Carbonyl Compounds Flashcards
Describe the reduction of aldehydes using NaBH4.
Aldehydes are reduced to primary alcohols by NaBH4.
Describe the reduction of ketones by NaBH4.
Ketones are reduced by NaBH4 to form secondary alcohols.
Outline the mechanism for nucleophilic addition reactions of aldehydes and ketones with hydrides (eg NaBH4)
- the hydride ion attacks the delta positive carbon atom and forms a bond.
- an intermediate forms.
- the intermediate donates an electron pair to the H atom of a H2O molecule, forming a dative covalent bond with it and OH-.
- the organic product is an alcohol.
Describe the use of 2,4-dinophenylhydrazine
- aldehydes and ketones can be detected shining 2,4-DNPH.
- it is mixed with methanol and sulphuric acid to make “Brady’s reagent”.
- when added to an aldehyde or a ketone, a yellow/orange precipitate is formed. This precipitate is called the 2,4-DNPH derivative.
- to identify what carbonyl compound it is, the precipitate is purified, the melting point is taken and it is compared with known values.
What is tollen’s reagent?
Ammoniacal silver nitrate.
What is tollen’s reagent used for?
Once a compound has been identified as a carbonyl compound by 2,4-DNPH, the compound can be further identified as either an aldehyde or a ketone.
Aldehydes are easily oxidised by tollen’s reagent.
Ketones are not oxidised by tollen’s reagent.
How is tollen’s reagent made?
- aqueous sodium hydroxide + aqueous silver nitrate until a brown precipitate of silver oxide forms.
- dilute ammonia added until the precipitate dissolves
Describe the oxidation of an aldehyde by tollen’s reagent.
Aldehyde + [0] -> carboxylic acid
eg: CH3CHO + [0] -> CH3COOH
Silver ions are also reduced to form silver metal in a redox reaction.
Ag+(aq) + e- -> Ag(s)
What’s the difference between a ketone and an aldehyde?
In a ketone, the carbonyl group (C=O) can be on ANY position on the chain except at the end.
In an aldehyde, the carbonyl group is at the end of the chain.
