AS-Level Halogenoalkanes

  • Carbon-halogen bonds are highly polar.

  • Due to the high electronegativity of the halogens, the carbon becomes slightly positively charged and the halogen slightly 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 forms alcohols.

  • Halogenoalkanes with ammonia in ethanolic conditions forms amines.

  • Halogenoalkanes with cyanide ions in ethanolic conditions forms nitriles.

    • ​Ethanolic conditions are needed instead of aqueous conditions otherwise alcohols would form.

QUICK NOTES

Nucleophilic Substitution of Halogenoalkanes 

 

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

 

 

 

 

 

 

 

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.

 

 

 

 

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

 

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 (NH ) 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.

 

 

 

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.

 

 

 

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

3