Aldehydes And Ketones Flashcards
What does Tollens’ reagent test for, and what is the positive result?
= Aldehyde
+ve Silver mirror (slight heating of the aldehyde and reagent is required)
How is Fehling’s reagent made?
Mix a copper (II) containing solution with sodium hydroxide
What does Fehling’s reagent test for, and what is a positive result?
= Aldehyde
+ve Dark blue solution forms a red-brown precipitate
Why are aldehydes able to react with Tollens’ and Fehling’s but ketones are not?
The hydrogen atom of the aldehyde group is quite reactive and allows aldehydes to act as reducing agents.
How do you make Tollens’ reagent?
Add aqueous ammonia to an aqueous solution of silver nitrate until brown precipitate dissolves to give a colourless solution.
Is it possible to distinguish between aldehydes and ketones using acidified potassium dichromate (VI)?
An excess of aldehyde will turn acidified potassium dichromate solution from orange to green.
Even in excess, a ketone will not change the colour of the solution; it will remain orange.
This also applies for acidified potassium manganate (VII)
Explain why the reaction between 2,4-dinitrophenylhydrazine and aldehydes/ketones is described as a condensation reaction and give the alternative name.
An addition occurs, followed by the elimination of a water molecule, hence the name condensation. Alternative name; Nucleophilic addition-elimination reaction
What is Brady’s reagent?
2,4-DNPH dissolved in methanol and concentrated sulphuric acid, this is how it is used in reactions.
Describe the test to determine specific aldehydes and ketones using 2,4-DNPH.
- Filter and wash precipitate with methanol
- Recrystallise from a suitable solvent (a mixture of water and ethanol is suitable for smaller compounds)
- Dissolve crystals in a minimum volume of hot solvent
- Cool and re-filter to remove any soluble impurities
- Find melting temperatures and compare with book values
What is the positive result given with 2,4-DNPH?
Orange-red precipitate with aldehyde or ketone
What does the triiodomethane reaction detect?
CH3C=O group found in ketones and ethanal
Positive result is also given by CH3CH(OH) group as this is oxidised during the reaction
Describe one method of carrying out the triiodomethane reaction.
- React iodine with enough sodium/potassium hydroxide to remove the iodine colour
- Add a small amount of the test solution
(Heating may be required)
OR - Add a small amount of the test solution to an aqueous mixture of sodium (I) chlorate and potassium iodide
(Heating may be required)
What is observed for a positive result with the iodoform reaction?
Yellow precipitate of CHI3
How would you increase the rate of reaction between carbonyl compounds and HCN?
Add cyanide ions, or alkali which will neutralise HCN to produce more cyanide ions.
How would you ascend hydrogen cyanide up its homologus series? Note, in the lab hydrogen cyanide is not used as it is a poisonous gas.
React with an aldehyde or ketone; this will produce a hydroxynitrile.
In the lab, what may be used instead of hydrogen cyanide to produce a hydroxynitrile when added to a carbonyl compound?
Sodium/ potassium cyanide dissolved in water to which a little bit of sulphuric acid has been added.
Suggest how you would make an amino acid from a hydroxynitrile.
- Replace the OH group with a chlorine; by adding the hydroxynitrile to a solution of HCl and shaking at room temp.
- Convert the chlorine atom to an amine group, by reaction with ammonia.
- Hydrolyse the nitrile group to a carboxylic acid.
