alkenes a group of hydrocarbons with the general

alkenes § a group of hydrocarbons with the general formula, Cn. H 2 n 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, C 2 H 4

alkenes alkene homologous series § each member of the series differs from the next by an extra –CH 2 - group of atoms No. of Carbon Atoms in One Molecule 2 3 4 Alkene Member ethene (C 3 H 4) propene (C 3 H 6) butene (C 4 H 8) Relative Molecular Mass Structure of One Molecule H 28 H C -104 C H H H 42 56 H H Boiling Point (°C) H C C C H H -48 H H H C C H H -6 C C H

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

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.

H H H C C X X H

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

H H H C C Br Br 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 hydrogen molecule adds to the C=C bond of ethene molecule § used in producing margarine from vegetable oil C 2 H 4 + H 2 H H C C H H + H C 2 H 6 H H C H H

H H H C C H 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 C 2 H 4 + H 2 O H H C C H H + H CH 3 CH 2 OH OH H C OH H H C H H water molecule adds to the C=C bond of ethene molecule § alcohols are commonly used as solvents and to make other chemicals o

H H H C C H OH 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 H H H C C H H H

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) Extra atoms not added to their molecules in addition reactions Extra atoms added to their molecules in addition reactions have alkane-type formula of Cn. H 2 n+2 have alkene-type formula of Cn. H 2 n Reaction with aqueous bromine : : do not decolourise aqueous decolourises aqueous bromine

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 solution shake testing for liquid alkene bromine becomes colourles s

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 gaseous alkene bromine solution poly(ethene) bromine becomes colourles s testing for gaseous alkene

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

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

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

cracking manufacture of alkenes §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 big hydrocarbon molecules break up on the surface of the catalyst small hydrocarbon molecules

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

cracking of alkanes cracking § Uses: Hydrogen can be one of the products in the cracking of alkanes C 16 H 34 H 2 + C 6 H 12 + C 10 H 20 Hydrogen is used as fuel for big rockets and to make ammonia for fertilisers

cracking of alkanes cracking § Uses: o 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

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). H HH H H CC C C CC H H HH HH CC C CC HH HH H H H CC C C CC H H 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 hydrogen atom carbon atom model of polyethene molecule

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

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

polymers examples of addition polymer Chemical Name of Polymer Poly(ethene) or polythene Poly(chloroethene) or PVC Poly(phenylethene) or polystyrene Monomer Polymer H H C C H H n H Cl C C H H n H C 6 H 5 C C H H Uses Plastic film for wrapping fresh vegetables and meat; plastic bottles; plastic bags, buckets and pipes For water pipes, waterproof plastic sheet, insulation for electric cables and wires n For packaging and containers for electrical goods, disposable cups and food containers used in ‘fast food’ restaurants and hawker centres

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 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… 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

end of chapter 16 - alkenes