Topic 17: Carbonyl Compounds

Question 1

Definitions I:
(a) Aldehyde
(b) Ketone
(c) Nucleophilic addition

Answer 1

Definitions I:
(a) Carbonyl compound with C=O at chain end, e.g., RCHO.
(b) Carbonyl compound with C=O within chain, e.g., RCOR’.
(c) Reaction where nucleophile adds to C=O, forming new bonds.

Question 2

Definitions II:
(a) Carbonyl compound
(b) Hydroxynitrile
(c) 2,4-DNPH

Answer 2

Definitions II:
(a) Organic compound with a C=O group, e.g., aldehydes, ketones.
(b) Compound with -OH and -CN on same carbon, e.g., RCH(OH)CN.
(c) 2,4-Dinitrophenylhydrazine, detects carbonyls with orange precipitate.

Question 3

Write equations for producing:
(a) CH₃CHO from ethanol [1 Mark]
(b) CH₃COCH₃ from propan-2-ol [1 Mark]

Answer 3

(a) CH₃CH₂OH + [O] → CH₃CHO + H₂O (K₂Cr₂O₇/H⁺, distil) (1 mark)
(b) CH₃CH(OH)CH₃ + [O] → CH₃COCH₃ + H₂O (KMnO₄/H⁺, distil) (1 mark)

Question 4

In a lab, ethanal reacts with HCN (KCN, heat). Show the mechanism with curly arrows. [3 Marks]

Answer 4

CN⁻ attacks C=O, arrow from CN⁻ to C (1 mark)
O⁻ forms, arrow from C=O to O (1 mark)
H⁺ adds, CH₃CH(OH)CN forms (1 mark)

Question 5

How does propanone (CH₃COCH₃) react with NaBH₄, and what’s the product? [2 Marks]

Answer 5

CH₃COCH₃ + 2[H] → CH₃CH(OH)CH₃ (NaBH₄) (1 mark)
Secondary alcohol, propan-2-ol (1 mark)

Question 6

In a test, a compound gives a silver mirror with Tollens’. Is it an aldehyde or ketone? [2 Marks]

Answer 6

Aldehyde, oxidised to RCOO⁻ (1 mark)
Ketones don’t react with Tollens’ (1 mark)

Question 7

Describe the reaction of CH₃CHO with alkaline I₂(aq) and identify the precipitate. [2 Marks]

Answer 7

CH₃CHO + 4I₂ + 6OH⁻ → CHI₃ + HCOO⁻ + 5I⁻ + 5H₂O (1 mark)
Yellow CHI₃ precipitate (1 mark)

Question 8

Write the equation for CH₃CH₂CHO + HCN (KCN, heat) forming a hydroxynitrile. [2 Marks]

Answer 8

CH₃CH₂CHO + HCN → CH₃CH₂CH(OH)CN (KCN, heat) (1 mark)
2-Hydroxybutanenitrile forms (1 mark)

Question 9

In a classroom demo, 2,4-DNPH turns orange with an unknown. What does this indicate? [2 Marks]

Answer 9

Presence of C=O group (1 mark)
Aldehyde or ketone confirmed (1 mark)

Question 10

Compare the oxidation of CH₃CH₂CHO and CH₃COCH₃ with Fehling’s solution. [3 Marks]

Answer 10

CH₃CH₂CHO + Fehling’s → CH₃CH₂COO⁻ + Cu₂O (red ppt) (1 mark)
CH₃COCH₃: no reaction, stays blue (1 mark)
Aldehydes oxidise, ketones don’t (1 mark)

Question 11

Predict the product when CH₃CHO is reduced with LiAlH₄ in dry ether. [2 Marks]

Answer 11

CH₃CHO + 2[H] → CH₃CH₂OH (LiAlH₄, dry ether) (1 mark)
Ethanol, primary alcohol (1 mark)

Question 12

How can you distinguish CH₃CHO from CH₃COCH₃ using a simple test? [2 Marks]

Answer 12

Tollens’: CH₃CHO gives silver mirror, CH₃COCH₃ doesn’t (1 mark)
Aldehyde oxidises, ketone doesn’t (1 mark)