Carbonyl compounds (Aldehyde and ketones)

Question 1

Physical properties of aldehydes and ketones

Answer 1

Structure: Simple molecular structure
1. Higher BP than alkanes (pdpd>idid)
2. BP increases down the group
3. Soluble in water (Hydrogen bond between carbonyl molecule and water > pdpd between carbonyl molecules and hydrogen bond between water molecules)
4. Soluble in organic solvents
(Idid between carbonyl molecule and organic solvent molecule > idid between organic solvent molecules and pdpd between carbonyl molecules)

Question 2

Synthesis of aldehydes and ketones

Answer 2

1. Oxidation of primary alcohol to aldehyde
2. Oxidation of secondary alcohol to ketone
3. Oxidative cleavage of alkenes

Question 3

Oxidation of primary alcohol to aldehyde

Answer 3

K2Cr2O7, H2SO4(aq), heat with immediate distillation

Question 4

Oxidation of secondary alcohol to ketone

Answer 4

K2Cr2O7, H2SO4(aq), heat under reflux OR
KMnO4, H2SO4(aq), heat under reflux

Question 5

Oxidative cleavage of alkenes

Answer 5

Hot KMnO4, H2SO4(aq)

Question 6

Reactions of aldehydes and ketones

Answer 6

1. Nucleophilic addition of aldehydes and ketones
2. Reduction of aldehydes and ketones
3. Oxidation of aldehydes to form carboxylic acid

Question 7

Nucleophilic addition of aldehydes and ketones

Answer 7

HCN, trace NaCN or NaOH, cold (10-20 degrees)
–> step-up reaction

Question 8

Role of NaCN in nucleophilic addition of aldehydes and ketones

Answer 8

• HCN is a weak acid and ionises partially in water
• NaCN is a salt which completely dissociates to from free CN- ions

Question 9

Reduction of aldehydes and ketones

Answer 9

LiAlH4 in dry ether OR
H2 gas, Ni catalyst, heat OR
NaBH4 in methanol

Question 10

Why LiAlH4 and NaBH4 cannot reduce alkenes?

Answer 10

C=C in alkenes are electron rich due to presence of 2 pi electrons in double bond, which will repel incoming H- nucleophile

Question 11

Why is LiAlH4 a stronger reducing agent than NaBH4?

Answer 11

• Al less EN than B
• Al-H bond more polar than B-H bond
• H- nucleophile more readily produced

Question 12

Oxidation of aldehydes

Answer 12

K2Cr2O7/KMnO4(aq), H2SO4(aq), heat under reflux

Question 13

Distinguishing tests for aldehydes and ketones

Answer 13

1. 2,4-DNPH
   Aldehydes and ketones: Orange ppt of 2,4-dinitro-phenylhydrazone
2. Tollen’s reagent [Ag(NH3)2]+, heat
   Aldehydes: Silver mirror of Ag
   Ketones: No silver mirror of Ag
3. Fehling’s solution Cu2+
   Aliphatic aldehyde (alkyl chain): Red ppt of Cu2O
   Aromatic aldehyde and ketones: No red ppt

Question 14

Why are aldehydes more reactive than ketones in nucleophilic addition reactions?

Answer 14

1. Electronic factor:
   - Aldehydes have one electron-donating alkyl group while ketones have two electron-donating alkyl groups
   - Carbonyl carbon of aldehydes have higher partial positive charge, making it more susceptible to nucleophilic attack
2. Steric hindrance
   - Aldehydes have one bulky alkyl group attached to carbonyl carbon while ketones have two bulky alkyl group, which will less likely hinder the nucleophilic approach as it faces lesser steric hindrance