Reactions of Alkenes

Electrophilic Addition

  • π bond in the C=C bond breaks and atoms or groups join the carbon atom, this happens because the C=C bond is rich in electrons and is target for electrophiles

  • Examples of electrophiles - Polar molecules and halogens

Addition of Halogens

  • Halogens react with alkenes in an electrophilic addition reaction to form dihaloalkanes

  • E.g C2H4 + Br2 → C2H4Br2

  1. High electron density of C=C bond repels electrons in the bromine molecule, polarising the bond between bromine atoms

  2. Heterolytic fission of the bromine atom, where one atom takes the pair of electrons, the newly created positive bromide ion bonds to one of the C atom

  3. Positive charged carbocation forms, the negatively charged bromide ion joins the positive C ion

  4. The final product, 1,2-dibromoethane has a bromine atom bonded to each of the original alkene carbon atom

Addition of hydration halides

  • Hydrogen halides like HCl and HBr add across the C=C to form a haloalkane

  • In symmetrical alkenes the double bonded carbons are identical leading to a single product

  • In unsymmetrical alkene with two different alkyl groups, hydrogen halides create two isomers, e.g propene forms a mixture of 1-bromopropane and 2-bromopropane, the ratio of the products depend on the stability of the carbocation intermediate

  • e.g ethene + hydrogen bromide

    • C2H4 + HBr → C2H5Br

Minor and major products

  • The major product in a hydrogen halide addition to unsymmetrical alkenes can be predicted by carbocation stabilty

  • The stability of the carbocation increases with the number of alkyl groups

    • Primary Carbocations - One alkyl group attached to the positive carbon, least stable

    • Secondary Carbocations - Two alkyl groups attached to positive carbon (more stable)

    • Tertiary Carbocations - Three alkyl groups and is the most stable

  • Most stable carbocation forms the major product. e.g. propene + HBr forms the major product 2-bromopropane

Addition of concentrated sulfuric acid

  • Cold concentrated H2SO4 adds across the C=C forming an alkyl hydrogen sulfate

  • E.g ethene + H2SO4 → ethyl hydrogen sulfate

  • Reaction with unsymmetrical alkenes lead to a mix of products

Hydration to form alcohols

  • Under the influence of a phosphoric acid catalyst, alkenes react with steam at 300°C to form an alcohol

  • E.g ethene + H2O → ethanol