Chapter 12 — Carbonyl Compounds and Carboxylic Acids Exercise Solutions
This chapter is reaction-rich. Learn the chemical tests separately from the conversion reactions. Always identify the functional group first.
Key Concepts Quick Recap
| Term | Definition |
|---|
| Aldehyde (−CHO) | Carbonyl group at end of chain; electrophilic carbon |
| Ketone (C=O) | Carbonyl group in middle of chain; less reactive than aldehyde |
| Carboxylic acid (−COOH) | Acidic due to resonance stabilisation of −COO⁻ |
| Nucleophilic addition | Nu⁻ attacks electrophilic C=O carbon → addition product |
| Aldol condensation | Self-condensation of aldehydes/ketones with α-H in dil. NaOH |
| Cannizzaro reaction | Disproportionation of aldehydes without α-H (e.g. HCHO, C₆H₅CHO) |
| Clemmensen reduction | C=O → CH₂ using Zn-Hg/HCl |
| Wolff-Kishner reduction | C=O → CH₂ using N₂H₄/KOH/ethylene glycol |
| Fischer esterification | RCOOH + R'OH ⇌ RCOOR' + H₂O (H⁺ catalyst) |
| Acidity of carboxylic acids | HCOOH > RCOOH; electron-withdrawing groups increase acidity |
Important Tests
| Test | Reagent | Positive Result | What It Detects |
|---|
| Tollens' test | [Ag(NH₃)₂]⁺ (silver-ammonia complex) | Silver mirror on tube wall | Aldehydes only |
| Fehling's test | Fehling A (CuSO₄) + Fehling B (Rochelle salt) | Brick-red Cu₂O precipitate | Aliphatic aldehydes; not aromatic or ketones |
| 2,4-DNP test | 2,4-dinitrophenylhydrazine | Yellow/orange ppt | All aldehydes and ketones |
| Schiff's test | Fuchsin + SO₂ (decolourised fuchsin) | Pink/magenta colour | Aldehydes only |
| Iodoform test | I₂/NaOH | Yellow CHI₃ precipitate | CH₃CO− group (methyl ketones + ethanol) |
I. Choose the Best Answer (MCQs)
1. Tollens' reagent gives a positive test with: (a) Acetone (b) Benzaldehyde (c) Cyclohexanone (d) Diethyl ketone
Answer(b) Benzaldehyde Tollens' test is positive for aldehydes only. Benzaldehyde (C₆H₅CHO) is an aromatic aldehyde. Acetone and cyclohexanone are ketones — they do not respond.
2. The iodoform test is positive for: (a) Ethanol (b) Methanol (c) Pentan-3-ol (d) Benzyl alcohol
Answer(a) Ethanol Iodoform test is positive for CH₃CO− (methyl ketone) and CH₃CH(OH)− groups. Ethanol (CH₃CH₂OH) has the CH₃CHOH structure → oxidises to CH₃CHO → positive. Methanol (HCHO after oxidation) is negative.
3. Aldol condensation requires: (a) Acid catalyst (b) An α-hydrogen in the carbonyl compound (c) High temperature always (d) A ketone only
Answer(b) An α-hydrogen The α-carbon (carbon next to C=O) must have at least one hydrogen for aldol reaction to proceed. HCHO and C₆H₅CHO have no α-H → they undergo Cannizzaro reaction instead.
4. Which acid is stronger — formic acid (HCOOH) or acetic acid (CH₃COOH)? (a) Acetic acid (b) They are equal (c) Formic acid (d) Depends on temperature
Answer(c) Formic acid HCOOH (Ka = 1.77×10⁻⁴) > CH₃COOH (Ka = 1.8×10⁻⁵). The CH₃ group in acetic acid is electron-donating, destabilising the carboxylate ion — making it a weaker acid.
5. The product of Fischer esterification of acetic acid with ethanol is: (a) Ethyl acetate (b) Acetic anhydride (c) Methyl acetate (d) Ethyl formate
Answer(a) Ethyl acetate CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O (H⁺, heat)
6. Cannizzaro reaction is given by: (a) Acetaldehyde (b) Formaldehyde (c) Acetone (d) Propionaldehyde
Answer(b) Formaldehyde Cannizzaro reaction occurs in aldehydes without α-hydrogen. HCHO has no α-H: 2HCHO + NaOH → HCOONa + CH₃OH (disproportionation).
7. The IUPAC name of CH₃COCH₂CH₃ is: (a) Propan-2-one (b) Butan-2-one (c) 2-Methylpropanal (d) Butanal
Answer(b) Butan-2-one CH₃CO−CH₂CH₃: 4 carbon chain, keto group at C-2 → butan-2-one (methyl ethyl ketone).
II. Short Answer Questions
Q1. Explain why aldehydes are more reactive than ketones in nucleophilic addition.
Answer
In nucleophilic addition, a nucleophile attacks the electrophilic carbonyl carbon (C=O):
- Aldehydes (RCHO): One alkyl group (R) attached to C=O. Alkyl groups donate electrons slightly but do not strongly shield the C. The carbonyl carbon is relatively electrophilic.
- Ketones (RCOR'): Two alkyl groups. Greater electron donation → reduced electrophilicity of carbonyl carbon. Also, two bulky groups create steric hindrance, making nucleophilic attack harder.
