Carbonyls Flashcards
1
Q
describe the oxidation of an aldehyde
A
- aldehydes can be oxidised to carboxylic acids
- reflux with potassium dichromate (K2Cr2O7) and dilute sulfuric acid
2
Q
How to make tollens reagent?
A
- add silver nitrate solution (colourless) to a test tube
- add a few drops of NaOH
- pale brown precipitate formed
- add few drops of dilute ammonia until precipitate dissolves.
3
Q
How to distinguish between an aldehyde and a ketone ?
A
- Add aldehyde/ketone to tollens reagent
- place in a hot water bath
- don’t use a Bunsen burner because aldehydes and ketones are flammable
- If aldehyde is present: tollens reagent reduced to silver mirror + aldehyde oxidised to CA
- if ketone present : no precipitate formed.
4
Q
How to test for aldehydes and ketones?
A
- 2,4-DNP (BR)
- brady’s reagent dissolved in concentrated sulfuric acid and methanol
- added to the substance
- if carbonyl group exists a bright orange precipitate is formed.
- only reacts with C=O in ketones + aldehydes NOT in carboxylic acids.
5
Q
Describe the reduction of ketones and aldehydes
A
- NaBH4 (sodium borohydride) is used as a reducing agent
- NaBH4 is dissolved in methanol and water
- aldehyde is reduced to a primary alcohol
- ketone is reduced to a secondary alcohol.
6
Q
What is the mechanism of reduction of aldehydes and ketones?
A
- nucleophilic addition
H- ion comes from reducing agent (C+ and O-) - H+ from water added to reducing reagent then reacts with intermediate.
- primary/secondary alcohol formed
7
Q
What is a hydroxnitriles?
A
- molecule that contains OH and CN groups
8
Q
Describe cyanide reactions with carbonyl groups
A
- potassium cyanide reacts with carbonyl compounds to form hydroxy nitriles
- nucleophile CN- attack the C=O
- KCN is used to form CN- ions. when dissolved in acidic solution(sulfuric acid) it dissociates into K+ and CN-
9
Q
What is the mechanism between cyanide and a carbonyl compound?
A
- Nucleophilic addition
- C is attacked by CN-
- oxygen reacts with H+ (from acidified solution)
10
Q
What are the risks + preventions of potassium cyanide?
A
- is an irritant
- dangerous is ingested/inhaled
- When KCN reacts with moisture it can form the toxic gas HCN
- to prevent : wear gloves, safety goggles, use a fume cupboard (prevent exposure to toxic fumes)
11
Q
Why is the reaction with HCN useful
A
- increases the length of the carbon chain