A2- Aldehydes and Ketones
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What is a carbonyl group?
C=O
Where is the carbonyl group in an aldehyde?
At the end of the C chain
Where is the carbonyl group located on a ketone?
in the middle of the C chain
What are aldehydes oxidised to?
carboxylic acids
What is butanal oxidised to?
Butanoic acid
What are ketones oxidised to?
NOTHING
What test can be used to distinguish between an aldehyde and a ketone?
Tollens’ -> positive for aldehyde = silver mirror
Why does Tollens’ reagent allow you to distinguish between aldehydes and ketones?
aldehydes can be oxidised, so can act as the reducing agent for [Ag(NH3)2], so it essentially becomes Ag. Ketones cannot be reduced, so there is no change
What reducing agent is used to reduce aldehydes and ketones?
NaBH4 (sodium tetrahydrifoborate)
What is the nucleophile in the reducing agent used in the nucleophilic addition of aldehydes and ketones?
the :H- in the NaBH4
What happens to the lone pair on the oxygen atom (in the intermediate stage)?
the electrons are donated to a H+, which forms an alcohol
Where is the - drawn on a cyanide ion?
on the C :-CN
What can be added as a source of :-CN ions?
KCN
Why can nucleophilic attack happen on carbonyl groups?
due to the difference in electronegativity between C and O
What is formed when an aldehyde or ketone reacts with KCN?
a hydroxynitrile (eg. 2-hydroxybutanenitrile)
How would you oxidise an aldehyde?
heat with K2Cr2O7 under reflux (forms COOH, turns orange to green)
Why aren’t ketones readily oxidised but aldehydes are?
as no H readily available
Why doesn’t NaBH4 reduce C=C?
• H- atom • C=C is electron dense • so repulsion