Aldehydes & Ketones ORGANIC

Question 1

What do aldehydes and ketones have in common?

Answer 1

Carbonyl functional group (C=O)

Question 2

What is the difference between aldehydes and ketones?

Answer 2

Aldehyde has carbonyl group located at end of chain
Ketone has carbonyl group located within chain

Question 3

How can aldehydes and ketones be prepared?

Answer 3

By oxidising primary and secondary alcohols

Question 4

What happens during oxidation of primary alcohol?

Answer 4

Apparatus set up to distill off aldehyde as produced
Further oxidation can take place to form carboxylic acids

Question 5

How is primary alcohol oxidised straight to carboxylic acid?

Answer 5

Reaction mixture heated under reflux
Aldehyde still produced, evaporates and condenses back into reaction mixture to be further oxidised to carboxylic acid

Question 6

What oxidising agent is used for aldehydes and ketones?

Answer 6

Acidified potassium dichromate

Question 7

Why are ketones resistant to oxidation?

Answer 7

No readily available hydrogen atom

Question 8

What can be used to distinguish between aldehyde and ketone?

Answer 8

Weak oxidising agents
Aldehydes will be oxidised to carboxylic acid, ketone will not undergo oxidation

Question 9

What two reagents are used to distinguish between aldehydes and ketones?

Answer 9

Tollen’s reagent
Fehling’s solution

Question 10

What is Tollen’s reagent?

Answer 10

Aqueous ammonia in solution of silver nitrate

Question 11

What is observed when Tollen’s reagent is gently warmed with an aldehyde?

Answer 11

Aldehyde is oxidised
Silver(I) complex ions reduced to solid metallic silver to produce “silver mirror”

Question 12

What observation is a positive test result with Tollen’s reagent?

Answer 12

Silver mirror

Question 13

What is observed when Tollen’s reagent is gently warmed with a ketone?

Answer 13

No silver mirror
Ketone cannot be oxidised by Tollen’s reagent

Question 14

What is Fehling’s solution?

Answer 14

An alkaline solution containing copper(II) ions which act as oxidising agents

Question 15

What is observed when Fehling’s solution is gently warmed with an aldehyde?

Answer 15

Aldehyde is oxidised and a colour change will take place

Question 16

What colour is Fehling’s solution? Why?

Answer 16

Blue
Due to copper(II) complex ions present

Question 17

Why is a colour change seen when aldehyde is oxidised by Fehling’s solution?

Answer 17

Blue Cu²⁺ ions are reduced to Cu⁺ ions and red precipitate is formed

Question 18

What is observed when Fehling’s solution is gently warmed with an aldehyde?

Answer 18

No reaction
Ketone will not be oxidised

Question 19

What is observed when acidified potassium dichromate is heated with an aldehyde?

Answer 19

Aldehyde oxidised
Colour change orange to green

Question 20

What is observed when acidified potassium dichromate is heated with a ketone?

Answer 20

Ketone not oxidised
No colour change

Question 21

What are aldehydes and ketones both reduced to?

Answer 21

Alcohols

Question 22

What are aldehydes reduced to?

Answer 22

Primary alcohols

Question 23

What are ketones reduced to?

Answer 23

Secondary alcohols

Question 24

What is the most common reducing agent for reduction of carbonyls?

Answer 24

Sodium tetrahydridoborate, NaBH₄

Question 25

How are aldehydes and ketones reduced by NaBH₄?

Answer 25

In aqueous solution, NaBH₄ generates hydride ion nucleophile H- Hydride ion will reduce carbonyl group but not C=C

Question 26

What is the reduction of aldehydes and ketones by NaBH₄ an example of?

Answer 26

Nucleophilic addition

Question 27

Where is electron density drawn in carbonyl group?

Answer 27

Towards oxygen atom as more electronegative than carbon

Question 28

What is susceptible in aldehydes and ketones to attack by nucleophile? Why?

Answer 28

Carbonyl carbon Slightly positively charged due to more electronegative oxygen

Question 29

Describe the nucleophilic addition of hydrogen cyanide to carbonyl compound.

Answer 29

Cyanide ion attacks carbonyl carbon to form negatively charged intermediate Negatively charged oxygen atom in reactive intermediate reacts with aqueous H+ to form 2-hydroxynitrile compounds

Question 30

Why is the nucleophilic addition of hydrogen cyanide to carbonyl compounds important to organic synthesis?

Answer 30

Increases carbon chain length

Question 31

What kind of reaction mixture is produced in nucleophilic addition of hydrogen cyanide to carbonyl compounds? Why?

Answer 31

50:50 racemic mixture 50:50 chance of attack on either side