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alkenes

alkenes . alkenes. a group of hydrocarbons with the general formula, C n H 2n where n is the number of carbon atoms in one molecule contain the C=C functional group (unsaturated hydrocarbons) names of alkenes end with -ene are molecular compounds

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alkenes

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  1. alkenes alkenes • a group of hydrocarbons with the general formula, CnH2n where n is the number of carbon atoms in one molecule • contain the C=C functional group (unsaturated hydrocarbons) • names of alkenes end with -ene • are molecular compounds • have low boiling points; gases under room conditions a ball-and-stick model of ethene, C2H4

  2. H H C C H H H H H C C C H H H H H H C H C C C H H H H alkenes alkene homologous series • each member of the series differs from the next by an extra –CH2- group of atoms

  3. alkenes reactions of alkenes: combustion • alkenes burn in air to form CO2 and H2O • incomplete combustion results in the formation of carbon monoxide, CO and soot, C • alkenes tend to produce more soot than alkanes when they burn in air • example: combustion of ethene C2H4 + 3O2 2CO2 + 2H2O

  4. alkenes reactions of alkenes: addition reactions • Alkenes undergo addition reaction • This means that a molecule is added to the alkene to make one product • This is possible because of the carbon-carbon double bond (C=C ) in an alkene.

  5. H H H H C C X X

  6. H H H H C Br C Br + Br H C C Br H H H alkenes reactions of alkenes: addition reactions • addition of bromine (bromination) • bromine molecule adds to the C=C bond of ethene molecule • example: addition reaction of bromine to ethene C2H4 + Br2 C2H4Br2 • reaction with aqueous bromine is used as a test for alkenes and other unsaturated compounds • alkene is shaken with or bubbled through a solution of bromine in water; alkene quickly decolourises the reddish-brown bromine solution

  7. H H H H C C Br Br

  8. H H H H C H C H + H H C C H H H H alkenes reactions of alkenes: addition reactions • addition of hydrogen (hydrogenation) • alkenes react with hydrogen to produce alkanes in the presence of nickel metal (catalyst) • example: addition reaction of hydrogen to ethene • hydrogen molecule adds to the C=C bond of ethene molecule • used in producing margarine from vegetable oil C2H4 + H2 C2H6

  9. H H H H C C H H

  10. H H H C OH C OH + C H H C H H H H alkenes reactions of alkenes: addition reactions • addition of water (hydration) • alkenes react with steam over a catalyst to produce alcohols • example: addition reaction of water to ethene C2H4 + H2O CH3CH2OH H • water molecule adds to the C=C bond of ethene molecule • alcohols are commonly used as solvents and to make other chemicals

  11. H H H H C C H OH

  12. H H H H C C H H H H C C H H alkenes saturated and unsaturated hydrocarbons • alkanes are saturated because it is not possible to add extra atoms to their molecules • alkenes are unsaturated because extra atoms can be added to their molecules in addition reactions

  13. alkenes saturated and unsaturated hydrocarbons • differences between saturated and unsaturated hydrocarbons: Contains carbon-carbon single bonds (C-C) Contains carbon-carbon double bonds (C = C) Extra atoms added to their molecules in addition reactions Extra atoms not added to their molecules in addition reactions have alkane-type formula of CnH2n+2 have alkene-type formula of CnH2n Reaction with aqueous bromine : do not decolourise aqueous bromine Reaction with aqueous bromine : decolourises aqueous bromine

  14. alkenes testing for unsaturated compounds like alkenes • for liquids: add a solution of bromine in water to an alkene and shake them; reddish-brown colour of bromine disappears in the presence of alkene liquid alkene bromine becomes colourless bromine solution shake testing for liquid alkene

  15. alkenes testing for unsaturated compounds like alkenes • for gas: bubble the gas through a solution of bromine in water; colour of bromine disappears in the presence of alkene gas gaseous alkene poly(ethene) bromine becomes colourless bromine solution testing for gaseous alkene

  16. Demo - to distinguish between an alkane and an alkene (decolourisation of bromine!). hexene hexane hexane hexene bromine bromine

  17. Qn: What will happen if I bubble the gas from a bottle filled with bananas to the bromine water?

  18. Demo – while ripening, bananas give off ethene gas! bananas bananas bromine bromine

  19. manufacture of alkenes cracking • Alkene molecules can be made by cracking alkanes • a process to break up big alkane molecules into smaller molecules; done by passing big alkane molecules over a solid catalyst at a high temperature • example: cracking of big petroleum (hydrocarbon) molecules CATALYST hydrocarbon molecules break up on the surface of the catalyst big hydrocarbon molecules small hydrocarbon molecules

  20. C20H42 C12H26 + C8H16 an alkane molecule an alkene molecule big alkane C18H38 X + C10H20 an ________ molecule big alkane cracking of alkanes cracking an ________molecule

  21. C16H34 H2 + C6H12 + C10H20 cracking of alkanes cracking • Uses: Hydrogen can be one of the products in the cracking of alkanes Hydrogen is used as fuel for big rockets and to make ammonia for fertilisers

  22. cracking of alkanes cracking • Uses: • cracking of big petroleum molecules into smaller • hydrocarbon molecules to produce fuel for motor • vehicles; • This is important as the amount of petrol produced from fractional distillation of petroleum is insufficient. So the extra petrol needed is produced by cracking

  23. C C C C C C C C C C C C C C C C C C H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H polymers polymerisation Macromolecules are very large numbers of small molecules joined together. Polymerisation is the chemical reaction when large numbers of similar small molecules (monomers) are joined together to form a big molecule (polymer).

  24. hydrogen atom carbon atom polymers polyethene (polythene) • one of the simplest addition polymer • used to make ‘cling film’ for wrapping fresh vegetables and meat, plastic bottles, plastic bags, buckets and pipes • can be made up of 500 to 20,000 ethene molecules model of polyethene molecule

  25. H H H H H H H H H H C C C C C C C C C C H H H H H H H H H H H H H H H H H H H H C C C C C C C C C C H H H H H H H H H H polymers polyethene (polythene) • deriving the polymerisation equation: • write out the structural formulae of several ethene molecules in a row • open the C=C double bonds

  26. H H H H H H H H H H H H C C C C C C C C C C C C H H H H H H H H H H H H H H C C H H n polymers polyethene (polythene) • deriving the polymerisation equation: • join the molecules together • formula of polyethene can be simply written as:

  27. H H H H H H Cl C6H5 H Cl C C C C C C C C C C H H H H H H H H H H n n H C6H5 C C H H n polymers examples of addition polymer

  28. unsaturated foods • polyunsaturatedmeans that organic molecules contain C=C bonds • example: vegetable oils are polyunsaturated oils • manufacture of margarine by addition reaction; vegetableoil reacts with hydrogen in presence of nickel catalyst; H2 molecules add to the C=C bonds of vegetable oil • after addition reaction, vegetable oil has more hydrogen atoms and hence, greater relative molecular mass and higher melting point; becomes solid(solidify) at room conditions margarine (a healthy product) made from vegetable oil by addition reaction

  29. Demo - Testing for unsaturation bromine

  30. After five droplets of each to bromine solution, shaken and stirred… butter margarine ghee olive oil Butter - did not decolorized Br soln (very minimal) - mostly saturated Margarine - decolorized Br soln (largely) - mostly polyunsaturated Ghee - did not decolorized Br soln(least) - highly saturated Olive oil -decolorized Br soln(completely) - highly monounsaturated

  31. end of chapter 16 - alkenes

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