1 of 34 © Boardworks Ltd 2009 2 of 34 © Boardworks Ltd 2009 What are alkenes? The alkenes are a homologous series of hydrocarbons with the general formula CnH2n and names ending –ene. Alkenes contain a carbon – carbon double bond and so are unsaturated. 3 of 34 No. of carbon atoms Molecular formula 2 C2H4 ethene 3 C3H6 propene 4 C4H8 butene 5 C5H10 pentene 6 C6H12 hexene Name © Boardworks Ltd 2009 Naming alkenes Alkenes with four or more carbon atoms display positional isomerism because the carbon–carbon double bond may appear between different carbon atoms. If there are two or more possible positions for the double bond, a number is used before the –ene to indicate the first carbon involved. but-1-ene 4 of 34 but-2-ene © Boardworks Ltd 2009 Structure of alkenes 5 of 34 © Boardworks Ltd 2009 Bonding in alkenes 6 of 34 © Boardworks Ltd 2009 Isomerism in alkenes Rotation around the double bond in alkenes is restricted by the presence of the bond. This leads to E–Z stereoisomerism in some alkenes. E-pent-2-ene Z-pent-2-ene The two E-Z stereoisomers have the same structure; the only difference between them is the arrangement of the atoms in space. 7 of 34 © Boardworks Ltd 2009 Structure of alkenes summary 8 of 34 © Boardworks Ltd 2009 9 of 34 © Boardworks Ltd 2009 Double bonds and electrophiles The double bond of an alkene is an area of high electron density, and therefore an area of high negative charge. The negative charge of the double bond may be attacked by electron-deficient species, which will accept a lone pair of electrons. These species have either a full positive charge or slight positive charge on one or more of their atoms. They are called electrophiles, meaning ‘electron loving’. Alkenes undergo addition reactions when attacked by electrophiles. This is called electrophilic addition. 10 of 34 © Boardworks Ltd 2009 Electrophilic addition mechanism: 1 In the first stage of electrophilic addition, the positive charge on the electrophile is attracted to the electron density in the double bond. δ+ As the electrophile approaches the double bond, electrons in the A–B bond are repelled towards B. δ- The pi bond breaks, and A bonds to the carbon, forming a carbocation – an ion with a positively-charge carbon atom. The two electrons in the A–B bond move to B forming a B- ion. 11 of 34 © Boardworks Ltd 2009 Electrophilic addition mechanism: 2 In the second stage of electrophilic addition, the B- ion acts as a nucleophile and attacks the carbocation. The lone pair of electrons on the B- ion are attracted towards the positivelycharged carbon in the carbocation, causing B to bond to it. Because both electrons in the bond that joins B- to the carbocation ion come from B-, the bond is a co-ordinate bond. 12 of 34 © Boardworks Ltd 2009 What is hydrogenation? Hydrogen can be added to the carbon–carbon double bond using a nickel catalyst in a process called hydrogenation. C2H4 + H2 C2H6 Vegetable oils are unsaturated and may be hydrogenated to make margarine, which has a higher melting point. As well as a nickel catalyst, this requires a temperature of 200 °C and a pressure of 1000 kPa. 13 of 34 © Boardworks Ltd 2009 Testing for alkenes The presence of unsaturation (a carbon– carbon double bond) can be detected using bromine water, a red/orange coloured solution of bromine. A few drops of bromine water are added to the test liquid and shaken. If a carbon–carbon double bond is present, the bromine adds across it and the solution turns colourless. 14 of 34 © Boardworks Ltd 2009 More on the bromine water test A simple equation for the bromine water test with ethene is: CH2=CH2 + Br2 + H2O CH2BrCH2Br + H2O However, because water is present in such a large amount, a water molecule (which has a lone pair) adds to one of the carbon atoms, followed by the loss of a H+ ion. CH2=CH2 + Br2 + H2O CH2BrCH2OH + HBr The major product of the test is not 1,2-dibromoethane (CH2BrCH2Br) but 2-bromoethan-1-ol (CH2BrCH2OH). 