3.3.8 aldehydes and ketones

Question 1

what does an aldehyde reduce to?

Answer 1

primary alcohol

Question 2

what does a ketone reduce to?

Answer 2

secondary alcohol

Question 3

what is the reducing agent involved in the reduction of aldehydes and ketones?

Answer 3

aqueous sodium borotetrahydride
represented by [H]

Question 4

why do we use NaBH4?

Answer 4

because it is a source of hydride ions and in aqueous solution it is a source of H+ ions

Question 5

write an equation for the reduction of butanal using [H] to represent reducing agent

Answer 5

CH3CH2CH2CHO + 2[H] — CH3CH2CH2CH2OH

Question 6

what is the name of the mechanism for reduction?

Answer 6

nucleophilic addition

Question 7

how are hydroxynitriles formed?

Answer 7

aldehydes and ketones undergo an addition reaction with hydrogen cyanide followed by addition of H2SO4

Question 8

why don’t we just use HCN instead of KCN and H2SO4 separately?

Answer 8

HCN is toxic

Question 9

what are the reagents used when forming hydroxynitriles?

Answer 9

KCN and H2SO4

Question 10

how do you distinguish between aldehydes and ketones using tollens reagent?

Answer 10

using tollens reagent
when tollens reagent is gently warmed with aldehyde the silver mirror is formed

Question 11

how do you distinguish between aldehydes and ketones using fehlings solution?

Answer 11

an aldehyde is oxidised and a colour change from a blue solution to a brick red precipitate is seen
this forms a carboxylic acid