NEET Exam > NEET Notes > Short Notes > Short Notes: Aldehydes, Ketones and Carboxylic Acids
Short Notes: Aldehydes, Ketones and Carboxylic Acids
3.1 Structure and Nomenclature
| Functional Group | Structure | Suffix | Example |
|---|---|---|---|
| Aldehyde | R-CHO (-C=O with H) | -al | CH₃CHO (Ethanal/Acetaldehyde) |
| Ketone | R-CO-R' (-C=O between carbons) | -one | CH₃COCH₃ (Propanone/Acetone) |
| Carboxylic Acid | R-COOH | -oic acid | CH₃COOH (Ethanoic acid/Acetic acid) |
3.2 Preparation of Aldehydes and Ketones
| Method | Reaction | Product |
|---|---|---|
| Oxidation of 1° Alcohol | RCH₂OH + [O] → RCHO (PCC or CrO₃/pyridine) | Aldehyde |
| Oxidation of 2° Alcohol | R₂CHOH + [O] → R₂CO (K₂Cr₂O₇/H⁺) | Ketone |
| Dehydrogenation | RCH₂OH → RCHO + H₂ (Cu, 573K) R₂CHOH → R₂CO + H₂ | Aldehyde/Ketone |
| Ozonolysis of Alkenes | R-CH=CH-R → RCHO + R'CHO (O₃, then Zn/H₂O) | Aldehydes/Ketones |
| From Acid Chlorides | R-COCl + H₂ → RCHO (Pd-BaSO₄, Rosenmund reduction) | Aldehyde |
| From Nitriles | R-CN + SnCl₂/HCl → RCHO (Stephen reduction) | Aldehyde |
| From Benzene (Ketone) | C₆H₆ + RCOCl → C₆H₅COR (AlCl₃, Friedel-Crafts) | Aromatic ketone |
| From Alkynes (Ketone) | RC≡CH + H₂O → RCOCH₃ (H₂SO₄, HgSO₄) | Methyl ketone |
3.3 Physical Properties
- Polarity: C=O bond is polar; dipole-dipole interactions
- Boiling points: Higher than hydrocarbons, lower than alcohols (no H-bonding between molecules)
- Solubility: Lower members soluble in water (H-bonding with water)
- Odor: Lower aldehydes - pungent; ketones - pleasant
3.4 Chemical Reactions - Nucleophilic Addition
| Reaction | Reagent | Product |
|---|---|---|
| Addition of HCN | HCN | Cyanohydrin: R₂C(OH)CN |
| Addition of NaHSO₃ | NaHSO₃ | Bisulfite addition product (crystalline) |
| Addition of Alcohols | ROH (dry HCl) | Hemiacetal → Acetal |
| Addition of Ammonia derivatives | NH₂OH (Hydroxylamine) | Oxime: R₂C=NOH |
| NH₂NH₂ (Hydrazine) | Hydrazone: R₂C=NNH₂ | |
| NH₂NHCONH₂ (Semicarbazide) | Semicarbazone: R₂C=NNHCONH₂ | |
| C₆H₅NHNH₂ (Phenylhydrazine) | Phenylhydrazone: R₂C=NNHC₆H₅ | |
| Addition of Grignard | RMgX then H₃O⁺ | Alcohol (1°, 2°, or 3° depending on substrate) |
3.5 Reduction Reactions
| Method | Reaction |
|---|---|
| To Alcohols | RCHO + LiAlH₄ → RCH₂OH R₂CO + LiAlH₄ → R₂CHOH |
| Clemmensen Reduction | RCHO/R₂CO + Zn-Hg/HCl → RCH₃/R₂CH₂ |
| Wolff-Kishner Reduction | RCHO/R₂CO + NH₂NH₂/KOH → RCH₃/R₂CH₂ |
3.6 Oxidation Reactions
| Compound | Reagent | Product |
|---|---|---|
| Aldehyde | K₂Cr₂O₇/H⁺ or KMnO₄ | Carboxylic acid (RCOOH) |
| Ketone | Strong oxidizing agents | No reaction (resistant to oxidation) |
3.7 Tests for Aldehydes vs Ketones
| Test | Aldehyde | Ketone |
|---|---|---|
| Tollen's Test (Ammoniacal AgNO₃) | Silver mirror forms | No reaction |
| Fehling's Test (Cu²⁺ in alkaline) | Red precipitate (Cu₂O) | No reaction (except α-hydroxy ketones) |
| Benedict's Test | Red/orange precipitate | No reaction |
| Schiff's Reagent | Pink/magenta color | No color |
| Iodoform Test | CH₃CHO gives yellow ppt | CH₃COR gives yellow ppt (CHI₃) |
Iodoform Test: Positive for compounds with CH₃CO- or CH₃CH(OH)- group
3.8 Special Reactions
| Reaction | Details |
|---|---|
| Aldol Condensation | 2CH₃CHO → CH₃CH(OH)CH₂CHO (dil. NaOH) β-hydroxy aldehyde (Aldol) On heating: CH₃CH=CHCHO (α,β-unsaturated aldehyde) |
| Cross Aldol | Between two different aldehydes/ketones One should not have α-hydrogen |
