Aldehydes and Ketones ( 3.3.8)

Question 1

What can aldehydes be oxidised to

Answer 1

Carboxylic acids

Question 2

What can ketones oxidise to

Answer 2

they can’t get oxidised any further

Question 3

What do aldehydes get reduced to

Answer 3

A primary alcohol

Question 4

What does a ketone get reduced to

Answer 4

A secondary alcohol

Question 5

What condition should oxidising a primary alcohol to an aldehyde be in

Answer 5

Reagent : K2Cr2O7 ( acidified potassium dichromate ) or H2SO4 ( dilute sulfuric acid )
Condition : heat and distil off product

Question 6

What condition should oxidising a primary alcohol to a carboxylic acid be in

Answer 6

reagent : K2Cr2O7 ( acidified potassium dichromate ) or H2SO4 ( dilute sulfuric acid )
condition : reflux

Question 7

What condition would oxidising an aldehyde into a carboxylic acid be in

Answer 7

Reagent :
K2Cr2O7 ( acidified potassium dichromate )
H2SO4 ( diltute sulfuric acid )
Or Tollens reagent ( heat )
Or Fehlings solution ( heat )
Condition : heat or reflux

Question 8

What condition should oxidising a secondary alcohol to a ketone be in

Answer 8

Reagent :
K2Cr2O7 ( acidified potassium dichromate )
H2SO4 ( dilute sulfuric acid )
Condition : heat or reflux

Question 9

What is the reducing agent used to reduce aldehydes and ketones

Answer 9

NaBH4

Question 10

what is the mechanism called for reducing aldehydes and ketones

Answer 10

Nucleophilic addition

Question 11

why is nucleophilic addition used

Answer 11

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).

Question 12

Why is NaBH4 used

Answer 12

acts as a source of hydride ions ( H-)

Question 13

What is the conditions needed for reduction of aldehydes and ketones

Answer 13

Reagent : NaBH4 or KCN followed by dilute acids
Conditions : aqueous , heat

Question 14

What products is formed when an aldehyde or a ketone is reduced with the addition of a CN- ion

Answer 14

Hydroxynitrile

Question 15

Why is KCN used and not HCN

Answer 15

HCN is very poisonous

Question 16

what would be formed when adding CN- ion to aldehydes and ketones

Answer 16

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

Question 17

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)

Answer 17

• Filter
• Dissolve in small volume
• of hot solvent
• Cool and filter
• Wash with cold water and dry

Question 18

EQ: State the hazard associated with the use of KCN (1)

Answer 18

toxic

Question 19

EQ : Suggest a reason , other than safety , why KCN is used instead of HCN. (1)

Answer 19

HCN is weak

Question 20

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)

Answer 20

H- ion is attracted to (delta) + C
C=C is electron rich
C=C only attacked by electrophiles

Question 21

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)

Answer 21

Reagent
Tollens reagents
Observation
Silver mirror

Question 22

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)

Answer 22

CH3CH2CHOCH3 + 2 [H] => CH3CH2CH(OH)CH3

Question 23

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)

Answer 23

• 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