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ALKENES The alkenes are a homologous series of unsaturated hydrocarbons which all have the general formula CnH2n. Alkenes contain two hydrogen atoms less than the corresponding alkane and are therefore described as unsaturated. They contain one carbon-carbon double covalent bond per molecule. The first members of this homologous series are: H H C C H CH3 C H ethene CH3 C H CH3 H H CH3 E-but-2-ene CH2CH3 C H C H H C H C propene CH3 C H C H Z-but-2-ene H but-1-ene Naming alkenes (the E/Z system) You may see alkenes with the words cis or trans at the start of their name. This naming system is not on the A level syllabus. The Cahn Ingold Prelog (CIP) or E-Z naming system, which is now preferred, works by assigning priority to each of the groups bonded to the carbons in the C=C based on atomic mass of the bonded group. If the two highest priority groups lie on the same side of the molecule it is given the prefix Z(from the German word zusammen meaning together). If the two highest priority groups are on different sides of the molecule it is given the prefix E- (from the German word entgegen meaning opposite). TOPIC 12.14: ORGANIC CHEMISTRY - ALKENES 1 E-3-methylpent-2-ene Z-3-methylpent-2-ene In the above example the two groups on the left hand carbon are H and CH 3. Since a C high a higher atomic mass than H it is given the higher priority. In the case of the right hand carbon both atoms attached are C atoms and so we move along the bonds to the next atom. When we find aonther C atom in the case of the C 2H5 group this is given a higher priority than the CH3 group. The arrows indicate the highest priority groups on each C atom of the C=C. Bonding In, for example, ethene C2H4 both carbon atoms form three -bonds (by overlap of their three sp2 hybrid orbitals): one bond to each of two hydrogen atoms and the third to the second carbon atom. The remaining electron on each carbon atom is in a 2p-orbital. The two 2p-orbitals overlap sideways to form a -bond. H H 2p 2p C C H H H H C H H H C -orbital C C H H Since all six atoms lie in the same plane, the molecule is described as planar. 120o The bond angle is 120o H For other alkenes, the double bond and the four atoms attached to it are planar. C C TOPIC 12.14: ORGANIC CHEMISTRY - ALKENES 2 Reduction Alkenes are reduced to alkanes by catalytic hydrogenation. This involves cis-addition of hydrogen across the double bond. The conditions used are: finely divided nickel catalyst temperature: 150oC On the surface of the catalyst, hydrogen molecules (H2) split into hydrogen atoms (H). Two hydrogen atoms are then added to the same side of the double bond (cis-addition) simultaneously. H H C + C H H H2 H H H C C H H H Vegetable oils (liquids) are esters of glycerol and unsaturated fatty acids (long-chain carboxylic acids). To convert these oils to margarine, they are catalytically hydrogenated (hardened) to partially saturate the double bonds. The products are fats (solids). The greater the degree of saturation, the harder (higher m.p.) the fat will be. By adjusting the extent of satuartion, hard or soft margarines can be produced. Electrophilic Addition The -orbital in alkenes is a centre of high electron density which is readily polarisable. This means that alkenes will be reactive towards reagents which are electron deficient; these are known as electrophiles. ELECTROPHILES are ELECTRON PAIR ACCEPTORS. They are particles which have centres of low electron density and which are attacked by centres of high electron density in other molecules or ions. Common electrophiles are: H+, NO2+, >C=O, R+, RCO+, RCOCl The principal reaction of alkenes is electrophilic addition. This can be represented in general terms by the equation: C C + X Y C C X Y The double covalent bond is replaced by two new single covalent bonds; the product is saturated. TOPIC 12.14: ORGANIC CHEMISTRY - ALKENES 3 1. Reaction with bromine water When an alkene is shaken with bromine water, the red-brown colour of the bromine water is discharged to give a colourless solution. This reaction is used as a test for unsaturation. H H H H C + C H H Br2 H ethene (unsaturated) C C Br Br H 1,2-dibromoethane (saturated) Under these conditions, alkanes do not react with bromine, so the reaction can be used to distinguish alkenes from alkanes. Mechanism: The Br2 molecule is polarised by close approach to the alkene -bond. The positive end of the dipole is then attacked. H2C=CH2 + - Br Br The carbocation which forms is then attacked by the nucleophile, Br-. :Br- Br + H2C-CH2 The final product is 1,2-dibromoethane H2C-CH2 Br Br NUCLEOPHILES are ELECTRON PAIR DONORS. They are particles which have centres of high electron density and which attack centres of low electron density in other molecules or ions. Common nucleophiles are: CN-, H-, Br-, OH-, NH3 , OR-, HSO4-, H2O, RNH2 2. Reaction with hydrogen bromide Alkenes react with hydrogen bromide both in concentrated aqueous solution and in the gas phase. The product is the corresponding bromoalkane. H H C + C H HBr H ethene (unsaturated) TOPIC 12.14: ORGANIC CHEMISTRY - ALKENES 4 H H H C C H Br bromoethane (saturated) H Mechanism: The mechanism is similar to that of bromination. Owing to the difference in electronegativity of hydrogen and bromine, HBr in the gas phase has a permanent dipole. Hydrogen, the positive end of the dipole, is attacked by the -orbital on the alkene. In aqueous solution, HBr dissociates into ions. The H+(aq) ions are electrophiles and are similarly attacked by the -orbital on the alkene. : Br H2C=CH2 + H - Br H2C-CH2 + H2C-CH2 Br H