26.2 Reactions of carbonyl group in aldehydes and ketones Flashcards
Many reactions are nucleophilic addition reactions with hydrogen cyanide or potassium cyanide
What is the product
Explain why this reaction isn’t carried out in a lab
. HCl is added after the KCN ions to acidify it
. Hydroxynitriles are the products
CN- ions are toxic
Describe nucleophilic addition of KCN- ions with propanal
. The CN- ions have a lone pair so give it to the C f+
. Then as a result the C=O bond breaks so arrow goes from bond to oxygen
. Then the O:- now has a lone pair that goes to the H+ ions from the HCl acid
. So a hydrogen joins the molecule
. The product is
2-hydroxybutanenitrile
Is the product of this reaction racemic
Yes it is a racemic mixture of the two optical isomers (enantiomers)
This is because the CN- ion may attack from above or below the flat C=O group
Why is this reaction important in organic synthesis
Because it allows you to add a carbon to the chain
Describe oxidation of aldehydes and ketones
Aldehydes can be oxidised to carboxylic acids but ketones can’t
Ketones can’t because a C-C bond will have to break for reaction to occur as there are no C-H bonds
However very strong oxidising agents may be able to oxidise them
What is Fehling’s solution
Fehling’s solution is a mix of two solutions, Fehling’s A and Fehling’s B
A contains Cu2+ ions so makes it blue, whilst B contains an alkali and complexing agent
How does Fehling’s solution test for aldehydes and ketones
Warm aldehyde with fehling’s solution and a brick red precipitate of copper oxide will form as the copper (I) oxidises the aldehyde to carboxylic acid and reduces itself to copper (I)
Ketones don’t react so solution remains blue
What complex does Tollens reagent contain
[Ag(NH3)2]+ which is formed when aqueous ammonia is added to a solution of aqueous silver nitrate
How does tollens reagent react with aldehydes and ketones
With aldehydes, a silver mirror will form because the aldehyde can be oxidised to a carboxylic acid.
So the Ag+ ions are reduced to metallic silver
[Ag(NH3)2]+ + e- —> Ag + 2NH3
Ketones don’t react because they can’t be oxidised
What do reducing agents do to aldehydes and ketones
They reduce them to primary and secondary alcohols
How does reducing agent NaBH4 work
In aqueous solution, it releases :H- ions called hydride ions
It reduces the C=O bond which is polar, as it is attracted to C f+ but it doesn’t reduce C=C bond due to high electron density in it
Describe mechanism for reducing an aldehyde
Nucleophilic addition
and ketone
The :H- ion gives its lone pair to the C f+ in the C=O bond, and so one of these double bonds break and give lone pair to oxygen
Then there are H+ ions in solution because it is acidified beforehand so the oxygen is now :O- so this lone pair is given to a H+ ion.
So they join to make OH
This makes a primary alcohol
Overall, the reaction is aldehyde + 2[H] –> RCH2OH
Ketone is the same except the OH ends up in the middle of carbon chain
