Aldehydes and ketones

Question 1

How are aldehydes and ketones produced?

Answer 1

Oxidation of primary alcohols: Aldehydes
* Warm primary alcohol added to oxidising agent
* Aldehyde forms
* Aldehyde distilled of as it has lower boiling point than alcohol
* As further oxidising would form carboxylic acid
Oxidation of secondary alcohols: Ketones
* Warm seconday alcohol added to oxidising agent
* Ketone distilled off

Oxidisinga agent: acidified K2Cr2O7 / acidified KMnO4

Question 2

State the reactions of carbonyl coumpounds

Answer 2

1. Reduction
2. Nucleophilic addition with HCN

Question 3

State the reduction reaction of carbonyls

Answer 3

REDUCTION OF ALDEHYDES AND KETONES
1. reduction of aldehydes
aldehydes + 2[H] —> primary alcohol
2. reduction of ketones
ketones + 2[H] —> secondary alcohol
Conditions: LiAlH4/ NaBH4

Question 4

catalyst, condition, property

State the nucleophilic addition of carbonyls with HCN

Answer 4

Catalyst: KCN
Condition: Heat
Property: Length of carbon chain increases by one in the end

Question 5

State the mechanism of the nucleophilic addition with HCN

Answer 5

1. C=O polarised causes partial charges beause oxygen is more electronegative
2. CN- acts as a nucleophile and is attracted to the C=O, it attackes the partialy positive carbon forming an intermediate
3. Intermediate reacts with H+ (aq) from HCN (Sometimes H+ from water or dilute acid)
4. Now electronegtaive O- attracted to Partially positive H+ forming a hydroxy group

Question 6

Two ways to identify aldehydes

Answer 6

1. Fehlings solution
2. Tollens reagant

Question 7

Fehlings solution

Answer 7

Fehlings solution has Cu2+ ions it is blue and is warmed with an aldehyde
When oxidised:
Positive test would result in:
Opaque red CU2O ppt formed:
* CU2+ —> CU+
* Aldehyde turns into carboxylic acid

Clear Blue —> Opaque Red

Question 8

Tollens reagant

Answer 8

Tollens reagant: has silver nitrate in excess ammonia

When oxidised:
Positive test would result in:
Silver mirror:
Ag+ —> Ag
Aldehyde —> Carboxylic acid

Colorless —> Silver mirror

Question 9

Explain how methyl ketones are detected.
Mention what else this reaction can detect

Answer 9

Iodoform reaction: Detects Methyl ketones and Ethanal
Reagant:
I2, NaOH (aq)
Condition: Heat
Steps:
1. Halogenation:
CH3 from methyl ketones —> CI3
2. Hydrolysis of intermediate:
Forms: RCO2Na+ CHI3 ppt
CHI3 => Yellow ppt: Tri-iodomethane

Question 10

State how both aldehydes and ketones can be tested

Answer 10

Reaction with 2,4 DNPH
* 2,4 DNPH added to solution containing aldehyde or ketone
IF positive:
Deep orange ppt formed by condensation therefore water is also product

2,4 DNPH + Carbonyl —> Deep orange ppt + Water

Question 11

Can carboxylic acids and esters be tested with 2,4 DNPH?

Answer 11

Nope, only aldehydes and ketones