hapter 10 ydrocarbons: alkenes Worksheet 1: Electrophilic addition Worksheet 2: Additional information on polymers Revision sheet: Alkenes Practical: Using bromine to detect the presence of a double bond in an alkene End-of-chapter test Marking scheme: Worksheet 1 Marking scheme: End-of-chapter test
Worksheet 1 Electrophilic addition [5.2.4e,f] 1 Ethene and bromine react together to give 1,2-dibromoethane. a Write a balanced chemical equation for this reaction. b Explain the mechanism for this reaction using curly arrows. You should include: the instantaneous dipole on the bromine molecule at the start the attack on the bromine molecule by an electron pair (curly arrow) from the π bond the breaking of the bromine bromine bond (curly arrow) the positive ion formed as an intermediate the bond forming (curly arrow) from the bromide ion to this positive ion. [6] 2 Ethene and hydrogen bromide react together to give bromoethane. a Write a balanced chemical equation for this reaction. b Explain the mechanism for this reaction using curly arrows. You should include: the dipole on the hydrogen bromide molecule at the start the attack on the hydrogen bromide molecule by an electron pair (curly arrow) from the π bond the breaking of the hydrogen bromine bond (curly arrow) the positive ion formed as an intermediate the bond forming (curly arrow) from the bromide ion to this positive ion. [6] 3 Propene and steam react together to give two isomeric products. a Write a balanced chemical equation for this reaction. b Name the catalyst that is used to speed up this reaction. c Give names and full structural formulae of the two isomeric products. d Explain the mechanism for this reaction using curly arrows. You should include: the dipole on the water molecule at the start the attack on the water molecule by an electron pair (curly arrow) from the π bond the breaking of the hydrogen oxygen bond (curly arrow) the positive ion formed as an intermediate the bond forming (curly arrow) from the hydroxide ion to this positive ion. [11] 4 Use one of the reactions above as a source of examples to explain what is meant by: a Electrophile b Addition c eterolytic fission [6] Total: / 29 Score: % 76 ambridge University Press 2005 10 ydrocarbons: alkenes
Worksheet 2 Additional information on polymers [5.2.4g m] 1 Section 5.2.4g,h,i, Polymers are often discussed along with alkenes because addition polymers are made using alkenes as the monomers. Learn the diagrams for ethene / poly(ethene), propene / poly(propene), chloroethene / poly(chloroethene), tetrafluoroethene / poly(tetrafluoroethene) and styrene / polystyrene, knowing how to show the repeat unit / monomer residue in the polymer diagram. For each one you should draw three monomer molecules in your first diagram, then your second diagram should show the length of the polymer chain formed from these three monomer molecules. The repeat unit should be marked on the second diagram. Now repeat this exercise for the following monomers. You will be less familiar with these but may get this sort of thing in your exam. heck your answers with your teacher. a But-1-ene b But-2-ene c 1,2-Dichloroethene d 1,1-Dichloroethene 2 Section 5.2.4k,l Disposing of such polymers is difficult, mainly because: i they are non-biodegradable, i.e. they don t rot naturally due to bacterial/fungal activity this is because they are new substances and evolution is slow. ii when they are burnt the combustion products may be toxic, e.g. polystyrene can release benzene when burnt, poly(chloroethene) releases l when burnt, polyurethane (foam seat cushions) releases toxic fumes. These problems are being dealt with in three main ways: Recycling, through melting and remoulding. This tends to produce poor quality products, often with dark colour added to mask poor appearance, e.g. flower pots, garden furniture, dustbins, food trays. ollection of waste plastic is tricky it doesn t smash when collected, so plastic banks end up full of air. ombustion. Using furnaces that have enhanced air input and high operating temperatures can produce complete combustion to O 2 and water with the release of useful energy. Such furnaces are expensive. It is still necessary to install some sort of scrubber to remove gases like l. Use as feedstock for cracking. The backbone of an addition polymer is basically a very long chain alkane. This can be cracked to give useful alkenes and shorter alkanes. Polystyrene would yield benzene. We assume that the reasons why this potentially very useful process (less pollution, increased lifetime for crude oil supplies) seems little used is that it is uneconomical and difficult to do. Perhaps other substances added to the polymers, e.g. colours, poison the catalysts that are necessary. 