CIE A Level Chemistry (9701) 2019-2021

Revision Notes

7.6.2 Basicity of Amines

Basicity of Aqueous Solutions of Amines

  • The nitrogen atom in ammonia and amine molecules can accept a proton (H+ ion)
  • They can therefore act as bases in aqueous solutions by donating its lone pair of electrons to a proton and form a dative bond
    • For example, ammonia undergoes an acid-base reaction with dilute hydrochloric acid (HCl) to form a salt

NH3 + HCl → NH4+Cl

base      acid            salt

Nitrogen Compounds - Ammonia and Amines as Bases, downloadable AS & A Level Chemistry revision notes

The nitrogen atom in ammonia and amines can donate its lone pair of electrons to form a bond with a proton and therefore act as a base

 Strength of ammonia and amines as bases

  • The strength of amines depends on the availability of the lone pair of electrons on the nitrogen atom to form a dative covalent bond with a proton
  • The more readily this lone pair of electrons is available, the stronger the base is
  • Factors that may affect the basicity of amines include:
    • Positive inductive effect – Some groups such as alkyl groups donate electron density to the nitrogen atom causing the lone pair of electrons to become more available and therefore increasing the amine’s basicity
    • Delocalisation – The presence of aromatic rings such as the benzene ring causes the lone pair of electrons on the nitrogen atom to be delocalised into the benzene ring
    • The lone pair becomes less available to form a dative covalent bond with ammonia and hence decreases the amine’s basicity
  • For example, ethylamine (which has an electron-donating ethyl group) is more basic than phenylamine (which has an electron-withdrawing benzene ring)

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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