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Alkenes - Introduction

Alkenes - Introduction. Alkene - a hydrocarbon that contains a carbon-carbon double bond . (hence unsaturated ). Occur abundantly in nature. Ethene – plant hormone that induces ripening in fruit. Alkenes. Limonene. citral. Alkenes. Natural rubber. Beta-carotene. Vitamin A.

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Alkenes - Introduction

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  1. Alkenes - Introduction • Alkene - a hydrocarbon that contains a carbon-carbon double bond. (hence unsaturated). • Occur abundantly in nature. Ethene – plant hormone that induces ripening in fruit.

  2. Alkenes Limonene citral

  3. Alkenes Natural rubber Beta-carotene Vitamin A

  4. General Formula • Alkenes are a homologous series of unsaturated hydrocarbon with general formula CnH2n. • The first 3 members are: • Ethene, with 2 carbon atoms • Molecular formula: C2H4 • Full structural formula: see white board • Condensed structural formula: • CH2=CH2 • Gas, b.p = -104C

  5. Alkenes The 2nd member is: 2) Propene, with 3 carbon atoms Molecular formula: C3H6 Condensed structural formula: see white board Gas; Bp = -48 C

  6. Alkenes The 3rd member is: 2) butene, with 4 carbon atoms Molecular formula: C4H8 Condensed structural formula: see white board Gas; Bp = -6C

  7. Alkenes As the no. of carbon atoms in a molecule increases, the boiling point increases. Alkenes are unsaturated; they contain carbon-carbon double bond. Functional group of alkenes is thus the carbon-carbon double bond. Trigonal planar (3 bond pairs)

  8. Chemical Reactions, Pg 2 Alkenes undergo complete combustion to give water and carbon dioxide, just like alkanes. E.g combustion of butene. C4H8 + 6O2 4CO2 + 4H2O

  9. Chemical Reactions, Pg 2 Alkenes are unsaturated, they contain a carbon-carbon double bond which allows other atoms to add across it. Unsaturated compounds such as alkenes undergo addition reactions. An addition reaction is a reaction in which an unsaturated organic compound combines with another element or compound to form a single new compound.

  10. Chemical Reactions, Pg 3 General equation is: • Note: • double bond has become single and the product becomes saturated. • A single new compound is formed, characteristic of addition reaction.

  11. Chemical Reactions, Pg 3 • Addition of hydrogen – hydrogenation • Alkenes + H2 alkanes • Or C2H4 + H2 C2H6 • Conditions: • Temp of 200°C • Nickel catalyst

  12. Vegetable oil polyunsaturated margarine Industrial Application of Hydrogenation, Pg 3 Addition of hydrogen is used to change vegetable oil into margarine over a nickel catalyst at 180°C at 5 atm. Higher Mr, because it has more H, higher mp, hence solid Lower Mr, lower mp, hence liquid

  13. b) Addition of steam-hydration • Alkene + steam  alcohol • catalyst of phosphoric (V) acid, • H3PO4 • temp of 300°C and pressure of 60 atm • E.g. C2H4 + H2O  C2H5OH

  14. **Addition of Br2-Bromination 1,2-dibromoethane Or C2H4 + Br2 C2H4Br2 The reaction can distinguish alkenes from alkanes. When an alkene is added to aqueous bromine, the reddish-brown bromine turns colourlessimmediately. In the absence of UV, alkanes will not react with Br2and the reddish-brown bromine remains unchanged.

  15. polymerisation polymer monomers d) Addition polymerisation General equation for polymerisaiton:

  16. d) Addition polymerisation At high temp and pressure, alkene monomers undergo addition polymerisation to form a polymer. Addition polymerisation is a process whereby monomers link together to form a giant molecule without any gain or loss of material.

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