Chemical Tests
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What does tollens reagent do?
Weak oxidising agent that oxidises aldehydes but not ketones. Ammoniacal silver nitrate. Forms a silver mirror. Ag+ is reduced to silver metal. Ag+ (aq) + e- —> Ag(s)
What does Brady’s reagent do?
Confirms presence of a carbonyl compound, only aldehyde or ketone. Forms yellow/orange precipitate. 2,4-diphenylhydrazine.
How can you identify the exact carbonyl compound?
2,4-DNP derivative is purified and recrystallised. The melting point is measured and recorded, then compared to a database.
How is tollens reagent made?
Aqueous silver nitrate is added to aqueous sodium hydroxide until a brown precipitate of silver oxide forms. Dilute aqueous ammonia is added until the precipitate dissolves.
How do these metals react with aqueous sodium hydroxide? Cu2+ Co2+ Fe2+ Fe3+
Cu2+ + 2OH- —> Cu(OH)2 Pale blue —> pale blue precipitate. Copper (II) hydroxide Co2+ + 2OH- —> Co(OH)2 Pink —> blue, beige in air. Colbalt (II) hydroxide Fe2+ + 2OH- —> Fe(OH)2 Pale green —> green, rusty brown when exposed to air (oxidised) Fe3+ + 3OH- —> Fe(OH)3 Pale yellow —> rusty brown
Describe the reaction if aqueous copper (II) ions and ammonia.
[Cu(H2O)6]2+ + 4NH3 <><> [Cu(NH3)4(H2O2)]2+ + 4H2O Octahedral product Pale blue —> deep blue solution Blue precipitate of Cu(OH)2 initially forms.
Describe the reaction of copper (II) ions and hydrochloric acid.
[Cu(H2O)6]2+ + 4Cl- <><> [CuCl4]2- + 6H2O Pale blue —> green —> yellow Chlorine larger than water, stronger repulsions. Tetrahedral shape, like CH4.
Describe the reaction of cobalt (II) ions and conc HCl.
[Co(H2O)6]2+ + 4Cl- <><> [CoCl4]2- + 6H2O Pink —> blue