Characteristics of Carbonyl Compounds

Question 1

what can be used to identify aldehydes and ketones?

Answer 1

• 2.4-dinitrophenylhydrazine or 2,4-DNP

Question 2

what happens when 2,4-DNP reacts with aldehydes and ketones?

Answer 2

• an orange precipitate forms
• when 2,4-DNP reacts with aldehydes and ketones they form a functional group called a hydrazone

Question 3

what is 2,4-dinitrophenylhydrazine a mixture of?

Answer 3

• methanol
• sulfuric acid
  = this is known as Brady’s reagent

Question 4

what is the method to working out whether a compound has an aldehyde or ketone?

Answer 4

• add 2,4-dinitrophenylhydrazine
• an orange precipitate should form if the carbonyl is present
• filter the solution and recrystallise the product
• measure the melting point of the product
• compare the melting point to a database of known carbonyls

Question 5

what can be used to test the presence of an aldehyde?

Answer 5

• Tollens’ reagent = ammoniacal silver nitrate

Question 6

what is Tollens’ reagent?

Answer 6

a weak oxidising agent

Question 7

what happens when Tollens’ reagent is used?

Answer 7

• aldehydes are easily oxidised with Tollens’ reagent
• ketones and alcohols don’t react with Tollens’ reagent
• silver ions are reduced to silver metal, which is on the walls of the flask forming a silver mirror
• Ag+(aq) –> Ag(s)

Question 8

how is Tollens’ reagent made?

Answer 8

• aqueous sodium hydroxide is added to aqueous silver nitrate until a precipitate appears (silver oxide)
• dilute ammonia is then added until the precipitate dissolves