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

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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.
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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.
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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.
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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.
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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
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7
Q

What is a hydroxnitriles?

A
  • molecule that contains OH and CN groups
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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-
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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)
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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)
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11
Q

Why is the reaction with HCN useful

A
  • increases the length of the carbon chain
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