aldehydes and ketones Flashcards
What are aldehydes?
Carbonyl compounds with the functional group C=O
The carbonyl group is at the end of the carbon chain
What are ketones?
Carbonyl compounds with the C=O group
The carbonyl group is in the middle of the carbon chain
Why are aldehydes able to be oxidised but ketones aren’t?
In aldehydes there is a hydrogen atom attached to the carbonyl group, but in ketones, the only way to oxidise them would be to break a carbon-carbon bond so they’re not easily oxidised
What is tollen’s reagent and what can it be used for?
Colourless solution of silver nitrate dissolved in aqueous ammonia. When heated in a test tube with an aldehyde it forms a silver mirror as the aldehyde is oxidised
What is Fehling’s solution?
A blue solution of complexed copper(II) ions dissolved in NaOH. If heated with an aldehyde the copper ions are reduced to a brick red precipitate of copper(I) oxide
What reducing agent is used to reduce aldehydes and ketones?
NaBH4 - sodium tetrahydridoborate
What are ketones reduced to?
Secondary alcohols
What are aldehydes reduced to?
Primary alcohols
Describe a nucleophilic addition mechanism:
- C=O bond is polar so the carbon is ∂+ and attracts the negatively charged lone pair on the H- ion
- H- ion donates its lone pair and forms bond with the ∂+ carbon
- The C=O bond breaks and the lone pair goes to the O atom
- Oxygen donates its electrons to a H+ ion (usually from water or an added weak acid)
What is a hydroxynitrile?
Molecules which contain a hydroxyl group (OH) and a nitrile group (CN)
How are hydroxynitriles produced?
By reacting aldehydes or ketones with potassium cyanide, followed by dilute acid. This is also a nucleophilic addition reaction. Can also you hydrogen cyanide, which is a source of H+ ions so doesn’t need to be followed with dilute acid
Why will hydroxynitriles form a racemic mixture?
The C=O bond is planar so when the nucleophiles attack the aldehyde/ketone they can attack from each side of the bond, producing enantiomers
