Aldehydes and Ketones ( 3.3.8) Flashcards
What can aldehydes be oxidised to
Carboxylic acids
What can ketones oxidise to
they can’t get oxidised any further
What do aldehydes get reduced to
A primary alcohol
What does a ketone get reduced to
A secondary alcohol
What condition should oxidising a primary alcohol to an aldehyde be in
Reagent : K2Cr2O7 ( acidified potassium dichromate ) or H2SO4 ( dilute sulfuric acid )
Condition : heat and distil off product
What condition should oxidising a primary alcohol to a carboxylic acid be in
reagent : K2Cr2O7 ( acidified potassium dichromate ) or H2SO4 ( dilute sulfuric acid )
condition : reflux
What condition would oxidising an aldehyde into a carboxylic acid be in
Reagent :
K2Cr2O7 ( acidified potassium dichromate )
H2SO4 ( diltute sulfuric acid )
Or Tollens reagent ( heat )
Or Fehlings solution ( heat )
Condition : heat or reflux
What condition should oxidising a secondary alcohol to a ketone be in
Reagent :
K2Cr2O7 ( acidified potassium dichromate )
H2SO4 ( dilute sulfuric acid )
Condition : heat or reflux
What is the reducing agent used to reduce aldehydes and ketones
NaBH4
what is the mechanism called for reducing aldehydes and ketones
Nucleophilic addition
why is nucleophilic addition used
The C= O bond is polar
O is more electronegative than the C . This means that the C is susceptible to attack by nucleophiles ( electron pair donors).
Why is NaBH4 used
acts as a source of hydride ions ( H-)
What is the conditions needed for reduction of aldehydes and ketones
Reagent : NaBH4 or KCN followed by dilute acids
Conditions : aqueous , heat
What products is formed when an aldehyde or a ketone is reduced with the addition of a CN- ion
Hydroxynitrile
Why is KCN used and not HCN
HCN is very poisonous
what would be formed when adding CN- ion to aldehydes and ketones
Racemic mixture ( 50/50 mixture of the two enantiomers)
This is due to the planar nature of the C=O group , the CN- ion had an equal chance of attaching the C (+) from above or below
EQ : This question is about ketones .
Solution X reacts with liquid ketones to form a crystalline solid.
This reaction can be used to identify a ketone if the crystalline solid is separated , purified by recrystallisation , and the melting point determined.
Describe how the crystalline solid is separated and purified (5)
• Filter
• Dissolve in small volume
• of hot solvent
• Cool and filter
• Wash with cold water and dry
EQ: State the hazard associated with the use of KCN (1)
toxic
EQ : Suggest a reason , other than safety , why KCN is used instead of HCN. (1)
HCN is weak
EQ : Aqueous NaBH4 reduces aldehydes but does not reduce alkenes.
Explain why NaBH4 reduces 2-methylbutanal but has no reaction with 2-methyl but-1-ene. (3)
H- ion is attracted to (delta) + C
C=C is electron rich
C=C only attacked by electrophiles
EQ : a student attempted to reduce a sample of 2-methylbutanal but added insufficient NaBH4.
The student confirmed that the reduction was incomplete by using a chemical test.
Give the reagent and observation for the chemical test (2)
Reagent
Tollens reagents
Observation
Silver mirror
EQ : Butanone is reduced in a two step reaction using NaBH4 followed by dilute hydrochloric acid .
Write an overall equation for the reduction of butanone using [H] to represent the reductant (1)
CH3CH2CHOCH3 + 2 [H] => CH3CH2CH(OH)CH3
EQ : Ethanol can be oxidised by potassium dichromate to ethanoic acid in a two step process. In order to ensure that the oxidation to ethanoic acid is complete , the reaction is carried out under reflux . Describe what happens when a reaction mixture is refluxed and why it is necessary in this case for complete oxidation to ethanoic acid . (3)
- mixture of liquids is heated to boiling point for a prolonged time.
- vapour is formed which escapes from the liquid mixture , is changed back into liquid and is returned to mixture .
- Any ethanol or ethanal that initially evaporates can then be oxidised
