AQA A Level Chemistry复习笔记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:

 

7.6-Nitrogen-Compounds-Ammonia-and-Amines-as-Bases

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)

7.6-Nitrogen-Compounds-Ethylamine-_-Phenylamine

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

 

7.6-Nitrogen-Compounds-Strength-of-Bases

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.

 

 

转载自savemyexams

 

 

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