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  have low boiling points; gases under room conditions  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  have low boiling points; gases under room conditions alkenes a ball-and-stick model of ethene, C 2 H 4

alkenes  each member of the series differs from the next by an extra –CH 2 - group of atoms alkene homologous series No. of Carbon Atoms in One Molecule Alkene Member Relative Molecular Mass Structure of One Molecule Boiling Point (°C) 2 ethene (C 3 H 4 ) propene (C 3 H 6 ) butene (C 4 H 8 ) 56-6 H H C H H C H H C H H HC H C H H HC H H C H C H H C

 alkenes burn in air to form CO 2 and H 2 O alkenes C 2 H 4 + 3O 2 2CO 2 + 2H 2 O reactions of alkenes: combustion  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  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

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.  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.

CC XX HH H H

alkenes reactions of alkenes: addition reactions  addition of bromine (bromination) C 2 H 4 + Br 2 C 2 H 4 Br 2 o example: addition reaction of bromine to ethene o bromine molecule adds to the C=C bond of ethene molecule HH C HH C + Br C H H H H C  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  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

CC Br HH H H

alkenes reactions of alkenes: addition reactions  addition of hydrogen (hydrogenation) o alkenes react with hydrogen to produce alkanes in the presence of nickel metal (catalyst) o example: addition reaction of hydrogen to ethene o alkenes react with hydrogen to produce alkanes in the presence of nickel metal (catalyst) o example: addition reaction of hydrogen to ethene o hydrogen molecule adds to the C=C bond of ethene molecule C 2 H 4 + H 2 C 2 H 6 HH C HH C + H H C H H H H C H H  used in producing margarine from vegetable oil

CC HH HH H H

alkenes reactions of alkenes: addition reactions  addition of water (hydration) o alkenes react with steam over a catalyst to produce alcohols o example: addition reaction of water to ethene o alkenes react with steam over a catalyst to produce alcohols o example: addition reaction of water to ethene o water molecule adds to the C=C bond of ethene molecule C 2 H 4 + H 2 O CH 3 CH 2 OH H HH C HH C + OH H C H H H C H  alcohols are commonly used as solvents and to make other chemicals

CC HOH HH H H

alkenes saturated and unsaturated hydrocarbons  alkenes are unsaturated because extra atoms can be added to their molecules in addition reactions CC HH H H CC HH H H H H  alkanes are saturated because it is not possible to add extra atoms to their molecules

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

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 bromine becomes colourless shake liquid alkene bromine solution testing for liquid alkene

poly(ethene) 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 testing for gaseous alkene bromine becomes colourless gaseous alkene bromine solution

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

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

Demonstration: Ripening, bananas give off ethene gas bananas bromine

manufacture of alkenes  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  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 big hydrocarbon molecules hydrocarbon molecules break up on the surface of the catalyst small hydrocarbon molecules CATALYST cracking  Alkene molecules can be made by cracking alkanes

an ________ molecule C 18 H 38 X + C 10 H 20 an ________ molecule big alkane cracking cracking of alkanes C 20 H 42 C 12 H 26 + C 8 H 16 an alkene molecule an alkane molecule big alkane

 Uses: C 16 H 34 H 2 + C 6 H 12 + C 10 H 20 cracking cracking of alkanes 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

 Uses: o cracking of big petroleum molecules into smaller hydrocarbon molecules to produce fuel for motor vehicles; o cracking of big petroleum molecules into smaller hydrocarbon molecules to produce fuel for motor vehicles; cracking cracking of alkanes 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

polymers 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). 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). polymerisation H H CC H H H H CC H H H H CC H H C H H C H H C H H C H H C H H C H H C H H C H H C H H C H H C H H C H H

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  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 carbon atom hydrogen atom

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

polymers polyethene (polythene)  deriving the polymerisation equation: o join the molecules together H H H H C C C C H H H H H H H H C C C C H H H H H H H H C C C C H H H H H H H H C C C C H H H H H H C C C C H H H H H H H H C C C C H H H H H H  formula of polyethene can be simply written as: CCHHCCHHHHHH nn

polymers examples of addition polymer Chemical Name of Polymer MonomerPolymerUses Poly(ethene) or polythene Plastic film for wrapping fresh vegetables and meat; plastic bottles; plastic bags, buckets and pipes Poly(chloroethene) or PVC For water pipes, waterproof plastic sheet, insulation for electric cables and wires Poly(phenylethene) or polystyrene For packaging and containers for electrical goods, disposable cups and food containers used in ‘fast food’ restaurants and hawker centres H H C ClCl H C H H C C6H5C6H5 H C H H C ClCl H C n H H C C6H5C6H5 H C n H H C H H C H H C H H C n

unsaturated foods  polyunsaturated means that organic molecules contain C=C bonds  example: vegetable oils are polyunsaturated oils  manufacture of margarine by addition reaction; vegetable oil reacts with hydrogen in presence of nickel catalyst; H 2 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  polyunsaturated means that organic molecules contain C=C bonds  example: vegetable oils are polyunsaturated oils  manufacture of margarine by addition reaction; vegetable oil reacts with hydrogen in presence of nickel catalyst; H 2 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

Demo - Testing for unsaturation bromine

After five droplets of each to bromine solution, shaken and stirred… buttermargarineghee 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