Edexcel A Level Chemistry:复习笔记7.5.3 Amine Basicity

• The nitrogen atom in ammonia and amine molecules can accept a proton (H+ ion)
• They can therefore act as Bronsted-Lowry bases in aqueous solutions by donating its lone pair of electrons to a proton and form a dative bond

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

• 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
• Primary aliphatic amines are stronger bases than ammonia as the alkyl groups are electron releasing and push electrons towards the nitrogen atom and so make it a stronger base
• Secondary amines are stronger bases than primary amines because they have more alkyl groups that are substituted onto the nitrogen atom in place of hydrogen atoms
  • Therefore more electron density is pushed onto the nitrogen atom (as the inductive effect of alkyl groups is greater than that of hydrogen atoms)
    

Base strength of aromatic amines

• Primary aromatic amines such as phenylamine do not form basic solutions because the lone pair of electrons on the nitrogen delocalise with the ring of electrons in the benzene ring
• This means the nitrogen is less able to accept protons
• 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