Structure of alkenes

  • Unsaturated hydrocarbons containing at least one carbon to carbon double bond. 
  • Have the general formula CnH2n.
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The double bond

  • For each carbon atom of the double bond, three out of four electrons are used in three sigma bonds, one to the other carbon of the double bond and the other two to two other atoms.
  • One electron is not involved in the sigma bonds and is instead involved in a pi bond which is formed by the overlap of two p orbitals. 
  • Each carbon atom in the double bond contributes one electron to the pi bond. 
  • The pi electron density is concentrated above and below the line joining the nuclei of the bonding atoms. 
  • The pi bond locks the two carbon atoms in position and prevents them from rotating around the double bond. 
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E/Z isomerism

  • If a molecule has a C=C double bond and different groups attached to each carbon atom of the double bond it will have E/Z isomerism. 
  • If the groups of high priority are on the same side of the double bond the compund is the Z isomer.
  • If the high priority groups are diagonally across from one another the compound is the E isomer. 
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Cis-trans isomerism

  • Special type of E/Z isomerism where one of the groups attached to each carbon must be hydrogen. 
  • If the hydrogen atoms are on the same side of the double bond then it is the cis isomer.
  • If the hydrogen atoms are diagonally across form one another than it is the trans isomer.
  • The cis isomer is the Z isomer.
  • The trans isomer is the E isomer. 
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Assigning priority

  • Examine the atoms directly bonded to the carbon atoms and decide which has the higher priority. The one with the highest priority will have the highest atomic number. 
  • If the two atoms attached are the same then you will need to find the first point of difference. The group that has the highest atomic number at the first point of difference will have the highest priority. 
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Reactivity of alkenes

  • Much more reactive than alkanes due to the presence of the double bond. 
  • The pi electrons are on the outside of the double bond so are more exposed. Because of this a pi bond breaks more readily than a sigma bond. 
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Hydrogenation of alkenes

  • When an alkene is mixed with hydrogen and passed over a nickel catalyst at 423K, an addition reaction takes place to form an alkane. 

C3H6 + H2 ---> C3H8

Propene + hydrogen ---> propane

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Halogenation of alkenes

  • Alkenes undergo a rapid reaction with chlorine and bromine at room temperature. 
  • This reaction can be used to identify if there is a C=C bond present. 
  • When bromine water is added dropwise to a sample of an alkene, bromine adds across the double bond. 
  • The orange colour disappears and goes colourless. 

C3H6 + Br2 ---> CH3CHBrCHBrH

Propene + bromine ---> 1,2-dibromopropane 

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Reaction with hydrogen halides

  • React with gaseous hydrogen halides at room temperature to form haloalkanes. 

Propene + hydrogenchloride ---> 1-chloropropane

Propene + hydrogenchloride ---> 2-chloropropane

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Hydration of alkenes

  • Alcohols are formed when alkenes react with steam in the presence of a phosphoric acid catalyst. 
  • The steam adds across the double bond. 

Propene + steam ---> Propan-1-ol

Propene + steam ---> Propan-2-ol 

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