CbD - Carbonyl compounds Flashcards
How can aldehydes and ketones be made?
By oxidising alcohols.
Examples of carbonyl compounds
Aldehydes and ketones.
How do you make an aldehyde?
Gently heat a primary alcohol with an oxidising agent.
What oxidising agent is usually used when making an aldehyde?
Acidified potassium dichromate(VI).
What do aldehydes oxidise further to make?
Carboxylic acids.
What do you have to do make an aldehyde not oxidise further into carboxylic acids?
You’ve got to distil the aldehyde out of the reaction as soon as it’s formed to stop it oxidising further.
How do you make a ketone?
Reflux a secondary alcohol with an oxidising agent.
What can you use to test for aldehydes and ketones?
Fehling’s solution and Tollens reagent.
What can Fehling’s solution be used to test for?
Aldehydes and ketones.
What is Fehling’s solution?
A blue solution of complexed copper(II) ions dissolved in sodium hydroxide. Fehling’s A and Fehling’s B.
(Fehling’s A = Cu2+ weak oxidising agent. Fehling’s B = alkaline solution containing a bidentate ligand.)
What does Fehling’s solution do to aldehydes?
Oxidises them to carboxylic acids.
Result when Fehling’s solution is mixed with an aldehyde
Blue solution –> Brick red precipitate
Result when Fehling’s solution is mixed with a ketone
Nothing happens
Explain how you’d use Fehling’s solution to test for aldehydes and ketones
1) Add 2cm3 of Fehling’s solution to a test tube (or Benedict’s solution). It should be a clear blue solution.
2) Add 5 drops of the aldehyde or ketone to the test tube.
3) Put the test tube in a hot water bath to warm it for a few minutes.
Aldehyde - blue solution to brick red precipitate.
Ketone - nothing happens.
Explain what is happening when you add Fehling’s solution to an aldehyde
Blue copper(II) ions (Cu^2+) in the Fehling’s solution are reduced (the electrons come from the oxidation of the aldehyde) forming copper(I) ions (Cu+) which then form a brick red precipitate of copper(I) oxide (Cu2O).
How do you make Tollens reagent?
1) Put 2cm3 of 0.1 mol dm-3 (colourless) silver nitrate solution in a test tube.
2) Add a few drops of dilute sodium hydroxide solution. A light brown precipitate should form (silver oxide).
3) Add drops of dilute ammonia solution until the brown precipitate dissolves completely. This is now Tollens reagent.
What is Tollens reagent?
Dissolved silver(I) oxide (Ag2O) in ammonia solution to form the [Ag(NH3)2]+ (aq) complex ion.
What can Tollens reagent be used to test for?
Aldehydes and ketones.
How do you use Tollens reagent to test for aldehydes and ketones?
- Place the test tube with Tollens reagent in a hot water bath and add 10 drops of aldehyde or ketone.
- Wait for a few minutes.
Aldehyde - a silver mirror (a thin coating of silver) forms on the walls of the test tube.
Ketone - nothing happens.
Explain what is happening when you add Tollens reagent to an aldehyde
Diamminesilver ions in the Tollens reagent (the silver(I) ions) are reduced (the electrons come from the oxidation of the aldehyde) to elemental silver, which appears as a silvery layer on the inner surface of the test tube.
Why must the test tubes be heated in a water bath rather than using a bunsen when testing for the aldehydes and ketones?
Aldehydes and ketones are flammable.
Equation of what is happening when testing for aldehydes using Fehling’s solution
Cu^2+ (aq) + e- –> Cu+ (s)
What is the aldehyde oxidised into when it reacts with Fehling’s solution/Tollens reagent?
RCHO is oxidised to the carboxylate ion RCOO-.
Equation of what is happening when testing for aldehydes using Tollens reagent
[Ag(NH3)2]+ (aq) + e- –>Ag (s) + 2NH3 (aq)
Colour change when an aldehyde reacts with acidified dichromate
orange –> green
Colour change when an aldehyde reacts with Fehling’s solution
blue –> brick-red
Colour change when an aldehyde reacts with Tollens reagent
clear –> silver (silver mirror effect)
Colour change when a ketone reacts with acidified dichromate
stays orange
Colour change when a ketone reacts with Fehling’s solution
stays blue
Colour change when a ketone reacts with Tollens reagent
no change (colourless still)
What is the original colour of acidified dichromate?
orange
What is the original colour of Fehling’s solution?
blue
What is the original colour of Tollens reagent?
colourless
Through what reaction will hydrogen cyanide react with carbonyls?
Nucleophilic addition
What is produced when hydrogen cyanide reacts with carbonyl compounds?
Cyanohydrins
What are cyanohydrins?
Molecules with a -CN and an -OH group.
What is hydrogen cyanide?
A weak acid
What does it mean that hydrogen cyanide is a weak acid?
It partially dissociates in water to form H+ ions and CN- ions.
How does hydrogen cyanide react with carbonyls by nucleophilic addition?
1) The CN- group attacks the partially positive carbon atom and donates a pair of electrons. Two electrons from the double bond transfer to the oxygen.
2) H+ (from either hydrogen cyanide or water) bonds to the oxygen to form the hydroxyl group (-OH).
Why is hydrogen cyanide bad?
It is a highly poisonous gas.
How does the nucleophilic addition of hydrogen cyanide with carbonyl compounds need to be carried out?
In a fume cupboard - as it is a highly poisonous gas.
What are cyanohydrins also known as?
Hydroxynitriles.
What is the cyano- functional group (nitrite group)?
N≡C^-
What is the cyano- group as an ion in an inorganic salt called?
Cyanide
What is the cyano- group in an organic compound called?
A cyano- group or nitrile group
