OCR AS Chemistry

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4.5.1 Reactions of Haloalkanes

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Substitution Reactions of Haloalkanes

Reactivity of Haloalkanes 

  • Haloalkanes are much more reactive than alkanes due to the presence of the electronegative halogens
  • The carbon-halogen bond is polar causing the carbon to carry a partial positive and the halogen a partial negative charge

Halogen Compounds Polarity of the C-X bond, downloadable AS & A Level Chemistry revision notes

Due to the large difference in electronegativity between the carbon and halogen atom, the C-X bond is polar

  • Because of this, haloalkanes will undergo two key types of reaction
  • Nucleophilic substitution reactions
    • A halogen is substituted for another atom or group of atoms
    • The products formed when haloalkanes undergo this type of reaction are alcohols, amines and nitriles

  • Elimination reactions
    • A hydrogen halide is eliminated during the reaction
    • The key product formed from this type of reaction is an alkene

Formation of alcohols

  • The nucleophile in this reaction is the hydroxide, OH- ion
  • An aqueous solution of sodium hydroxide (NaOH) or potassium hydroxide (KOH) with ethanol is used
  • Halogenoalkanes are generally insoluble in water
    • This means that there would be an organic layer of the halogenoalkane and an aqueous layer of sodium hydroxide
    • Any reaction would only occur where the two layers meet
    • The use of ethanol allows the aqueous sodium hydroxide layer and the organic halogenoalkane layer to mix, allowing the reaction to proceed
  • This reaction is very slow at room temperature, so the reaction mixture is warmed
  • This is an example of a hydrolysis reaction and the product is an alcohol
    • The rate of this reaction depends on the type of halogen in the haloalkanes
    • The stronger the C-X bond, the slower the rate of the reaction
    • In terms of bond enthalpy, C-F > C-Cl > C-Br > C-I
    • Fluoroalkanes do not react at all, but iodoalkanes have a very fast rate of reaction

Halogen Compounds Electrophilic Substitution by NaOH, downloadable AS & A Level Chemistry revision notes

The halogen is replaced by the nucleophile, OH- 

  • This reaction could also be done with water as the nucleophile, but it is very slow
    • The hydroxide ion is a better nucleophile than water as it carries a full negative charge
    • In water, the oxygen atom only carries a partial charge

Halogen Compounds Nucleophilicity, downloadable AS & A Level Chemistry revision notes

A hydroxide ion is a better nucleophile as it has a full formal negative charge whereas the oxygen atom in water only carries a partial negative charge; this causes the nucleophilic substitution reaction with water to be much slower than the aqueous alkali

Measuring the rate of hydrolysis

  • Acidified silver nitrate can be used to measure the rate of hydrolysis of halogenoalkanes
  • Set up three test tubes in a 50 oC water bath, with a mixture of ethanol and acidified silver nitrate
  • Add a few drops of a chloroalkane, bromoalkane and an iodoalkane to each test tube and start a stop watch
  • Time how long it takes for the precipitates to form
  • The precipitate will form as the reaction progresses and the halide ions are formed
  • A white precipitate will form from the chloroalkane, a cream precipitate will form from the bromoalkane and a yellow precipitate will form from the iodoalkane
    • The yellow precipitate will form the fastest
    • This is because the C-I bond has the lowest bond enthalpy, so it is the easiest to break and will cause the I- ions to form the fastest
    • The white precipitate will form the slowest
    • This is because the C-Cl bond has the highest bond enthalpy, so it is the hardest to break and will cause the Cl- ions to form the slowest

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Philippa

Author: Philippa

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.