Aldehydes and ketonesEdexcel A-Level Chemistry: Mind map

Exam questions on Aldehydes and ketones

1. A forensic analyst is given two colourless liquids. One is propanal and the other is propanone, both with molecular formula C₃H₆O. She has Tollens' reagent, Fehling's solution, 2,4-dinitrophenylhydrazine solution and acidified potassium dichromate(VI) available.
   A positive result with 2,4-dinitrophenylhydrazine shows that both liquids are carbonyl compounds. Describe how the analyst could use this reagent to confirm that one of the liquids is propanone.2 marks
2. Ethanal reacts with hydrogen cyanide, HCN, in the presence of a small amount of potassium cyanide, KCN, to form a hydroxynitrile. The product contains a carbon atom bonded to four different groups, but the product mixture shows no effect on plane-polarised light.
   Explain why the product mixture does not rotate the plane of plane-polarised light.2 marks
3. A research chemist studies the reduction of propanal and propanone, and also investigates a compound X of molecular formula C₄H₈O. Compound X gives an orange precipitate with 2,4-dinitrophenylhydrazine, gives no change with Tollens' reagent and gives a pale yellow precipitate when warmed with iodine and aqueous sodium hydroxide.
   Propanal and propanone are each reduced by lithium tetrahydridoaluminate, LiAlH₄, in dry ether. State the organic product from each and explain why the ether must be dry.3 marks