Addition Reactions
Section: 11. Organic Chemistry | Syllabus: Cambridge AS Level Physics 9702
Addition Reactions Addition reactions are chemical reactions where two or more molecules combine to form a single larger molecule with no other products. For alkenes, these are the most important reactions.
Addition Reaction A reaction where two or more molecules combine to form a single product with no by-products. For alkenes, the C=C double bond breaks and atoms add across it. General Pattern C=C + X-Y → C-C with X and Y added The double bond (C=C) breaks and becomes a single bond (C-C), with the two parts of the reagent adding to each carbon atom that was part of the double bond.
Why Alkenes Undergo Addition Reactions The structure of the C=C double bond makes alkenes highly reactive toward addition reactions. The C=C double bond has a region of high electron density The π (pi) bond is weaker than the σ (sigma) bond The π bond breaks relatively easily, allowing atoms to add across the double bond The product is more stable (saturated) than the reactant (unsaturated) Key Concept Addition reactions convert unsaturated compounds (with C=C) into saturated compounds (with only C-C single bonds).
Types of Addition Reactions 1. Hydrogenation (Addition of Hydrogen) Alkene reacts with hydrogen gas to form an alkane. Conditions Nickel (Ni) or platinum (Pt) catalyst Temperature: 150-200°C General Equation: Alkene + H₂ → Alkane Example - Ethene to Ethane: CH₂=CH₂ + H₂ → CH₃-CH₃ (ethene + hydrogen → ethane) Example - Propene to Propane: CH₃-CH=CH₂ + H₂ → CH₃-CH₂-CH₃ Uses: Converting vegetable oils (unsaturated) to margarine/solid fats (saturated) Hardening of oils in food industry Making alkanes from alkenes for fuel 2.
Halogenation (Addition of Halogens) Alkene reacts with a halogen (Cl₂, Br₂, I₂) to form a dihalogenoalkane. Conditions Room temperature No catalyst needed (unlike alkanes which need UV light) Reaction occurs quickly and spontaneously General Equation: Alkene + X₂ → Dihalogenoalkane (where X = halogen: Cl, Br, or I) Addition of Bromine (Bromine Water Test): CH₂=CH₂ + Br₂ → CH₂Br-CH₂Br (ethene + bromine → 1,2-dibromoethane) Orange/brown bromine water becomes colorless This is the test for unsaturation (presence of C=C) Addition of Chlorine: CH₂=CH₂ + Cl₂ → CH₂Cl-CH₂Cl (ethene + chlorine → 1,2-dichloroethane) Uses: Testing for C=C double bonds Making halogenated compounds used as solvents 3.
Hydration (Addition of Water) Alkene reacts with steam (water vapor) to form an alcohol. Conditions Steam (H₂O gas) Concentrated phosphoric acid (H₃PO₄) catalyst Temperature: 300°C Pressure: 60-70 atmospheres General Equation: Alkene + H₂O → Alcohol Example - Ethene to Ethanol: CH₂=CH₂ + H₂O → CH₃-CH₂OH (ethene + steam → ethanol) Example - Propene to Propanol: CH₃-CH=CH₂ + H₂O → CH₃-CH₂-CH₂OH (propene + steam → propan-1-ol) Uses: Industrial production of ethanol for use as fuel, solvent, and in alcoholic drinks Making other alcohols for industrial use 4.
Addition of Hydrogen Halides Alkene reacts with hydrogen halides (HCl, HBr, HI) to form halogenoalkanes. Conditions Room temperature Gaseous HX or dissolved in solvent General Equation: Alkene + HX → Halogenoalkane (where X = Cl, Br, or I) Example - Ethene with Hydrogen Bromide: CH₂=CH₂ + HBr → CH₃-CH₂Br (ethene + hydrogen bromide → bromoethane) Example - Ethene with Hydrogen Chloride: CH₂=CH₂ + HCl → CH₃-CH₂Cl (ethene + hydrogen chloride → chloroethane) Markovnikov's Rule (For Unsymmetrical Alkenes) When adding HX or H₂O to an unsymmetrical alkene (like propene), two products are possible.
Markovnikov's Rule predicts which product is the major one: Markovnikov's Rule The hydrogen atom adds to the carbon of the double bond that already has MORE hydrogen atoms attached to it. Example - Propene + HBr: Propene: CH₃-CH=CH₂ Left carbon of C=C has 1 H (from CH₃-CH=) Right carbon of C=C has 2 H's (from =CH₂) H adds to the carbon with more H's (the right carbon) Br adds to the other carbon (the left carbon) Major Product: CH₃-CH=CH₂ + HBr → CH₃-CHBr-CH₃ (2-bromopropane - MAJOR) Minor Product: CH₃-CH=CH₂ + HBr → CH₃-CH₂-CH₂Br (1-bromopropane - minor) Memory Aid "The rich get richer" - the carbon with more hydrogens gets another hydrogen.
Addition Polymerization Many small alkene molecules (monomers) can join together through addition reactions to form very large molecules called polymers. This is covered in detail in the Polymers topic.
Example: n(CH₂=CH₂) → -(CH₂-CH₂)ₙ- (many ethene molecules → poly(ethene)/polythene) Comparing Addition vs Substitution Feature Addition Reactions Substitution Reactions What happens Atoms/groups ADD to molecule One atom/group REPLACES another Number of products One main product Two products (main + small molecule) Which compounds Unsaturated (alkenes) Saturated (alkanes, benzene) Saturation change Unsaturated → Saturated Saturation stays the same Bond change Double bond → Single bond Single bonds only Example C₂H₄ + Br₂ → C₂H₄Br₂ CH₄ + Cl₂ → CH₃Cl + HCl (needs UV) Summary of Addition Reactions Reaction Type Reagent Conditions Product Examp…
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