Tutorial 8 — Aldehydes and Ketones

Learning Outcomes

Identify aldehydes and ketones using chemical tests
Understand nucleophilic addition reactions
Learn the iodoform test and its applications
Compare reactivity of aldehydes and ketones

Part A: Identification Tests

Question 1

a) How would you distinguish between an aldehyde and a ketone using simple chemical tests?

b) Describe the results you would expect with:

Tollens' reagent
Fehling's solution
2,4-dinitrophenylhydrazine (2,4-DNP)

c) Which of the above tests would give a positive result with both aliphatic and aromatic aldehydes?

Question 2

Consider the following carbonyl compounds:

Ethanal (acetaldehyde)
Propanone (acetone)
Benzaldehyde
Pentan-2-one

a) Identify which compound(s) would give a positive result with each reagent:

Tollens' reagent
Fehling's solution
2,4-DNP

b) Which compounds contain the methyl ketone group (CH₃CO-)?

Extracted note: identify the presence of CH₃CO-

c) Which aldehyde is the only one that gives a positive iodoform test?

Extracted answer: Ethanal is the only aldehyde that give +ve result

Part B: Iodoform Test

Question 3

a) What is the iodoform test? Write the reaction showing the formation of iodoform (CHI₃).

b) Which of the following compounds would give a positive iodoform test?

Ethanol
Propan-2-ol
Propan-1-ol
Acetophenone
Benzophenone
Ethanal

c) What are compounds A, B, and E in the context of the iodoform test?

Extracted note: A B E — likely referring to compounds in a multiple choice or matching question

Question 4

a) Write the mechanism for the iodoform reaction starting from ethanol.

b) Explain why the iodoform test is specific for compounds containing the CH₃CH(OH)- or CH₃CO- groups.

c) A compound C₄H₈O gives a positive iodoform test but does not reduce Tollens' reagent. Identify the compound.

Part C: Nucleophilic Addition Reactions

Question 5

a) Explain why aldehydes are generally more reactive than ketones in nucleophilic addition reactions.

b) Arrange the following in order of increasing reactivity toward nucleophilic addition:

Formaldehyde
Acetaldehyde
Acetone
Benzaldehyde
Acetophenone

c) Explain your reasoning for the order.

Question 6

a) Complete the following reactions:

Ethanal + HCN →
Propanone + NaHSO₃ →
Benzaldehyde + Grignard reagent (CH₃MgBr) →
Acetaldehyde + 2,4-DNP →

b) Write the mechanism for the addition of HCN to acetone.

Part D: Classification

Question 7

a) Classify the following as aliphatic or aromatic carbonyl compounds:

Extracted note: Aliphatic

b) Define:

Aliphatic aldehyde
Aromatic aldehyde
Aliphatic ketone
Aromatic ketone

c) Give one example of each type.

Part E: Advanced Reactions

Question 8

a) Explain the Wolff-Kishner reduction and Clemmensen reduction. When would you use each?

b) What is the Cannizzaro reaction? Give an example.

c) Write the mechanism for the Aldol condensation between two molecules of acetaldehyde.

Key Concepts

Aldehyde — Carbonyl compound with -CHO group
Ketone — Carbonyl compound with R-CO-R' structure
Nucleophilic Addition — Characteristic reaction of carbonyl compounds
Iodoform Test — Test for CH₃CO- or CH₃CH(OH)- groups
Tollens' Reagent — [Ag(NH₃)₂]⁺ used to test for aldehydes
Fehling's Solution — Cu²⁺ complex used to test for reducing sugars/aldehydes

Quick Reference: Chemical Tests

Compound	Tollens'	Fehling's	2,4-DNP	Iodoform
Aliphatic aldehyde	✓	✓	✓	Sometimes
Aromatic aldehyde	✓	✗	✓	Sometimes
Methyl ketone	✗	✗	✓	✓
Other ketones	✗	✗	✓	✗

FAD1018 Tutorial 8 — Aldehydes and Ketones