Section 12b

B. Classification based on change in structure of a reactant

  1. Decomposition: A substance breaks down to smaller species.
    Example:
    Decomposition of limestone to calcium oxide (quicklime) and carbon dioxide on heating.

    Many reactions may be thought to be between a substrate and a reagent. The classification is based on the change in structure of the substrate.

  2. Addition: The reagent adds to the substrate, often across a double or triple bond.
    Examples:

    1. Bromine adds across the double bond of ethene (ethylene).

    2. Water adds across the carbonyl double bond of methanal (formaldhyde).

    3. xCH2=CH2 CH2-CH2-CH2-CH2-CH2-
      Molecules add to each other to give a molecule of large molar mass called a polymer. The process is called polymerisation.

  3. Elimination: Atoms or groups are lost from neighbouring atoms of the substrate with the formation of a double or triple bond. Effectively a small molecule is eliminated.

    1. H and Br are eliminated from adjacent carbon atoms.

    2. Hydrogen is eliminated from adjacent carbon and oxygen atoms. This loss of two H atoms is more commonly called oxidation. See D, redox reactions.

  4. Substitution: One group on the substrate is replaced by (substituted for) another.
    Examples:

    Br replaced by OH


    H replaced by Cl photochemically (i.e. with light).

  5. Condensation: Two molecules are joined and a small molecule is eliminated. Condensation can be thought of as particular type of substitution.
    Example:

    OH is replaced by OCH2CH3 and water is eliminated.