3 Section 5.2.4m Outline the role of chemists in minimising damage to the environment by, for example, the removal of toxic waste products such as l during combustion of halogenated plastics (such as pvc). Firstly, be aware that pvc is the old name for poly(chloroethene). Secondly, it is difficult to see what will be in the exam beyond what it says. They want us to realise that smelly old physicists or engineers would just lob a match onto a pile of plastics, while clever chemists, knowing that l is acidic, pass the waste gases through an alkaline slurry. (A slurry is a solid/liquid mixture, like calcium hydroxide and water.) 10 ydrocarbons: alkenes ambridge University Press 2005 77
4 Occurrences in past papers of material from Sections 5.2.4k,l,m [OR Module 2812: hains and Rings] Jan 2001 has one question on this: State two difficulties in the disposal of polypropene. [2] Jan 2002 has one question on this about non-biodegradability and disposal by combustion of polypropene. [3] Jan 2003 has two questions: Describe difficulties in the disposal of polymers in general and pvc specifically. [3] (The three marks were for non-biodegradable, toxic fumes when burnt, l when burnt.) Outline the role of chemists in minimising environmental damage during the disposal process. [1] (There are various ways of scoring the mark: recycling, using energy from combustion for domestic heating, use of alkaline spray, manufacture of biodegradable alternatives.) June 2001, June 2002, June 2003, Jan 2004 and June 2004 had no such questions. 78 ambridge University Press 2005 10 ydrocarbons: alkenes
Revision sheet Alkenes [5.2.4d] Part ii gives the answers to part i. Part i gives the answers to part ii. ut or fold the sheet in half and do each half separately. i Write the reagents and reaction conditions next to the arrows. Br Br Br O n [5] ii Draw the products at the end of the arrows. Br 2 (l ) Br steam 3 PO 4 2, Ni polymerisation 10 ydrocarbons: alkenes ambridge University Press 2005 79 [5]
Practical Using bromine to detect the presence of a double bond in an alkene [5.2.4d(ii)] Safety Always refer to the departmental risk assessment before carrying out any practical work. See the Part 2 Notes on practical work, found in the Additional resources section, for additional guidance and azcard references. Wear eye protection at all times. Apparatus per student 3 clean test-tubes with stoppers or bungs to fit Test-tube rack 4 clean teat pipettes Bromine water Ethanol yclohexane yclohexene Eye protection Procedure 1 Place 1cm 3 of ethanol into one test-tube, 1 cm 3 of cyclohexane into a second test-tube, and 1 cm 3 of cyclohexene into a third test-tube. 2 Add 1cm 3 of bromine water to each test-tube, add a stopper and shake. 3 Note down your observations and write up the experiment. Write a balanced chemical equation for any reactions that occur. 80 ambridge University Press 2005 10 ydrocarbons: alkenes
End-of-chapter test 1 Propene, cis-pent-2-ene and trans-pent-2-ene are alkenes. a For each one give i its molecular formula, ii its structural formula, iii its displayed formula, and iv its skeletal formula. [12] b Give the general formula that is used to represent alkenes. [1] c d e Two of these alkenes are isomers of each other. Identify which two and identify the type of isomerism they show. [2] Why is it not possible for these two isomers to simply interconvert at normal temperatures? [2] Give displayed formulae and the names for the four alkenes with molecular formula 4 8. [8] f 3-Methylpent-2-ene has two isomers. Draw them and name the type of isomerism involved. [3] 2 Using structural formulae throughout, give balanced chemical equations for the following reactions: a Propene with bromine. [1] b c Propene with hydrogen. Name the catalyst used. Which industrial process uses a similar reaction and the same catalyst? [3] Propene with hydrogen bromide. Give structural formulae for both possible products. [3] d Propene with steam. Give the formula of the catalyst used. Give structural formulae for both possible products. [2] 3 a Alkenes are unsaturated hydrocarbons. Explain both words in inverted commas. [2] b Describe the bonding between the two carbon atoms in ethene. [2] c d Describe and draw the shape of an ethene molecule, labelling all bond angles. [2] Explain the meaning of the term functional group. Which functional group is present in all alkenes? [2] e Describe a simple chemical test to determine whether an unknown hydrocarbon is an alkene. Describe the result if the test