Therefore, aldehydes are more reactive than ketones in nucleophilic addition.
Q2. Why are carboxylic acids more acidic than alcohols or phenols?
Answer
Carboxylic acids ionise to give carboxylate ions (−COO⁻) that are stabilised by resonance:
R−COOH ⇌ R−COO⁻ + H⁺
The negative charge in −COO⁻ is delocalised equally over both oxygen atoms (two equivalent resonance structures). This resonance stabilisation makes the carboxylate ion very stable → equilibrium shifts right → stronger acid.
Alcohols and phenols have less stable anions — alcohols (−O⁻) have no resonance; phenols have some (limited), but less than carboxylic acids.
Q3. Explain aldol condensation with a suitable example.
Answer
Aldol condensation is a reaction between two aldehyde (or ketone) molecules with α-H in the presence of dilute NaOH:
Step 1: Dil. NaOH removes an α-H → enolate ion forms. Step 2: Enolate attacks the carbonyl carbon of another molecule → β-hydroxy aldehyde (aldol product). Step 3: On heating, the aldol loses water → α,β-unsaturated carbonyl compound.
Example:
2CH₃CHO →(dil. NaOH) CH₃CH(OH)CH₂CHO (3-hydroxybutanal, the "aldol")
→(heat) CH₃CH=CHCHO + H₂O (but-2-enal, crotonaldehyde)
Q4. Write the reactions of carboxylic acids with (a) PCl₅ and (b) SOCl₂.
Answer
These reactions convert –COOH to acid chlorides (–COCl):
(a) CH₃COOH + PCl₅ → CH₃COCl + POCl₃ + HCl
(Acetyl chloride formed; POCl₃ and HCl are by-products)
(b) CH₃COOH + SOCl₂ → CH₃COCl + SO₂ + HCl
(SOCl₂ preferred in lab — gaseous by-products SO₂ and HCl escape; easy purification)
Worked Examples
Example 1 — IUPAC nomenclature
HCHO → methanal (formaldehyde)
CH₃CHO → ethanal (acetaldehyde)
CH₃COCH₃ → propan-2-one (acetone)
CH₃CH₂COCH₃ → butan-2-one (methyl ethyl ketone)
HCOOH → methanoic acid (formic acid)
CH₃COOH → ethanoic acid (acetic acid)
C₆H₅CHO → benzaldehyde
C₆H₅COOH → benzoic acid
Example 2 — Tollens' test (distinguishing aldehyde from ketone)
CH₃CHO + 2[Ag(NH₃)₂]⁺ + 2OH⁻ → CH₃COO⁻ + 2Ag↓ + 4NH₃ + H₂O
Silver mirror deposited on the inner wall of the test tube.
Ketones (CH₃COCH₃) → NO silver mirror formed.
Use Tollens' test to distinguish between acetaldehyde and acetone.
Example 3 — Iodoform test
Positive for: CH₃COR (methyl ketones) and CH₃CH(OH)R groups
CH₃COCH₃ + 3I₂ + 4NaOH → CHI₃↓ (yellow) + CH₃COONa + 3NaI + 3H₂O
Ethanol is also positive (oxidised to CH₃CHO, then iodoform forms).
Methanol (HCOOH after oxidation) → NEGATIVE.
Example 4 — Aldol condensation of benzaldehyde + acetophenone (Claisen-Schmidt)
C₆H₅CHO + CH₃COC₆H₅ →(dil. NaOH) C₆H₅CH=CHCOC₆H₅ + H₂O
(Benzaldehyde has no α-H; acetophenone provides the α-H)
Product: dibenzylideneacetone (undergoes dehydration directly)
Example 5 — Acidity order of carboxylic acids
Effect of substituents on acidity (pKa):
Cl₃CCOOH < Cl₂CHCOOH < ClCH₂COOH < CH₃COOH < HCOOH
(strongest) (weakest among these)
Rule:
- Electron-withdrawing groups (Cl, NO₂, CN) increase acidity (stabilise −COO⁻).
- Electron-donating groups (CH₃, C₂H₅) decrease acidity (destabilise −COO⁻).
- Effect decreases with distance from −COOH.
Conversion Pattern
For organic conversion questions: 1. Identify the functional group in starting material and product. 2. Find the key reaction that changes that group. 3. Write the reagent and condition. 4. Show the product clearly.
Example: CH₃OH → CH₃CHO → CH₃COOH
- CH₃OH →(PCC or CrO₃) CH₃CHO (primary alcohol to aldehyde — controlled oxidation)
- CH₃CHO →(KMnO₄ or K₂Cr₂O₇/H⁺) CH₃COOH (aldehyde to carboxylic acid)
Common Mistakes
- Saying all ketones give iodoform test — only methyl ketones (CH₃CO−) do.
- Forgetting that Fehling's test is negative for aromatic aldehydes (benzaldehyde).
- Missing the observation (colour change, precipitate) in test questions.
- Writing aldol product without the β-hydroxy group.
- Not writing structures in conversion questions.
- Confusing Tollens' (silver mirror) and Fehling's (brick-red precipitate) observations.
Exam Checklist
- Functional group in starting material is identified.
- Test reagent and observation are both written (not just one).
- Product structure or IUPAC name is correct.
- Reason for acidity/reactivity difference is included.
- Conditions (dil. NaOH, heat, H⁺) are specified for reactions.
