AQA A Level Chemistry复习笔记7.5.2 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:

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)

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