Alkenes

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  • Alkenes
    • A covalent bond is a shared pair of electrons
      • So, when two atoms form a covalent bond 'a singly occupied orbital on one atom overlaps with a singly occupied orbital on another atom'
      • When two s orbitals each with one electron overlap to form a sigma bond
      • 2 p orbitals, each with one electron overlap end over end to form a sigma bond
      • 2 p orbitals, each with one electron overlap sideways to form a pi bond
      • When a carbon forms a single bond, this will be a sigma bond since this type of overlap leads to a lower energy orbital and hence a stronger bond than a pi bond
        • In a double bond, one electron pair forms a sigma-bond and the other forms a pi-bond
      • Key features of a double bond
        • Shape
          • Each carbon has one double bond and two other bonded pairs repelling each other in it's outer shell, so the geometry around each carbon is trigonal planar (bond angle 120 degrees)
        • Rigidity
          • Unlike a single bond, rotation is not possible around a double bond. This is because rotation would destroy the sideways overlap of the p-orbitals and hence break the pi-bond
        • Bond lengths and bond energies
          • A double bond is shorter and stronger than a single bond
            • Within the double bond, the pi-bonds is weaker than the sigma bond, so in reaction it is the pi-bond that tends to break
    • Alkenes are unsaturated hydrocarbons. They form a homologous series with the general formula CnH2n
      • Each alkene contains a C=C double bond
    • Isomerism in alkenes
      • Structural Isomerism
        • Like alkanes, alkenes can be straight or branched but now the position of the double bond may also vary
      • Stereoisomerism
        • Stereoisomers are compounds with the same structural formula but with a different arrangement in space
        • E/Z isomerism
          • E means that the two hydrogen's are on opposite sides of the double bond
            • Z means that the two hydrogen's are on the same side of the double bond
          • For E/Z isomerism to occur, each of the doubly bonded carbon atoms must be also bonded to two different groups
    • Preparation of Alkenes
      • Cracking of hydrocarbons
        • This is the main industrial source of alkenes
          • Long chain alkanes from petroleum are cracked to provide a variety of alkenes. These are major industrial chemicals
            • Industrial uses of alkenes include;
              • Polymerisation to form plastics e.g.poly(ethene)
              • Conversion to other organic chemicals e.g. alcohols
      • Dehydration of alcohols
        • Alkenes can be made starting from alcohols which are compounds that contain an -OH group
          • If a water molecule is lost from this molecule by eliminating the -OH from one carbon and a hydrogen from the adjacent carbon, it leaves a double bond between the two carbons
            • This type of reaction is called elimination
              • Conditions: Dehydration requires heating in the presence of an acid catalyst.
    • Chemical reactions of alkenes
      • In alkenes the functional group is the C=C double bond and this is called the alkene group
        • The characteristic reaction of alkenes is addition. An addition reaction is one in which two reactant molecules combine together to form one product molecule
          • In this case, the pi-bond of the double bond breaks and each carbon atom forms a new sigma bond to another atom
      • Addition of Hydrogen
        • Alkenes react with hydrogen gas in the presence of a nickel catalyst at 60 degrees centigrade
          • This reaction is the basis of the manufacture of margarine from liquid vegetable oils. These oils are unsaturated
            • On hydrogenation, some of these double bonds react with hydrogen and are converted into single bonds. The more saturated hydrocarbons obtained have higher melting points and are solid at room temperature, so can be used as margarine
      • Addition of halogens
        • Alkenes react readily with chlorine of bromine at room temperature and in the dark
          • The reaction with bromine is used as a test for unsaturation becuase the orange solution of bromine is immediately decolourised in the dark and at room temperature, as the product of the reaction is colourless
      • Addition of steam
        • This is the reverse of the dehydration reaction used to prepare alkenes in the laboratory. The reaction requires H2O in the form of steam and a phosphoric acid catalyst
          • The product of addition is an alcohol
            • Ethene and steam are passed at a high temperature and pressure (About 300 degrees centigrade and 60 atmospheres) over a phosphoric acid catalyst to make ethanol
              • Industrial ethanol is extensively used as a solvent
      • Addition of hydrogen halides
        • Hydrogen chloride, bromide and iodide undergo addition reactions with alkenes, forming the corresponding halogenoalkane
    • Polymerisation of alkenes
      • A polymer is a long molecule formed by chemically linking together many smaller molecules called monomers
        • The polymer is simply named by writing poly(name of alkene)
          • This is an addition reaction,  so is sometimes called an addition polymerisation
            • The bracketed fragment of the product polymer chain is called the repeat unit of the polymer
    • Disposal of Polymers
      • Alkene polymers are not biodegradable, so if we throw them away after use, waste plastics accumulate in the environment either on land or at sea. The will remain in landfill for hundreds of years
      • Combustion
        • Advantage
          • They are good fuels, like other hydrocarbons and burning them releases energy and reduces demand for landfill sites
        • Disadvantage
          • Toxic combustion products may be formed. This is particularly an issue for chlorinated plastics such as PVC. Toxic, acidic hydrogen chloride is formed when this burns, but may be removed from the waste gases by chemical treatment
      • Recycling
        • One difficulty is that the different types of polymers have to be separated if recycling is to produce a useful product and separation is a costly, labour-intensive process
      • Feedstock for Cracking
        • Polymers can be cracked just like other hydrocarbons to give a mixture of alkanes and alkenes
          • Alkanes can be used for petrol etc. and the alkenes can be polymerised to give more polymers. At present this is still an expensive process but is likely to become more important
      • Biodegradable polymers
        • Polymers are now being developd which are biodegradab;e
          • One example is where granules of starch are built into the plastic. Microbes in soil feed on the starch, breaking the plastic into small bits that will rot away more quickly e.g. starch sacks for garden/compost waste
    • Arenes
      • The simples arene is benzene. It has the molecular formula C6H6
        • Benzene does not react as if it had three C=C double bonds: the pi bonds interfere with one another and as a result have very different properties

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