Tests for Aldehydes + Ketones
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What is the key chemical difference between aldehydes and ketones in oxidation reactions?
Aldehydes can be oxidised to carboxylic acids, but ketones cannot be oxidised by gentle oxidising agents
What does an aldehyde become when gently oxidised?
A carboxylic acid (RCHO + [O] → RCOOH
Write the letter equation showing the oxidation of aldehyde and its final product
(RCHO + [O] → RCOOH
Outline the Tollens’ test when it tests for aldehyde
An aldehyde and Tollens’ reagent is added to a test tube and warmed up. If the test is positive a deposit of silver mirror forms
What happens when an aldehyde is warmed with Tollen’s reagent?
A silver mirror forms as Ag+ is reduced to Ag(s)
Outline the Tollens’ test, when it tests for ketones
A ketone and Tollens’ reagent is added to a test tube, then warmed in a water bath. If ketone is present, there will be no visible change so the solution will remain colourless
Why doesn’t Tollens’ reagent react with ketones?
Tollens’ reagent does not react with ketones, because it cannot be easily oxidised under mild conditions
What is Fehling’s reagent used for?
To distinguish aldehydes from ketones by oxidising aldehydes
Outline the Fehling’s test, when it tests for aldehydes
An aldehyde and Fehling’s reagent is added to a test tube, the tube is then warmed If the test is positive, a brick red precipitate forms
What is observed when an aldehyde reacts with Fehling’s solution?
The blue solution turns brick red due to precipitate of Cu2O (copper (I) oxide)
Outline the Fehling’s test, when it test for ketones
Add a ketone and Fehling’s reagent to a test tube, then warm the solution in a water bath If test is positive, there will be no visible change, the solution will remain blue
What happens when a ketone is tested with Fehling’s or Tollens’ reagent?
No reaction occurs, the solution remains unchanged