CD ik: Reactions of carbonyl compounds

Question 1

• State and explain whether it is the carbonyl group in aldehydes or ketones that can be oxidised
• Give the conditions required
• Name the organic product

Answer 1

• Aldehydes, since there is a hydrogen on the carbon adjacent to the carbonyl carbon (or reverse argument: oxidising ketones would involve breaking a strong C-C bond)
• Acidified dichromate(VI) solution, reflux
• Carboxylic acid

Question 2

State and explain what is observed when 2,3-dimethyl pentanal is reacted with acidified potassium dichromate(VI) under reflux.

Answer 2

• Mixture turns from orange to green
• Orange dichromate(VI) (Cr₂O₇^2-) ions are reduced to green chromium(III) (Cr³⁺) ions

Question 3

• Is Fehling’s solution used as a test for aldehydes or a test for ketones?
• What is Fehling’s solution?

Answer 3

• Aldehydes
• Mixture of Fehling’s A, containing Cu²⁺ ions, and Fehling’s B, containing alkali + a bidentate ligand

Question 4

• Describe how Fehling’s solution is used to test for aldehydes.
• State and explain the observation which would indicate a positive result.
• State the organic product formed.

Answer 4

• Warm Fehling’s A and B with test solution
• Blue solution → brick-red precipitate. Blue Cu²⁺ ions are oxidised to Cu⁺ ions, which precipitate as red Cu₂O
• Carboxylic acid

Question 5

• Is Tollen’s reagent used as a test for aldehydes or a test for ketones?
• What is Tollen’s reagent?

Answer 5

• Aldehydes
• Solution of [Ag(NH₃)]⁺ ions formed by dissolving Ag₂O in NH₃(aq)

Question 6

• Describe how Tollen’s reagent is used to test for aldehydes.
• State and explain the observation which would indicate a positive result.
• State the organic product formed.

Answer 6

• Warm Tollen’s reagent with test solution in test tube
• Silver mirror forms on inside of test tube. Ag⁺ (really [Ag(NH₃)]⁺ ions reduced to elemental silver
• Carboxylate ion

Question 7

Fill in the table.

Answer 7

Question 8

• Draw the CN^- ion, using lines to represent the covalent bonds
• What is the ion called when in inorganic salts?
• What is it called as a functional group in an organic compound?

Answer 8

• C≡N^-
• Cyanide
• Cyano / nitrile

Question 9

• What mechanism occurs when a carbonyl compound reacts with a cyanide ion?
• What is the name of the product?

Answer 9

• Nucleophilic addition
• Cyanohydrin (so called since cyano and hydroxyl group on same carbon)

Question 10

Draw the mechanism for the reaction between hydrogen cyanide and:

• ethanal
• benzaldehyde

Answer 10