Aldehydes, Ketones, and Carboxylic Acids
'The CARBONYL group (C=O) is the MOST VERSATILE functional group in organic chemistry — it is the key to aldehydes, ketones, acids, and their derivatives.'
1. Chapter Overview
This chapter covers three important classes of CARBONYL COMPOUNDS: aldehydes (carbonyl group at the END of a carbon chain), ketones (carbonyl group in the MIDDLE), and carboxylic acids (carbonyl group with an −OH group). Topics include: NOMENCLATURE, METHODS OF PREPARATION, PHYSICAL PROPERTIES, and CHEMICAL REACTIONS — with special emphasis on NUCLEOPHILIC ADDITION reactions of aldehydes and ketones (including ALDOL CONDENSATION and CANNIZZARO REACTION), and the ACIDITY and DERIVATIVES of carboxylic acids.
2. Nomenclature
Aldehydes
- IUPAC: Replace −e of alkane with −al. Aldehyde carbon = C1 (always).
- Examples: HCHO (methanal), CH₃CHO (ethanal), C₆H₅CHO (benzaldehyde).
Ketones
- IUPAC: Replace −e with −one. Number the chain to give the LOWEST number to the carbonyl carbon.
- Examples: CH₃COCH₃ (propanone), CH₃COCH₂CH₃ (butanone), C₆H₅COCH₃ (acetophenone).
Carboxylic Acids
- IUPAC: Replace −e with −oic acid. Carboxyl carbon = C1.
- Examples: HCOOH (methanoic acid), CH₃COOH (ethanoic acid), C₆H₅COOH (benzoic acid).
3. Preparation of Aldehydes and Ketones
From Alcohols
- 1° alcohols → Aldehydes (PCC, mild oxidation). 2° alcohols → Ketones (K₂Cr₂O₇/H⁺, CrO₃).
- Dehydrogenation: Passing alcohol vapour over Cu/Ag at 300°C.
From Hydrocarbons
- Alkenes: Ozonolysis — cleaves C=C to give aldehydes and/or ketones.
- Arenes: Friedel-Crafts acylation — ArH + RCOCl (AlCl₃) → ArCOR.
Special Methods
- Aldehydes from acyl chlorides: RCOCl + H₂ (Pd/BaSO₄) → RCHO (Rosenmund reduction).
- Ketones from alkyl nitriles: RCN + RMgX → RCOR' (after hydrolysis).
4. Physical Properties
| Property | Aldehydes | Ketones | Carboxylic Acids |
|---|---|---|---|
| Boiling point | Higher than alkanes, lower than alcohols | Slightly higher than aldehydes | MUCH HIGHER (dimerisation via H-bonds) |
| Solubility in water | Lower members MISCIBLE | Lower members MISCIBLE | Lower members MISCIBLE |
| bp trend | Methanal (gas at RT). Ethanal (bp 21°C). Acetone (bp 56°C). | Formic acid (bp 101°C), Acetic acid (bp 118°C) | |
| Odour | Pungent (except benzaldehyde — almond smell) | Pleasant (acetone — nail polish remover) | Pungent (vinegar smell for acetic acid) |
5. Chemical Reactions of Aldehydes and Ketones
5.1 Nucleophilic Addition Reactions
- 'The CARBONYL carbon is ELECTROPHILIC (partially positive due to the polar C=O bond). Nucleophiles ATTACK this carbon.'
- Mechanism: Nu⁻ attacks C=O → tetrahedral intermediate → protonation → product.
| Reagent | Product with Aldehyde | Product with Ketone |
|---|---|---|
| HCN | Cyanohydrin (RCHOHCN) | Cyanohydrin (RR'COHCN) |
| NaHSO₃ | Bisulphite addition product | Reacts SLOWLY (if at all) |
| NH₂OH | Oxime (RCH=NOH) | Oxime (RR'C=NOH) |
| NH₂NH₂ | Hydrazone | Hydrazone |
| 2,4-DNP | 2,4-Dinitrophenylhydrazone (ORANGE/YELLOW precipitate) | Same — TEST for carbonyl compounds |
| Alcohol (ROH) | Hemiacetal → Acetal | Hemiketal → Ketal |
5.2 Reduction
- Catalytic hydrogenation (H₂/Ni): Aldehyde → 1° alcohol. Ketone → 2° alcohol.
- Clemmensen reduction (Zn(Hg)/HCl): C=O → CH₂ (for aldehydes/ketones).
- Wolff-Kishner reduction (NH₂NH₂, KOH): C=O → CH₂.
5.3 Oxidation
- Aldehydes: Readily oxidised to CARBOXYLIC ACIDS (even by mild oxidants like Tollen's reagent, Fehling's solution).
- Ketones: RESIST oxidation — need STRONG oxidants (K₂Cr₂O₇/H⁺) under vigorous conditions.
5.4 Reactions Specific to Aldehydes (Tests)
| Test | Reagent | Positive Result | Which Aldehydes |
|---|---|---|---|
| Tollen's test | [Ag(NH₃)₂]⁺ | SILVER MIRROR on tube | ALL aldehydes |
| Fehling's test | Cu²⁺/tartrate | RED PRECIPITATE of Cu₂O | Aliphatic aldehydes |
| Iodoform test | I₂/NaOH | YELLOW precipitate of CHI₃ | CH₃CO− compounds and CH₃CHOH− compounds |
5.5 Aldol Condensation
- 'Two aldehyde (or ketone) molecules with α-hydrogen atoms couple in the presence of a BASE.'
- CH₃CHO + CH₃CHO (dilute NaOH) → CH₃CH(OH)CH₂CHO (3-hydroxybutanal).
- Mechanism: Enolate formation → attack on another carbonyl → dehydration.
