Aldehydes + ketones

Question 1

What type of compounds are aldehydes and ketones?

Answer 1

• carbonyl
  C=O

Question 2

What are the properties of aldehydes and ketones?

Answer 2

• don’t hydrogen bond with themsleves as they don’t have a polar O-H bond.
• therefore they have low boiling points
• small ketones and aldehydes can dissolve in water.

Question 3

How can aldehydes and ketones dissolve in water?

Answer 3

• Since they don’t have polar OH group, the lone pair on the oxygen atom be used to form hydrogen bonds.

Question 4

How can you test for the presence of an aldehyde?

Answer 4

Tollens reagent - colourless solution of silver nitrate dissolved in aqueous ammonia. If it is heated with an aldehyde, a silve rmirror forms.

Fehling’s solution -
Blue solution of copper (II) ions dissolved in sodium hydroxide.
Brick - red precipitate of copper oxide
Benedict - dissolve in sodium carbonate instead

Acidified dischromate ions.

Question 5

What are aldehydes and ketones reduced to?
What is the reducing agent?

Answer 5

aldehyde - primary alcohol
ketones - secondary alcohol
reducing agent - LiALH4 (lithiumm aluminium hydride) in dry ether.
very violent (bursts into flames)

Question 6

What is the test for carbonyls?

Answer 6

2,4 dinitrophenylhydrazine
- is dissolved in methanol and concentrated sulfuric acid.
- it reacts to form a bright orange precipitate if carbonyl present.

Melting point - mp of precipitate identifies the carbonyl group

Question 7

What carbonyls react wih iodine?

Answer 7

• carbonyls that contain a methyl group react when heated with iodine in the presence of an alkali. If there is a methyl carbonyl group, you get a yellow precipitate.
  methyl carbonate must be ethanal or a ketone with one methyl group.

Question 8

What is th formula for potassium dichromate?

Answer 8

K2Cr2O7

Question 9

What is the reaction from acidified potassium dichromate?

Answer 9

Cr2O7^2- + 14H^+ +6e^- produces 2Cr^3+ + 7H2O

Question 10

What is the reaction equation for Tollens reagent?

Answer 10

Ag(NH3)2(aq)^+ + e^- produces Ag(s) + 2NH3(aq)

Question 11

What is the equation for the oxidation of copper ions in Fehling’s/ Benedict’s?

Answer 11

Cu^2+(aq) +e- produces Cu+

Question 12

What does potassium cynaide produce when it reacts with carbonyls?

Answer 12

hydroxynitiriles