Aldehydes and ketones

Question 1

Why is is preferable to use KCN over HCN? [2]

Answer 1

Both are toxic, due to the cyanide ions.
However, HCN is far more difficult to control as it’s a gas.

Question 2

When is a racemic mixture formed during nucleophilic addition of carbonyls? Why is this the case?

Answer 2

• during nuc. addition to aldehydes or unsymmetrical ketones
• the trigonal planar carbonyl is approached in equal proportionsfrom both sides by the HCN attacking species.

Question 3

HCN reacts with carbonyls

Answer 3

Reagents: acidified KCN (generates HCN in situ)

Products: Hydroxynitrile

Question 4

State the mechanism for the reaction between propanone and NaBH4.

Answer 4

Nucleophilic Addition.

Question 5

Recall and explain whether aldehydes or ketones are more reactive.

Answer 5

• Alkyl groups push electrons away, so theδ+ charge on the C is reduced;makes C more stable and less reactive
• POSITIVE INDUCTIVE EFFECT
• Aldehydes are therefore more reactive that ketones

Question 6

Recall the reduction of ketones and aldehydes. Include:
- Reagent
- Difference in reactions

Answer 6

Reagent:
NaBH4(sodium tetrahydridoborate)
OR: LiAlH4(lithium tetrahybridoaluminate - in dry ether)
Both contain the hydride ion H-

Difference:
Aldehydes are reduced to primary alcohols
Ketones are reduced to secondary alcohols

Question 7

Recall the way of distinguishing between aldehydes and ketones. State the observations for each.

Answer 7

Fehling’s Solution
Warm with Fehling’s Solution
Aldehydes form brick red precipitate
Ketones show no change

Question 8

Recall the oxidation of aldehydes and ketones. Include:
Reagent
Observation
Difference in the reaciton

Answer 8

Reagent:acidified potassium dichromate (VI)

Observation: orange to green

Difference:
- Aldehydes are oxidised to carboxylic acids
- Ketones cannot be oxidised