[26.2] identifying aldehydes and ketones

a yellow / orange precipitate is formed

• acidified potassium dichromate (VI) • tollens’ reagent

• aldehydes can be oxidised but ketones can’t • for aldehydes, the colour will change from orange to green • for ketones, it will stay orange

• ammoniacal silver nitrate • mild oxidising agent which will oxidise aldehydes but not ketones • contains the diammine silver (I) ion: [Ag(NH₃)₂]⁺

• silver (I) ion is reduced to silver • Ag⁺ (aq) + e⁻ ➞ Ag (s)

• in a clean test tube, add 3ml depth of AgNO₃ (aq) • add NaOH (aq) to the silver nitrate until a brown precipitate of Ag₂O is formed • add dilute ammonia solution dropwise until the brown precipitate just dissolves to form a clear colourless solution

• pour 2ml depth of unknown solution into a clean test tube • add an equal volume of freshly prepared tollens’ reagent • leave the test tube to stand in a beaker of warm water at about 50°C for 10-15 mins • silver mirror indicates presence of aldehyde

• add 5ml of a solution of 2,4-DNP to a clean test tube. this is in excess • using a dropping pipete, add 3 drops of the unknown compound. leave to stand • if no crystals form, add a few drops of sulfuric acid • a yellow / orange precipitate indicates the presence of an aldehyde or ketone

• filter precipitate (impure 2,4-DNP derivative) • recrystallise by dissolving impure precipitate in a minimum amount of hot solvent, cooling quickly and crystallising the pure solid • measure the melting point of the purified solid derivative using a capillary tube and a thiele tube • compare to data values