguous representation showing how atoms are bonded together in a molecule.
• Unlike molecular formula (which only shows the number of each atom), the structural formula shows the exact arrangement of atoms.
• Examples:
  • CH₂=CH₂ → Ethene (two carbon atoms double bonded, each carbon bonded to two hydrogens)
  • CH₃CH₂OH → Ethanol (two carbon atoms single bonded, first carbon bonded to three hydrogens, second bonded to two hydrogens and an –OH group)
  • CH₃COOCH₃ → Methyl ethanoate (ester with a methyl group, a carbonyl, and an alkoxy group)

Displayed formula

• Shows all atoms and all bonds in a molecule.
• Every bond is represented by a line:
  • Single bond = one line (–)
  • Double bond = two lines (=)
  • Triple bond = three lines (≡)
• Example: Ethanol:

      H   H
      |   |
  H - C - C - O - H
      |   |
      H   H
  

• Displayed formulas are important for understanding reactivity and functional groups.

General formula of homologous series

• A general formula represents the composition of a series of compounds with similar chemical properties.
• Alkanes: CₙH₂ₙ₊₂
  • Example: n = 3 → C₃H₈ (propane)
• Alkenes: CₙH₂ₙ
  • Example: n = 4 → C₄H₈ (butene)
• Alcohols: CₙH₂ₙ₊₁OH
  • Example: n = 2 → C₂H₅OH (ethanol)
• Carboxylic acids: CₙH₂ₙ₊₁COOH
  • Example: n = 3 → C₃H₇COOH (butanoic acid)

Structural isomers

• Definition: Compounds with the same molecular formula but different structural arrangements.
• Types:
  1. Chain isomerism – different carbon chain arrangement
     • Example: C₄H₁₀:
       • Butane → CH₃CH₂CH₂CH₃
       • 2-Methylpropane → CH₃CH(CH₃)CH₃
  2. Position isomerism – same functional group in different positions
     • Example: C₄H₈:
       • But-1-ene → CH₂=CHCH₂CH₃
       • But-2-ene → CH₃CH=CHCH₃
• Isomerism affects physical and sometimes chemical properties.

Functional groups

• Definition: Specific atoms or groups of atoms in a molecule that determine its chemical properties.
• Examples:
  • Alkenes: C=C double bond
  • Alcohols: –OH
  • Carboxylic acids: –COOH
  • Esters: –COO–

General characteristics of a homologous series

• Same functional group → same type of reactions.
• Same general formula → predictable formula from n.
• Differ from each other by a –CH₂– unit → difference in relative molecular mass.
• Gradual change in physical properties → e.g., boiling points increase with chain length.
• Similar chemical properties → due to identical functional group.

Saturated vs unsaturated compounds

• Saturated compound:
  • All carbon–carbon bonds are single bonds.
  • Examples: alkanes (C–C).
  • Cannot undergo addition reactions with bromine water without UV light.
• Unsaturated compound:
  • Contains at least one carbon–carbon double bond (C=C) or triple bond (C≡C).
  • Examples: alkenes, alkynes.
  • Undergo addition reactions easily, e.g., decolourising bromine water.