Cambridge O Level Chemistry · Syllabus 5070 · Organic Chemistry
Addition Reaction
What is Addition Reaction?
A reaction in which a carbon-carbon double bond opens and atoms from another molecule attach to the two carbons that were sharing it, so that only one product is formed and nothing is displaced.
This definition is part of the Organic Chemistry chapter in Cambridge O Level Chemistry.
Common mistakes with Addition Reaction
- “Ethene plus bromine gives 1,2-dibromoethane and hydrogen bromide.” why it fails That second product is imported from substitution. In an addition reaction there is nothing left over to form it. correct An addition reaction forms only one product. Every atom of both reactants ends up in that single molecule. Taught in 11.5B.
- 9. “Ethene + Br2 → 1,2-dibromoethane + HBr.” defect A second product invented for an addition reaction. why Substitution thinking applied to addition. Nothing was displaced from ethene, so there is nothing to build a second molecule from — and the equation no longer balances. repair Delete the second product. Check by adding the reactant formulae: C2H4 + Br2 = C2H4Br2 exactly. transfer Say how many products form when ethene reacts with steam, and why.
Questions students ask about Addition Reaction
What is the difference between an addition reaction and a substitution reaction?
An addition reaction opens a carbon-carbon double bond so that atoms from another molecule join the two carbons, forming exactly one product with nothing displaced — as when ethene reacts with bromine. A substitution reaction replaces one atom or group in a molecule with a different one, and the displaced atom leaves as part of a second product — as when methane reacts with chlorine in ultraviolet light to give chloromethane and hydrogen chloride.