Aldehydes and Ketones: Chemical Tests Flashcards
Can aldehydes be oxidised?
- Yes, oxidised to form carboxylic acids
Can ketones be oxidised?
- Ketones are not easily oxidised
Why are ketones not easily oxidised?
- Oxidising ketones requires the breakdown of a carbon-carbon bond
Write a general equation, using structural formula, to show the oxidation of an aldehyde
- RC(=O)H + [O] → RC(=O)OH
Which chemical tests are used to distinguish between aldehydes and ketones?
- Acidified potassium dichromate (VI)
- Tollens’ reagent
- Fehling’s solution
What is Tollens’ reagent?
- A colourless solution of silver nitrate dissolved in aqueous ammonia
What can you observe during the Tollens’ reagent chemical test? Do both aldehydes and ketones lead to a positive result?
- Heat test tube with an aldehyde and Tollens’ reagent
- Solution turns from colourless to forming a silver mirror
- Ketones don’t react with Tollens’ reagent
Give an equation that shows how Tollens’ reagent reacts with aldehydes
- Ag(NH3)2^+ (aq) + e- → Ag(s) + 2NH3(aq)
- Diaminesilver ions in Tollens’ reagent are reduced
- Electrons come from oxidation of aldehyde
What is Fehling’s solution?
- Blue solution of complexed copper(II) ions dissolved in sodium hydroxide
What can you observe during the Fehling’s solution chemical test? Do both aldehydes and ketones lead to a positive result?
- Heat test tube with aldehyde and Fehling’s solution
- Solution turns from blue to forming a brick-red precipitate
- Ketones don’t react with Fehling’s solution
Give an equation that shows how Fehling’s solution reacts with aldehydes
- Cu2+(aq) + e- → Cu+ (s)
- Copper (II) ions in Fehling’s solution are reduced
- Electrons come from oxidation of aldehyde
- Copper (I) ions form a brick-red precipitate of copper (I) oxide
What happens to the ions in both Tollens’ reagent and Fehling’s solution when reacted with aldehydes?
- Ions are reduced
