CD.9 Flashcards
Reactions of aldehydes and ketones and their mechanisms
What is the formula of a cyanide ion?
C≡N-
What is the functional group known as when a cyanide group is present in organic molecules?
Nitrile.
What is the advantage of introducing a cyanide/nitrile group into a moleucle during synthesis?
It provides a way of lengthening the carbon chain in a molecule.
What are two things that can be done to a nitrile group once it is in a molecule?
- Hydrolysed to a carboxylic acid group.
- Reduced to an amine group.
What are two types of organic compounds that contain a carbonyl group, C=O?
Aldehyde and ketone.
What is the key difference betweent the structures of an aldehyde and a ketone?
Aldehydes contain a hydrogen atom attached to the carbonyl group, whereas a ketone doesn’t.
Can both aldehydes and ketones by oxidised?
Only aldehydes can be oxidised because it has that hydrogen atom on the carbonyl group.
What would you observe if a sample of aldehyde was heated under reflux with acidified potassium dichromate?
The solution turning from orange to green.
What would you observe if a sample of ketone was heated under reflux with acidfied potassium dichromate?
No change.
What does the acidified dichromate act as in the test for aldehydes?
An oxidising agent.
What other compounds can turn acidified dichromate from orange to green when heated under reflux?
Primary and secondary alcohols.
What are two tests for aldehydes?
Fehling’s test and Tollen’s solution.
What are aldehydes oxidised to in Fehling’s test?
Carboxylic acids.
What are three step to carrying out a Fehling’s test?
- Prepare Fehling’s solution by mixing together Fehling’s A and B.
- Add the solution containing presumablly aldehyde to the Fehling’s solution.
- Warm the solution in a water bath.
What would you observe if the solution contained an aldehyde?
The colour of the solution would change from transparent blue to cloudy brick red.
What would you observe if the solution contained a ketone?
No change, the solution remains blue.
What does Fehling’s A and B each contain?
Fehling’s A contains Cu2+ ions, which will act as a weak oxidising agent.
Fehling’s B is alkaline solution containing bidentate ligand.
What is the chemical change associated with the colour change in Fehling’s solution?
The blue Cu2+ ions are reduced to Cu+ ions with form a brick red precipitate of Cu2O.
What are aldehydes oxidised to by Tollen’s reagent?
Carboxylate ions.
How do you make a Tollen’s reagent?
By dissolving silver(I) oxide, Ag2O in ammonia solution to form a complex ion of [Ag(NH3)2]+.
How do you test an aldehyde using Tollen’s reagent?
Add the solution containing the aldehyde to the Tollen’s reagent and warm it using a water bath.
What would you observe if the solution contained an aldehyde?
A silver mirror forming on the inside of the test tube (silver-mirror effect).
What would you observe if the solution contained a ketone?
No change.
What is the chemical change associated with the silver mirror effect in Tollen’s reagent?
Silver(I) ions are reduced to elemental silver, which appears as a silvery layer on the inside of the test tube.
What compound will an aldehyde be reduced to?
Primary alcohol.
What compound will a ketone be reduced to?
Secondary alcohol.
What is required to reduce aldehydes and ketones back to alcohols?
A very powerful reducing agent such as sodium tetrahydridoborate, NaBH4, a metal hydride.
What type of reaction is it when aldehyde or ketone react with acidified cyanide?
Nucleophilic addition.
What type of compound is formed when aldehyde or ketone react with acidified cyanide?
Cyanohydrin, which contains a nitrile group and a hydroxyl group on the same carbon.
In practice, what is used instead of hydrogen cyanide, as it is too hazardous?
Acidified potassium cyanide for exmaple.
How many steps is mechanism of the nucleophilic addition between aldehyde/ketone and acidified cyanide?
2
What happens in the first step of the mechanism?
CN-, cyanide ion acts as a nucleophile and attacks the ẟ+ carbon in the carbonyl group. A carbon carbon bond is formed and a pair of electrons in the C=O moves onto the ẟ- oxygen, which now becomes negative. The bond between the carbon and the oxygen is now a single bond.
What happens in the second step of the mechanism?
The negatively charge oxygen picks up a H+ ion from the solution (acidic conditions or from HCN) and a hydroxyl group is formed.
