6.1.8: Characteristic tests for carbonyl compounds Flashcards
What is Brady’s reagent?
An orange transparent mixture of methanol, sulfuric acid and a solution of 2,4-dinitrophenylhydrazine (2,4-DNP)
What happens when Brady’s solution is added to an aldehyde or a ketone?
a yellow/orange precipitate of 2,4-dinitrophenylhydrazone derivative is seen.
What is observed Brady’s solution is added to a carboxylic acid or an ester?
No precipitation is observed despite these compounds also having a C=O bond.
After a positive Brady’s reagent test, how can further processing help to identify specific ketones and aldehydes?
- The 2,4-dinitrophenylhydrozone derivative precipitate can be collected by filtration and purified using recrystallisation.
- After drying, the accurate melting point of the pure product can then be measured through experiment.
- The aldehyde or ketone can be identified by comparing the melting point of the 2,4-dinitrophenylhydrazone derivative with a database.
Why is identifying an aldehyde or ketone by comparing the melting point of the 2,4-dinitrophenylbenzene derivative precipitate easier and more accurate than comparing the specific melting point of the pure chemical?
- Although every pure chemical has a specific melting and boiling point that would allow identification, this is experimentally difficult for similar ketones.
- Ketones with a similar chain length have very similar boiling points, making it challenging to experimentally distinguish between them.
- However, the 2,4-dinitrophenylhydrazone derivatives have very different melting points.
What is Tolen’s reagent also known as and what colour is it?
ammoniacal silver nitrate
Colourless chemical
Tollens reagent is made in a two-stage process. What are the two steps?
- Sodium hydroxide solution is added to silver nitrate solution until a brown precipitate is formed.
- Dilute ammonia is added drop-wise until the brown precipitate is redissolved.
Why can Tollens reagent be used to distinguish between an aldehyde and a ketone?
-It is a weak oxidising agent and can react with the carbonyl functional group in an aldehyde but not a ketone.
What happens when Tollens reagent is added to an aldehyde?
- When Tollens reagent is added to an aldehyde, a silver mirror is observed.
- A redox reaction occurs.
- The silver ions are reduced.
- The aldehyde functional group is oxidised.
What is the balanced symbol equation for the reduction of the silver ions?
Ag+(aq) + e- –> Ag(s)
Why does a silver mirror form on the inside of the reaction vessel?
The reaction causes silver metal to be precipitated out.
As the silver ions are being reduced, simultaneously, the aldehyde functional group is oxidised to a carboxylic acid. Write the equation of this. Summarise the oxidising agent as [O]
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