Amines react with aqueous copper(II) ions in two different ways, depending on how much amine is added. The reaction closely mirrors that of ammonia.Copper(II) sulfate solution contains the pale blue complex ion [Cu(H2O)6]2+.A few drops of amine. An aqueous amine such as butylamine is basic. It removes protons from two water ligands, and a pale blue precipitate of copper(II) hydroxide forms:[Cu(H2O)6]2+ + 2C4H9NH2 → [Cu(OH)2(H2O)4] + 2C4H9NH3+ Here the amine is acting as a base.Excess amine. The precipitate dissolves to give a deep blue solution. The amine now acts as a ligand: its nitrogen lone pair forms a dative covalent bond to the Cu2+ ion, replacing ligands in a ligand exchange reaction:[Cu(OH)2(H2O)4] + 4C4H9NH2 → [Cu(C4H9NH2)4(H2O)2]2+ + 2OH− + 2H2O The deep blue complex has four amine ligands and two water ligands and an overall charge of 2+. It is an octahedral complex with the same colour and shape as the ammonia complex [Cu(NH3)4(H2O)2]2+.So a single observation series shows both roles of the nitrogen lone pair: first accepting a proton (base), then donating to a metal ion (ligand).
Reactions of Amines with Copper(II) Ions
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Specification: Edexcel Chemistry (9CH0), A-level, Topic 18B: Amines, amides, amino acids and proteins. Topic: Amines, Amides, Amino Acids and Proteins.
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