AQA A Level Chemistry

Revision Notes

7.5.2 Basic Properties

Basic Properties

  • 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

  • Amines react with HCl to form amine salts, such as ethylammonium chloride

C2H5NH2 + HCl → C2H5NH3+Cl

base          acid                 salt

  • Like ammonium salts, amine salts are soluble ionic compounds, so when the alkyl group is small they are water soluble but become less so as the carbon chain increases
  • We can show the reaction in Brønsted-Lowry terms, where ammonia or the amine acts as a proton acceptor:

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 ability of the lone pair of electrons on the nitrogen atom to accept a proton and form a dative covalent bond
  • The more readily a proton is attracted, 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)

Nitrogen Compounds - Ethylamine _ Phenylamine, downloadable AS & A Level Chemistry revision notes

Ethylamine is more basic than phenylamine due to electron donating ethyl group which increases electron density on the nitrogen and makes it more attractive to protons

Nitrogen Compounds - Strength of Bases, downloadable AS & A Level Chemistry revision notes

Exam Tip

Amines have a fishy smell, which disappears as soon as you add an acid, because you are making an non-volatile salt. Decomposing fish give off amines which are produced by the bacterial decomposition of proteins.

Close

Join Save My Exams

Download all our Revision Notes as PDFs

Try a Free Sample of our revision notes as a printable PDF.

Join Now
Already a member?
Go to Top