bromoethane H 3. Reaction with sulphuric acid When alkenes are passed into cold, concentrated sulphuric acid, they react to form the corresponding alkyl hydrogensulphate. After adding water and warming, hydrolysis of the alkyl hydrogensulphate takes place to produce an alcohol. H H C + C H H H2SO4 H ethene (unsaturated) H H H C C H OSO3H H H H C C H OSO3H H ethyl hydrogensulphate (saturated) + H 2O H H H C C H OH H + H2SO4 ethanol Since the overall result of this reaction is the addition of water to ethene, it is known as the hydration of ethene. : O.SO3H Mechanism: H2C=CH2 + H - O,SO3H + H2C-CH2 H TOPIC 12.14: ORGANIC CHEMISTRY - ALKENES 5 O.SO3H H2C-CH2 H ethyl hydrogensulphate Direct Hydration of Ethene In industry, the hydration of ethene is carried out by the direct reaction of ethene and steam. The conditions used are: temperature: 300oC pressure: 65 atmospheres catalyst: phosphoric acid (H3PO4) adsorbed on celite H H C + C H H H 2O H H H C C H OH H Addition to Unsymmetrical Alkenes When an unsymmetrical molecule (e.g. HBr) adds to an unsymmetrical alkene such as propene, two addition products are possible. In practice, there is one major product, 2bromopropane. CH3.CH=CH2 CH3.CHBr.CH3 2-bromopropane X + HBr CH3.CH2.CH2Br 1-bromopropane The favoured product is the one which is formed via the more stable carbocation intermediate. The order of stability of carbocations is: R + C R > R + C H > H + C H R R R tertiary secondary primary This is due to the positive inductive effect. Alkyl groups (eg methyl group) have a tendency to push electron density towards the carbon atom with the positive charge thus stabilising it. The more alkyl groups there are, the greater the stabilising effect. + 2-bromopropane is formed via the secondary carbocation: CH3.CH.CH3 1-bromoproane is formed via the primary carbocation: CH3.CH2.CH2 + Since the secondary carbocation is the more stable, 2-bromopropane is formed as the major product. TOPIC 12.14: ORGANIC CHEMISTRY - ALKENES 6 In general, the direction of addition to an unsymmetrical alkene is given by Markovnikov’s rule: ‘The more positive end of the molecule being added bonds to the alkene carbon atom which is bonded to the larger number of hydrogen atoms.’ Markovnikov TOPIC 12.14: ORGANIC CHEMISTRY - ALKENES 7 POLYMERS Polymers are usually divided into two categories: Addition polymers Condensation polymers ADDITION POLYMERS In addition polymerisation, a large number of molecules of a monomer combine to produce a polymer, but no other product is formed. The polymer has the same empirical formula as the monomer but has a larger molecular mass, which could be as high as 500,000. Polymerisation is initiated by a radical or an ion (this depends on the catalyst system which is used) which adds to a carbon-carbon double bond. For example: . X + CH2=CH2 X . CH2CH2 Further molecules of monomer then add sequentially to the reactive end, and the chain grows in length. The overall reaction for addition polymerisation can be represented as: H n R C H ( C H H R C C H H )n The repeating unit is enclosed within the brackets. n is a large number, which typically lies in the range 100 to 10,000. The end groups constitute such a small fraction of the polymer molecule that they are usually omitted. Under the right conditions, ethene molecules will add to each other to form a giant molecule (a polymer) by the process of addition polymerisation. The backbone of a polyalkene molecule is made up of singly-bonded carbon atoms. The molecule is saturated and is chemically unreactive. These polymers are nonbiodegradable. The attractive forces between polymer chains are weak. This means that, on heating to relatively low temperatures, they soften and become mouldable: these are thermosoftening plastics. They can be recycled. The properties of polymers can be altered by adding plasticisers which push the polymer chains further apart thus weakening the intermolecular forces. This means that chains can slide past each other more easily and the polymer becomes more flexible. With the possible exception of p.v.c., these plastics are flammable. When heated, p.v.c. decomposes releasing HCl; this acts as a self-extinguisher. TOPIC 12.14: ORGANIC CHEMISTRY - ALKENES 8 H H n C C H H ethene (unsaturated) H H C C H H n poly(ethene) (saturated) There are two types of poly(ethene): low density poly(ethene) or LDPE and high density poly(ethene) or HDPE. The manufacturing conditions are: LDPE 200oC 2000 atmospheres trace of oxygen temperature: pressure: the polymer has an Mr of ~50,000 and a softening temperature of 120 oC temperature: 30oC pressure: 2 atmospheres catalyst: (C2H5)3Al / TiCl4 HDPE the polymer has an Mr of ~300,000 and a softening temperature of 130oC Other addition polymers include: CH3 n H C H H C H propene H C H Cl C H chloroethene (pvc) H n C C H H n H H C C H Cl n poly(chloroethene) H C H poly(propene) H n CH3 C H H H C C H phenylethene TOPIC 12.14: ORGANIC CHEMISTRY - ALKENES 9 n poly(phenylethene) (polystyrene) These polymers are thermosoftening polymers and can be recycled. Examples of addition polymers are: Monomer Polymer Use ethene, CH2=CH2 poly(ethene) bags, buckets propene, CH3CH=CH2 poly(propene) ropes, containers phenylethene, CH2=CHC6H5 poly(phenylethene) chloroethene, CH2=CHCl poly(chloroethene) (PVC) window frames, guttering, pipes propenenitrile, CH2=CHCN poly(propenenitrile) acrylic fibres tetrafluoroethene, CF2=CF2 poly(tetrafluoroethene) non-stick surface ‘vinyl’ records, drainpipes methyl 2-cyanopropenoate,CH2=C(CN)COOCH3 poly(methyl 2-cyanopropenoate) TOPIC 12.14: ORGANIC CHEMISTRY - ALKENES 10 super glue