is positive. [2] 4 Ethene reacts with bromine to give 1,2-dibromoethane as the only product. The mechanism for the reaction is electrophilic addition and involves heterolytic fission of chemical bonds. The bromine molecules behave as electrophiles in this reaction. Use this passage and your own knowledge to answer the questions that follow. a By what mechanism does bromine react with ethene? [1] b Write a balanced chemical equation for this reaction. [2] c Bonds break in this reaction. What type of bond breaking is involved? [1] d Describe the reaction mechanism as fully as you can, using curly arrows. [4] e Which substance behaves here as an electrophile? Explain what is meant by the term electrophile. [2] 10 ydrocarbons: alkenes ambridge University Press 2005 81
5 In a similar reaction to the one in Question 4, 2.80 g of ethene are reacted with chlorine. 8.91 g of dichloroethane are formed. A r values: = 1.0, = 12.0, l = 35.5. alculate: a The number of moles of ethene that were used. [2] b The number of moles of dichloroethane that were formed. [2] c The percentage yield. [2] d The number of grams of dichloroethane that would have formed if the percentage yield had been 80.0%. [2] 6 Alkenes are important industrial chemicals, particularly as raw materials for the manufacture of polymers. Ethene can be used to make poly(ethene). Ethene is used to make chloroethene, which is then used to make poly(chloroethene), and ethene is also used to make tetrafluoroethene, which is used to make poly(tetrafluoroethene). a Use displayed formulae to describe the making of poly(chloroethene) from chloroethene. Label the repeat unit of the poly(chloroethene). [3] b Give two reasons why waste (used) polymers are difficult to dispose of. [2] c d Give three of the ways of dealing with waste polymers which are now replacing dumping/landfill. [3] Burning halogenated polymers such as pvc can release toxic waste products such as l into the environment. ow can this be minimised? [2] Total: / 75 Score: % 82 ambridge University Press 2005 10 ydrocarbons: alkenes
Marking scheme Worksheet 1: Electrophilic addition 1 a 2 4 + Br 2 2 4 Br 2 [1]; b [1] for each correct step ([5] in total) 2 a 2 4 + Br 2 5 Br [1]; b [1] for each correct step ([5] in total) 3 a 3 6 + 2 O 3 7 O [1]; b phosphoric(v) acid [1]; c propan-1-ol [1], 3 2 2 O [1]; propan-2-ol [1], 3 O 3 [1]; d [1] for each correct step ([5] in total) 4 a electron-pair receiver [1], e.g. Br [1]; b two reactant molecules combine to form one product molecule [1], e.g. 2 4 + Br 2 5 Br [1]; c the breaking of a bond where one of the previously bonded atoms gets both of the bonding electrons [1], e.g. the breaking of the bromine bromine bond in Br 2 [1] 10 ydrocarbons: alkenes ambridge University Press 2005 83
84 ambridge University Press 2005 10 ydrocarbons: alkenes Marking scheme End-of-chapter test 1 a [1] for each answer (total [12]) b n 2n [1]; c cis- and trans-pent-2-ene [1], cis-trans isomerism [1]; d restricted rotation at double bond [1]; e but-1-ene [1], methylpropene [1] [1], [1], [1], [1], [1], [1] cis-but-2-ene [1] trans-but-2-ene [1] f cis-trans or geometric [1], i ii iii iv propene 3 6 3 = 2 cis-pent-2-ene 5 10 3 = 2 3 trans-pent-2-ene 5 10 3 = 2 3
2 a 3 = 2 + Br 2 3 Br 2 Br [1]; b 3 = 2 + 2 3 2 3 [1], nickel catalyst [1], manufacture of margarine [1]; c 3 = 2 + Br 3 2 2 Br or 3 Br 3 [3]; d 3 = 2 + 2 O 3 2 2 O or 3 O 3 [1], 3 PO 4 catalyst [1] 3 a aving one or more double bonds [1], compounds of and only [1]; b As well as a σ bond [1], there is a π bond caused by overlapping p orbitals [1]; c planar [1], all bond angles 120 [1] d A functional group gives particular chemical properties [1], = double bond [1]; e Add bromine water [1], alkenes decolorise it [1] 4 a electrophilic addition [1]; b 2 4 + Br 2 2 4 Br 2 [2]; c heterolytic fission [1]; d urly arrow from double bond [1], bond breaking in bromine molecule [1], bond forming from bromide ion [1], structure of product [1] Br Br δ- δ+ Br Br + Br Br e the Br 2 [1], an electrophile is an electron pair receiver [1] 5 a M r is 28.0 [1], 2.80 g is 2.80/28.0 mol = 0.100 mol [1]; b M r is 99.0 [1], 8.91 g is 8.91/99.0 mol = 0.0900 mol [1]; c 0.0900/0.100 [1] = 90.0% [1]; d 80.0% of 0.100 mol is 0.0800 mol [1], 0.0800 mol is 0.0800 99.0 g = 7.92 g [1] 6 a chloroethene [1], poly(chloroethene) [1]; repeat unit shown [1] repeat unit n l l n b give toxic fumes when burnt [1], non-biodegradable [1]; c recycling [1], burning for energy production [1], feedstock for cracking [1]; d l is acidic [1], can be removed by dissolving in water/alkali [1] 10 ydrocarbons: alkenes ambridge University Press 2005 85