Aldehydes, Ketones and Carboxylic Acids
🟢 Lite — Quick Review (1h–1d)
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Aldehydes, Ketones and Carboxylic Acids — Key Facts Aldehyde functional group: -CHO (carbonyl C bonded to H and R); e.g., methanal HCHO, ethanal CH₃CHO Ketone functional group: -C(=O)- (carbonyl C bonded to two R groups); e.g., propanone (acetone) CH₃COCH₃ Carboxylic acid functional group: -COOH; e.g., ethanoic acid (acetic acid) CH₃COOH IUPAC: aldehydes = -al (methanal, ethanal); ketones = -one (propanone, butanone); acids = -oic acid (ethanoic acid) ⚡ Exam tip: Aldehydes can be oxidised to acids; ketones cannot be oxidised without breaking C-C bonds
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Aldehydes, Ketones and Carboxylic Acids — JAMB Chemistry Study Guide
Aldehyde reactions:
- Tollens’ test (silver mirror): $RCHO + 2[Ag(NH_3)2]^+ + 3OH^- \rightarrow RCOO^- + 2Ag + 2H_2O + 4NH_3$ (silver mirror on test tube)
- Fehling’s test (red precipitate): $RCHO + 2Cu^{2+} + 4OH^- \rightarrow RCOO^- + Cu_2O$ (red) $+ 2H_2O$
- Reduction: $RCHO + H_2 \xrightarrow{Ni} RCH_2OH$ (primary alcohol)
- Addition of HCN: $RCHO + HCN \rightarrow RCH(OH)CN$ (cyanohydrin)
Ketone reactions:
- Cannot reduce easily with Tollens or Fehling’s (distinguishes from aldehydes)
- Reduction: $R_2C=O + H_2 \xrightarrow{Ni} R_2CHOH$ (secondary alcohol)
- Addition of HCN: $R_2C=O + HCN \rightarrow R_2C(OH)CN$ (less reactive than aldehydes)
Carboxylic acid reactions:
- Acidic: $CH_3COOH \rightleftharpoons CH_3COO^- + H^+$ ($K_a = 1.8 \times 10^{-5}$)
- Reaction with bases: $CH_3COOH + NaOH \rightarrow CH_3COONa + H_2O$
- Esterification: $CH_3COOH + C_2H_5OH \xrightarrow{H_2SO_4} CH_3COOC_2H_5 + H_2O$ (ethyl acetate)
- Halogenation: no reaction (unlike alcohols); carboxyl group is electron-withdrawing, makes alpha carbon less reactive
Ester formation: $RCOOH + R’OH \rightleftharpoons RCOOR’ + H_2O$ (Fischer esterification — reversible, needs acid catalyst)
Common student mistakes: confusing Tollens’ and Fehling’s reagents and their colour changes; thinking ketones give positive Tollens’ test (they don’t); forgetting that esterification is reversible.
🔴 Extended — Deep Study (3mo+)
Comprehensive coverage for students on a longer study timeline.
Aldehydes, Ketones and Carboxylic Acids — Comprehensive Chemistry Notes
Mechanism of nucleophilic addition to carbonyl: The carbonyl carbon is electrophilic (δ+) due to electron-withdrawing oxygen. Nucleophile attacks carbonyl carbon. The π bond breaks, oxygen takes electrons and becomes alkoxide, which is then protonated.
General mechanism:
- Nucleophilic attack: Nu⁻ attacks C=O carbon, forming tetrahedral intermediate
- Protonation of oxygen: alkoxide O⁻ is protonated to give alcohol
For aldehydes vs ketones reactivity order: Formaldehyde > acetaldehyde > propionaldehyde > … > ketones Reason: steric hindrance (ketones have two alkyl groups); also electronic (alkyl groups donate electrons, reducing electrophilicity).
Reduction with NaBH₄ vs LiAlH₄: NaBH₄: reduces aldehydes and ketones only (not carboxylic acids, esters); mild, can use in water/alcohol. LiAlH₄: reduces almost all carbonyl compounds (acids, esters, amides, nitriles); very strong; must use in dry ether.
Aldol condensation: Base-catalysed aldol addition between two aldehydes or two ketones. One molecule acts as nucleophile (enolate ion) attacking the carbonyl of another. Example: 2CH₃CHO → CH₃CH(OH)CH₂CHO (aldol) → CH₃CH=CHCHO + H₂O (after dehydration)
Decarboxylation: Carboxylic acids with α-hydrogens can undergo decarboxylation (loss of CO₂) when heated with soda lime. $CH_3COONa + NaOH \xrightarrow{CaO} CH_4 + Na_2CO_3$ (Kol electrolysis in methane from acetate)
JAMB exam patterns:
- 2022 JAMB: Compound with formula C₃H₆O gives silver mirror with Tollens’ reagent; identify compound
- 2021 JAMB: Write equation for reaction between ethanoic acid and ethanol with H₂SO₄ catalyst
- 2020 JAMB: Distinguish between ethanal and propanone using a chemical test
- 2019 JAMB: What is the IUPAC name of CH₃CH₂COCH₂CH₃?
Physical properties:
| Property | Aldehydes | Ketones | Carboxylic Acids |
|---|---|---|---|
| Boiling point | Higher than alkanes (polar) | Higher than alkanes | Much higher (H-bonding dimer) |
| Water solubility | Formaldehyde, acetaldehyde soluble | Propanone miscible | Lower acids miscible (acetic acid) |
| Odour | Formaldehyde: pungent; others fruity | Fruity (acetone) | Vinegary (acetic acid) |
📊 JAMB Exam Essentials
| Detail | Value |
|---|---|
| Questions | 180 MCQs (UTME) |
| Subjects | 4 subjects (language + 3 for course) |
| Time | 2 hours |
| Marking | +1 per correct answer |
| Score | 400 max (used for university admission) |
| Registration | January – February each year |
🎯 High-Yield Topics for JAMB
- Use of English (Grammar + Comprehension) — 60 marks
- Biology for Science students — 40 marks
- Chemistry (Organic + Physical) — 40 marks
- Physics (Mechanics + Optics) — 35 marks
- Mathematics (Algebra + Geometry) — 40 marks
📝 Previous Year Question Patterns
- Q: “The process of photosynthesis requires…” [2024 Biology]
- Q: “The electronic configuration of Fe is…” [2024 Chemistry]
- Q: “Find the value of x if 2x + 5 = 15…” [2024 Mathematics]
💡 Pro Tips
- Use of English carries the most weight — master grammar rules and comprehension strategies
- JAMB syllabus is your Bible — questions come directly from it. Download and use it.
- Past questions are highly predictive — repeat patterns appear every year
- For Science students, Biology and Chemistry are high-scoring if you study NCERT-level content
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📐 Diagram Reference
Clear scientific diagram of Aldehydes, Ketones and Carboxylic Acids with atom labels, molecular structure, reaction arrows, white background, color-coded bonds and groups, exam textbook style
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