15 of 34 © Boardworks Ltd 2009 Electrophilic addition reactions 16 of 34 © Boardworks Ltd 2009 Addition to unsymmetrical alkenes When an electrophile (e.g. HBr) attacks an alkene with three or more carbon atoms (e.g. propene), a mix of products is formed. This is because these alkenes are unsymmetrical. minor product: 1-bromopropane HBr major product: 2-bromopropane Unequal amounts of each product are formed due to the relative stabilities of the carbocation intermediates. 17 of 34 © Boardworks Ltd 2009 Structure of carbocations A chain of carbon atoms can be represented by R when drawing organic structures. This is an alkyl group (general formula CnH2n+1). Primary (1°) carbocations have one alkyl group attached to the positively-charged carbon. Secondary (2°) carbocations have two alkyl groups attached to the positively-charged carbon. Tertiary (3°) carbocations have three alkyl groups attached to the positively-charged carbon. 18 of 34 © Boardworks Ltd 2009 Stability of carbocations The stability of carbocations increases as the number of alkyl groups on the positively-charged carbon atom increases. primary secondary tertiary increasing stability The stability increases because alkyl groups contain a greater electron density than hydrogen atoms. This density is attracted towards, and reduces, the positive charge on the carbon atom, which has a stabilizing effect. 19 of 34 © Boardworks Ltd 2009 Structure of carbocations 20 of 34 © Boardworks Ltd 2009 Type of addition reaction • Hydrogenation • Halogenation • Hydration • Addition of hydrogen halide • Oxidation 21 of 34 © Boardworks Ltd 2009 Electrophiles: true or false? 22 of 34 © Boardworks Ltd 2009 23 of 34 © Boardworks Ltd 2009 Polyalkenes Alkenes can undergo addition reactions with themselves to form a long chain polymer molecule. This reaction is addition polymerization. The polymer can be represented by showing the repeating unit with square brackets around it. The n stands for a unspecified number of monomer units. 24 of 34 © Boardworks Ltd 2009 Polymerization of ethene 25 of 34 © Boardworks Ltd 2009 LDPE and HDPE The reaction conditions under which ethene polymerizes affect the structure and properties of the poly(e)thene. Low-density polythene (LDPE) is formed under a high pressure (1400 atm) and a temperature of about 170 °C. These conditions cause a high level of branching, meaning that the polymer chains cannot pack tightly together. High-density polythene (HDPE) is formed with a catalyst, a pressure of 2 atm and a temperature of about 70 °C. Little branching occurs under these conditions, resulting in chains that can pack tightly together to create a denser material. 26 of 34 © Boardworks Ltd 2009 Other polyalkenes Propene undergoes addition polymerization to form polyproprene: Chloroethene undergoes addition polymerization to form polychloroethene: 27 of 34 © Boardworks Ltd 2009 Which alkene? 28 of 34 © Boardworks Ltd 2009 More about LDPE LDPE is a soft, flexible and stretchy plastic, with a melting point of about 120 °C. It is used to make: plastic bags squeezable bottles, and general purpose containers and trays other items that need to be soft and flexible, such as tubing. LDPE has the recycling symbol ‘4’. 29 of 34 © Boardworks Ltd 2009 More about HDPE HDPE is a tough and flexible plastic, with a melting point of about 130 °C. It is used to make: containers such as milk and detergent bottles rigid items such as folding tables, chairs and pipes. HDPE has the recycling symbol ‘2’. 30 of 34 © Boardworks Ltd 2009 More about polypropene Polypropene is a tough and flexible plastic, with a melting point of about 160 °C. It is used to make: ropes, carpets, rugs and other textiles medical, laboratory and kitchen items that need to withstand temperatures in autoclaves and dishwashers certain bottles, buckets, containers and other items such as bottle tops and moulded fittings. Polypropene has the recycling symbol ‘5’. 31 of 34 © Boardworks Ltd 2009 32 of 34 © Boardworks Ltd 2009 Glossary 33 of 34 © Boardworks Ltd 2009 What’s the keyword? 34 of 34 © Boardworks Ltd 2009 Multiple-choice quiz 35 of 34 © Boardworks Ltd 2009
