Carbonyls

Question 1

describe the oxidation of an aldehyde

Answer 1

• aldehydes can be oxidised to carboxylic acids

- reflux with potassium dichromate (K2Cr2O7) and dilute sulfuric acid

Question 2

How to make tollens reagent?

Answer 2

• 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.

Question 3

How to distinguish between an aldehyde and a ketone ?

Answer 3

• 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.

Question 4

How to test for aldehydes and ketones?

Answer 4

• 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.

Question 5

Describe the reduction of ketones and aldehydes

Answer 5

• 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.

Question 6

What is the mechanism of reduction of aldehydes and ketones?

Answer 6

• 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

Question 7

What is a hydroxnitriles?

Answer 7

• molecule that contains OH and CN groups

Question 8

Describe cyanide reactions with carbonyl groups

Answer 8

• 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-

Question 9

What is the mechanism between cyanide and a carbonyl compound?

Answer 9

• Nucleophilic addition
• C is attacked by CN-
• oxygen reacts with H+ (from acidified solution)

Question 10

What are the risks + preventions of potassium cyanide?

Answer 10

• 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)

Question 11

Why is the reaction with HCN useful

Answer 11

• increases the length of the carbon chain