3.3.8 Aldehydes and ketones

Question 1

Whats the difference between the position of the functional group on an aldehyde and ketone

Answer 1

aldehydes have their carbonyl groups on the end carbon

ketones have their carbonyl group on an inner carbon

Question 2

How do you make tollens reagent

Answer 2

add silver nitrate solution
add few drops of NaOH (pale brown precipitate forms)
add few drops of dilute ammonia until precipitate dissolves

Question 3

How do you carry out tollens reagent testing

Answer 3

warm tollens reagent is added to aldehyde/ketone
place in hot water bath (no bunsen flame as its flammable)
if silver mirror forms then its aldehyde if not then ketone

Question 4

How do you carry out fehlings testing

Answer 4

blue fehlings solution (due to Cu2+) is added warm to aldehyde/ketone
place in hot water bath

if colour changes to form brick red precipitate its aldehyde
if no colour change then ketone

Question 5

Give an example of an agent which can reduce aldehydes and ketones

Answer 5

solution of NaBH4 (sodium borohydride) dissolved in methanol and water

Question 5

Give an example of an agent which can reduce aldehydes and ketones

Answer 5

solution of NaBH4 (sodium borohydride) dissolved in methanol and water

Question 6

How many sets of [H] (reducing agent) are required to reduce aldehydes or ketones?

Answer 6

2

Question 7

What mechanism does an aldehyde or ketone undergo when being reduced to a primary/secondary alcohol? (what are the reagents)

Answer 7

Nucleophilic addition
H- as the nucleophile coming from NaBH4
H+ from the acid or water that was added to the reducing agent in the beginning

Question 8

Draw out the mechanism for the reduction of propanal to propanol

Question 9

What is the product of KCN with carbonyl compounds?

Answer 9

hydroxynitriles

Question 10

What mechanism produces hydroxynitirles from carbonyl compounds?

Answer 10

nucleophilic addition

Question 11

How are nucleophiles formed from KCN?

Answer 11

dissolve in acidic solution
KCN dissociates to form K+ and CN- ions

Question 12

Draw the mechanism of KCN being added to propanal

Question 13

HCN can also be used to form hydroxynitriles but what is different from HCN and KCN?

Answer 13

HCN does not require acid

Question 14

what is the generic equation for an aldehyde and ketone reaction with KCN?

Question 15

if we use an unsymmetrical ketone or aldehyde (besides methanal), A MIXTURE OF ENANTIOMERS IS MADE

Question 16

What are the 2 risks that come with using KCN?

Answer 16

• its an irritant and dangerous if ingested or inhaled
• when reacts with moisture it can form HCN which is a TOXIC GAS