OCR AS Chemistry

Revision Notes

4.3.3 Addition Reactions of Alkenes

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Reactivity of Alkenes

  • Alkenes are more reactive than alkanes
    • This is due to the presence of the C=C bond or more specifically the π bond

Chemical Bonding Electron Density in Ethene, downloadable AS & A Level Chemistry revision notes

The σ bond between the two carbon atoms is in the centre of the C=C bond with the π bond concentrated above and below the plane of the σ bond, therefore, exposing the π electrons

  • A C-C single bond has a bond enthalpy value of 347 kJ mol-1 
    • This is equivalent to the σ bond portion of a C=C bond 
  • A C=C double bond has a bond enthalpy value of 612 kJ mol-1 
    • This value represents the bond enthalpy of the σ and π bond
  • Using these values, we can estimate the value of the π bond alone
    • 612 - 347 = 265 kJ mol-1 
  • This calculation shows that the π bond of the C=C double bond is weaker than a C-C single bond an explains why alkenes are more reactive
    • The π bond of the C=C bond requires less energy to break than a C-C single bond and consequently reacts more readily

Addition Reactions of Alkenes

  • Alkenes are very useful compounds as they can undergo many types of reactions
  • They can therefore be used as starting molecules when making new compounds

Hydrogenation

  • The reaction between an alkene and hydrogen is known as hydrogenation or reduction
  • As well as a nickel catalyst, this requires a temperature of 200 °C and a pressure of 1000 kPa

Catalytic hydrogenation converts alkenes into useful alkanes

  • One important application of this reaction is in the production of margarine from vegetable oils
  • Vegetable oils are unsaturated and may be hydrogenated to make margarine, which has a higher melting point due to stronger London Dispersion Forces
  • By controlling the conditions it is possible to restrict how many of the C=C bonds are broken and produce partially hydrogenated vegetable oils which have which have the desired properties and textures for margarine manufacture

Halogenation

  • The reaction between alkenes and halogens is known as halogenation
  • It is an example of an electrophilic addition where an electrophile ('electron seeker') joins onto to a double bond
  • The C=C double bond is broken, and a new single bond is formed from each of the two carbon atoms
  • The result of this reaction is a dihaloalkane
  • The reaction occurs readily at room temperature and is the basis for the test for unsaturation in molecules

Halogenation in alkenes

  • Halogens can be used to test if a molecule is unsaturated (i.e. contain a double bond)
  • Br2 is an orange or yellow solution, called bromine water
  • The unknown compound is shaken with the bromine water
  • If the compound is unsaturated, an addition reaction will take place and the coloured solution will decolourise

Hydrocarbons Bromine Water, downloadable AS & A Level Chemistry revision notes

The bromine water test is the standard test for unsaturation in alkenes

Hydrohalogenation

  • Alkenes will react readily with hydrogen halides such as HCl and HBr to produce halogenoalkanes
  • This reaction is known as hydrohalogenation 
  • It is also an electrophilic addition reaction that occurs quickly at room temperature

Hydrohalogenation reactions in alkenes

  • All the hydrogen halides react in this way, but the fastest reaction occurs in the order HI > HBr > HCl due to the increasing bond strength of the hydrogen-halogen bond, so the weakest bond reacts most easily

Hydration

  • When alkenes are treated with steam at 300 oC, a pressure of 60 atmospheres and sulfuric acid (H2SO4) or phosphoric acid (H3PO4) catalyst, the water is added across the double bond in a reaction known as hydration
  • An alkene is converted into an alcohol
  • The reaction processes via an intermediate in which H+ and HSO4 ions are added across the double bond
  • The intermediate is quickly hydrolysed by water, reforming the sulfuric acid

Hydration in Alkenes

  • This is a very important industrial reaction for producing large quantities of ethanol, a widely used solvent and fuel
  • The process is much faster and higher yielding that producing ethanol by fermentation

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