5.6 Cannizzaro Reaction
- 'Aldehydes WITHOUT α-hydrogen undergo SELF-OXIDATION and REDUCTION in concentrated alkali.'
- 2HCHO + conc. NaOH → CH₃OH (methanol) + HCOONa (sodium formate).
6. Carboxylic Acids
Acidity
- pKa: Carboxylic acids are MUCH STRONGER (pKa ≈ 4-5) than alcohols (pKa ≈ 16) and phenols (pKa ≈ 10).
- 'The acidity is due to RESONANCE STABILISATION of the carboxylate ion — the negative charge is DELOCALISED over two oxygen atoms.'
- Effect of substituents: Electron-withdrawing (EWG) groups INCREASE acidity. Electron-releasing (ERG) groups DECREASE acidity.
- Acidity order: HCOOH > CH₃COOH > C₂H₅COOH > C₃H₇COOH.
Derivatives
| Derivative | Formula | Preparation from Acid |
|---|---|---|
| Acyl chloride | RCOCl | RCOOH + SOCl₂ or PCl₅ |
| Acid anhydride | (RCO)₂O | RCOOH + RCOCl (pyridine) or dehydration |
| Ester | RCOOR' | RCOOH + R'OH (conc. H₂SO₄) |
| Amide | RCONH₂ | RCOCl + NH₃ |
Reduction
- RCOOH → RCH₂OH (LiAlH₄). 'LiAlH₄ reduces acids to alcohols — NaBH₄ does NOT.'
7. Comparison Table: Aldehydes vs Ketones Reactivity
| Feature | Aldehydes | Ketones |
|---|---|---|
| Carbonyl reactivity | HIGHER (less steric hindrance, less electron donation) | LOWER (more alkyl groups = more hindrance, +I effect reduces δ+ on C) |
| Nucleophilic addition | FASTER | SLOWER |
| Oxidation | EASY (even mild oxidants work) | DIFFICULT (requires strong conditions, C−C cleavage) |
| Tollen's test | POSITIVE (silver mirror) | NEGATIVE |
| Fehling's test | POSITIVE (aliphatic only) | NEGATIVE |
8. Common Mistakes
- Tollen's and Fehling's tests: Tollen's test works for ALL aldehydes (aliphatic and aromatic). Fehling's test works ONLY for ALIPHATIC aldehydes (benzaldehyde gives NEGATIVE Fehling's).
- Aldol condensation requires α-hydrogen: Aldehydes WITHOUT α-hydrogens (HCHO, C₆H₅CHO) undergo CANNIZZARO reaction instead.
- Cannizzaro reaction requires CONCENTRATED alkali: Dilute alkali gives aldol (if α-H is present).
- Iodoform test is for METHYL KETONES and SECONDARY ALCOHOLS with CH₃CHOH− group: Not for ALL carbonyl compounds.
- Reduction of acids: LiAlH₄ reduces −COOH to −CH₂OH. NaBH₄ does NOT reduce carboxylic acids.
9. CBSE Exam Focus
- Preparation of aldehydes and ketones — from alcohols, ozonolysis, Friedel-Crafts acylation
- Nucleophilic addition reactions — HCN, NaHSO₃, NH₂OH, 2,4-DNP
- Distinction tests — Tollen's, Fehling's, Iodoform, 2,4-DNP
- Aldol condensation and Cannizzaro reaction — mechanisms
- Acidity of carboxylic acids — effect of substituents, comparison with other O−H compounds
- Derivatives of carboxylic acids — acyl chloride, ester, amide
- Reduction reactions — Clemmensen, Wolff-Kishner, Rosenmund
10. Self-Test
Q1: Distinguish between ethanal and acetone. A1: Tollen's test: Ethanal → silver mirror (positive). Acetone → no reaction (negative).
Q2: Predict the product: C₆H₅CHO + conc. NaOH → ? A2: Cannizzaro reaction. Products: C₆H₅CH₂OH (benzyl alcohol) + C₆H₅COONa (sodium benzoate).
Q3: Which is more reactive towards nucleophilic addition — CH₃CHO or C₆H₅COCH₃? Why? A3: CH₃CHO (acetaldehyde) is MORE reactive. Ketones have TWO electron-releasing alkyl groups (or aryl groups) that stabilise the carbonyl carbon's partial positive charge and also cause more steric hindrance.
Q4: Arrange in increasing order of acidity: CH₃COOH, CCl₃COOH, HCOOH, C₆H₅COOH. A4: CCl₃COOH > HCOOH > C₆H₅COOH > CH₃COOH. (CCl₃ is strongly electron-withdrawing. HCOOH has no +I group. C₆H₅ is slightly withdrawing by induction. CH₃ is electron-releasing.)
Q5: What happens when acetaldehyde is treated with dilute NaOH? A5: Aldol condensation. Product: CH₃CH(OH)CH₂CHO (3-hydroxybutanal). On heating, it dehydrates to CH₃CH=CHCHO (but-2-enal).
11. Conclusion
Carbonyl compounds are the HEART of organic chemistry:
- ALDEHYDES: 'Reactive carbonyls — readily oxidised, excellent electrophiles. Tollen's and Fehling's tests identify them.'
- KETONES: 'Less reactive than aldehydes, but participate in aldol condensation and can be reduced to alkanes (Clemmensen, Wolff-Kishner).'
- CARBOXYLIC ACIDS: 'The most ACIDIC common organic compounds. Their derivatives (esters, amides) are everywhere in biology.'
- 'From the flavour of almonds (benzaldehyde) to the sting of ant bites (formic acid) — carbonyl compounds are the FLAVOUR and FRAGRANCE of organic chemistry.'
