Quick Notes Halogenoalkanes - Nucleophilic Substitution Reactions

  • Carbon-halogen bonds are highly polar.
    • Due to the high electronegativity of halogens, the carbon becomes partially positively charged and the halogen partially negatively charged.
  • Polarity of the carbon-halogen bonds decreases as you go down group 7.
  • The carbon in a carbon-halogen bond is easily attacked by electron donating species (nucleophiles) that swap places with the halogen in nucleophilic substitution reactions.
  • Key Reactions:
    • Halogenoalkanes with sodium hydroxide in aqueous conditions form alcohols.
    • Halogenoalkanes with ammonia in ethanolic conditions form amines.
    • Halogenoalkanes with cyanide ions in ethanolic conditions form nitriles.
      • Ethanolic conditions are needed instead of aqueous conditions, otherwise alcohols would form.

Full Notes Halogenoalkanes - Nucleophilic Substitution Reactions

Halogens form highly polar bonds when bonded to carbon. This polarity makes halogenoalkanes reactive.

functional group of halogenoalkanes polarity

Electronegativity increases as you go across a period in the periodic table, but decreases as you go down a group. This means fluorine is the most electronegative halogen, and iodine is the least electronegative halogen. Carbon-fluorine bonds are the most polar, and carbon-iodine are the least polar.

electronegativity of carbon halogen bond polarity decreases down group

The halogen in a carbon-halogen bond has a partial negative charge, and the carbon atom has a partial positive charge. This partially positive carbon atom attracts electron donating species (nucleophiles). Nucleophiles can donate their electrons to the carbon, forming a bond. The electrons in the carbon-halogen bond are forced to go to the halogen, which becomes a negatively charged ion with a lone pair of electrons and leaves the molecule.

nucleophilic substitution mechanism bromoethane with hydroxide ion to form ethanol

As the nucleophile is effectively ‘swapping’ places with the halogen, the reaction is a nucleophilic substation reaction. The mechanism shown is a common example: the hydrolysis reaction between a halogenoalkane and sodium hydroxide in the presence of water (aqueous conditions), to produce an alcohol.

Halogenoalkane with Ammonia

A type of functional group called amines can be produced when halogenoalkanes are reacted with ammonia (NH3) and ethanol under reflux conditions. The ethanol acts as a solvent. Water cannot be used as the solvent otherwise hydrolysis (see above) would take place.

reaction between halogenoalkane bromoethane and ammonia to form amine

The ammonia acts as a nucleophile and is substituted with the halogen, the reaction is a nucleophilic substitution.

Halogenoalkane with Cyanide Ions

A type of functional group called nitriles can be produced when halogenoalkanes are reacted with cyanide ions (:CN-) and ethanol under reflux conditions. As with a halogenoalkane and ammonia, ethanol acts as a solvent (instead of water) to ensure no hydrolysis takes place.

reaction between halogenoalkane bromoethane and nitrile ion

The cyanide ion acts as a nucleophile and is substituted for the halogen, the reaction is a nucleophilic substitution.