Hydrocarbons: Alkenes Summary

Hydrocarbons: Alkenes

  • Alkenes are hydrocarbons containing one or more C=C bonds.

  • They can form long-chain molecules such as polythene and polystyrene.

  • Polyunsaturated fats and oils contain multiple C=C bonds, essential in diet.

Structure and Properties

  • Alkenes are a homologous series with the general formula CnH{2n}.

  • Examples: Ethene (C2H4), Propene (C3H6), But-1-ene, But-2-ene (C4H8).

  • Alkenes possess no permanent dipole; they exhibit van der Waals forces.

  • Lighter alkenes are gases; heavier alkenes are solids due to increased van der Waals forces.

Structural Isomers

  • Isomers like but-1-ene and but-2-ene differ in the position of the C=C bond.

  • Naming follows IUPAC rules; requires numbering based on the lowest C=C bond position.

  • Example naming: Pent-1-ene and Pent-2-ene based on carbon chain.

C=C Functional Group

  • C=C contains a sigma (σ) bond and a pi (π) bond formed from p-orbital overlap.

  • π-bond is weaker than σ-bond, making alkenes more reactive than alkanes.

  • Geometric isomers exist due to restricted rotation around C=C bonds.

Geometric Isomers

  • Designated as cis-trans or E-Z based on arrangements of groups around C=C bond.

  • cis: Similar groups on the same side; trans: on opposite sides.

  • E-Z notation used when complex groups are present.

Reactions of Alkenes

  • Alkenes participate in addition reactions with hydrogen, halogens, and hydrogen halides.

  • Hydrogenation involves adding H2 across the C=C bond creates alkanes.

  • For example: Propene + H2 yields propane.

  • Halogenation forms halogenated alkanes, testing C=C presence by decolorizing bromine water.

  • Electrophilic addition using HBr follows Markovnikov's Rule, favoring stable carbocation formation.

Polymerization

  • Polyethene forms from the polymerization of ethene (addition reaction of monomers).

  • Similar process applies for other alkenes to form polypropene, polyvinylchloride, etc.

  • Addition polymerization is characterized by forming larger molecules from alkenes sharing double bonds.