Base Properties (Oxford AQA International A Level Chemistry)

Base Properties 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

• 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:

Ammonia acting as a Brønsted-Lowry 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

Ethylamine is more basic than phenylamine

Decreasing base strength from ethylamine to phenylamine