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Substitution Reactions

Chemistry: AQA A Level, topic 5.3.1

Halogenoalkanes

A halogenoalkane is a halogenated alkane. This means that one (or more) of the hydrogen atoms in an alkane has been replaced by a halogen atom.

Names of halogenoalkanes

Names of halogenoalkanes

  • The names of halogenoalkanes use the halogen names at the start.
  • E.g. Fluoromethane.
  • E.g. Chloromethane.
  • E.g. Bromomethane.
  • E.g. Iodomethane.
Properties of halogenoalkanes

Properties of halogenoalkanes

  • Halogenoalkanes are polar molecules and contain polar bonds.
  • The polar bonds are important in their reactions.
  • Because halogens are more electronegative than carbon, the carbon atoms carry a partial positive charge and the halogen atoms carry a partial negative charge.
  • Halogenoalkanes undergo substitution reactions with ammonia, hydroxide ions, and cyanide ions.
Nucleophiles and electrophiles

Nucleophiles and electrophiles

  • A nucleophile is a molecule that is attracted to a nucleus.
  • Nuclei have positive charges, so nucleophiles tend to be negatively charged.
  • Molecules with lone pairs are also nucleophilic.
  • An electrophile is a molecule that is attracted to a pair of electrons.
  • Electrons have negative charges, so electrophiles tend to be positively charged.

Nucleophilic Substitution

Halogenoalkanes react with nucleophiles in a nucleophilic substitution reaction. This mechanism is a one-step process.

Nucleophilic substitution

Nucleophilic substitution

  • The mechanism for nucleophilic substitution with hydroxide ions is shown above.
  • This makes an alcohol.
With ammonia

With ammonia

  • Nucleophilic substitution can be used to make amines. This is shown above.
With cyanide ions

With cyanide ions

  • Nucleophilic substitution can be used to make nitriles. This is shown above.
Reactivity

Reactivity

  • The reactivity of halogenoalkanes decreases in the order I>Br>Cl>F.
  • This is because the C-I bond is weakest and the C-F bond is strongest.
  • For the halogenoalkane to react, the C-X bond must break (X means any halogen).
  • This means that the activation energy is highest for a halogenoalkane with a carbon-fluorine bond and the reaction is therefore slowest.

Test yourself on Substitution Reactions

  • What controls halogenoalkanes' reactivity?

  • Halogenoalkanes tend to be involved in what type of reactions?

  • What is the most reactive halogenoalkane out of the following?

  • Which of the following could substitute for a chlorine atom in an organic molecule?

  • OH−, NH3 and CN− are all examples of what?

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