Carbonyls
Chemistry: Cambridge International A Level, topic 3.5.1
Aldehydes and Ketones
The carbonyl functional group is a carbon double bonded to an oxygen atom. It is present in aldehydes, ketones, carboxylic acids, and amides.
Redox behaviour
- Aldehydes can be oxidised to carboxylic acids or reduced to primary alcohols.
- Ketones cannot be oxidised but can be reduced to a secondary alcohol.
Identifying aldehydes and ketones
- There are three tests: Benedict’s solution, Fehling’s solution, or Tollen’s reagent.
- Benedict’s and Fehling’s solution will turn from blue to brick red in the presence of an aldehyde, but do nothing with a ketone.
- Tollen’s reagent will coat the test-tube in a silver mirror in the presence of an aldehyde, but stay clear with a ketone.
Carbonyls and Reduction
Carbonyls can be reduced to alcohols.
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Hydride reduction
- The mechanism for the reduction of an aldehyde by a hydride ion is shown.
- Note how the H− is acting as a nucleophile as it attacks the carbonyl.
- The mechanism is called a nucleophilic addition reaction.
- It is used here to reduce aldehydes and ketones to primary and secondary alcohols.
Sodium borohydride
- We get the H− from NaBH4 - sodium borohydride.
- It is called a hydride source.
Key points
- You can’t have a free hydride ion - it’s too reactive.
- The reactive ion is actually the B-H bond, but you can draw it as a H- ion in mechanisms.
- NaBH4 will only reduce carbonyls once because the alcohol is less reactive. The reasons for that are complex!
Test yourself on Carbonyls
What's a ketone reduced to?
Why can't you have a free hydride ion?