| Cannizzaro Reaction | 2RCHO → RCH₂OH + RCOONa (conc. NaOH) For aldehydes without α-hydrogen Disproportionation reaction |
| Haloform Reaction | CH₃COR + 3X₂ + 4NaOH → CHX₃ + RCOONa Yellow ppt of CHI₃ (iodoform) Also works with CH₃CH(OH)R |
| Benzoin Condensation | 2C₆H₅CHO → C₆H₅CH(OH)COC₆H₅ (CN⁻ catalyst) |
3.9 Preparation of Carboxylic Acids
| Method | Reaction |
|---|---|
| From Primary Alcohols | RCH₂OH + [O] → RCOOH (KMnO₄ or K₂Cr₂O₇) |
| From Aldehydes | RCHO + [O] → RCOOH |
| From Alkylbenzenes | C₆H₅CH₃ + KMnO₄ → C₆H₅COOH (vigorous oxidation) |
| From Nitriles | R-CN + H₂O/H⁺ → RCOOH + NH₃ (hydrolysis) |
| From Grignard Reagent | R-MgX + CO₂ → RCOOMgX → RCOOH (H₃O⁺) |
| Hydrolysis of Esters | RCOOR' + H₂O → RCOOH + R'OH (H⁺ or OH⁻) |
3.10 Physical Properties of Carboxylic Acids
- Hydrogen bonding: Strong intermolecular H-bonding → high boiling points
- Dimerization: Form dimers in vapor phase
- Solubility: Lower members soluble in water (up to C₄)
- Acidity: Weak acids (pKa ~4-5); stabilization of carboxylate ion by resonance
3.11 Acidity Order
Effect of Substituents on Acidity:
- Electron-withdrawing groups (EWG) increase acidity: -NO₂, -CN, -X (halogens)
- Electron-donating groups (EDG) decrease acidity: -CH₃, -OCH₃, -NH₂
- Closer the EWG to COOH, greater the acidity
Order: CCl₃COOH > CHCl₂COOH > CH₂ClCOOH > CH₃COOH
- Electron-withdrawing groups (EWG) increase acidity: -NO₂, -CN, -X (halogens)
- Electron-donating groups (EDG) decrease acidity: -CH₃, -OCH₃, -NH₂
- Closer the EWG to COOH, greater the acidity
Order: CCl₃COOH > CHCl₂COOH > CH₂ClCOOH > CH₃COOH
3.12 Chemical Reactions of Carboxylic Acids
| Reaction Type | Reaction | Product |
|---|---|---|
| Formation of Salts | RCOOH + NaOH → RCOONa + H₂O RCOOH + NaHCO₃ → RCOONa + CO₂ + H₂O | Carboxylate salt |
| Esterification | RCOOH + R'OH → RCOOR' + H₂O (H⁺) | Ester |
| Acid Chloride Formation | RCOOH + PCl₅ → RCOCl + POCl₃ + HCl RCOOH + SOCl₂ → RCOCl + SO₂ + HCl | Acid chloride (acyl chloride) |
| Anhydride Formation | 2RCOOH → (RCO)₂O + H₂O (P₂O₅) | Acid anhydride |
| Amide Formation | RCOOH + NH₃ → RCOONH₄ → RCONH₂ + H₂O (heat) | Amide |
| Reduction | RCOOH + LiAlH₄ → RCH₂OH | Primary alcohol |
| Decarboxylation | RCOONa + NaOH → RH + Na₂CO₃ (CaO, heat) | Alkane (soda lime) |
| Hell-Volhard-Zelinsky | RCOOH + Cl₂/Br₂ → RCH(X)COOH (Red P) | α-halo acid |
3.13 Derivatives of Carboxylic Acids
| Derivative | Structure | Example |
|---|---|---|
| Acid Chloride | R-COCl | CH₃COCl (Acetyl chloride) |
| Acid Anhydride | (RCO)₂O | (CH₃CO)₂O (Acetic anhydride) |
| Ester | R-COOR' | CH₃COOCH₃ (Methyl acetate) |
| Amide | R-CONH₂ | CH₃CONH₂ (Acetamide) |
Reactivity Order: Acid chloride > Acid anhydride > Ester > Amide
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FAQs on Short Notes: Aldehydes, Ketones and Carboxylic Acids
| 1. What are the general characteristics of aldehydes? |  |
Ans. Aldehydes are organic compounds that contain a carbonyl group (C=O) with at least one hydrogen atom attached to the carbon atom of the carbonyl. They are typically characterised by their distinctive odours and are often used in flavourings and fragrances. Aldehydes are generally more reactive than ketones due to the presence of the hydrogen atom, which makes them susceptible to oxidation and reduction reactions.
| 2. How do ketones differ from aldehydes in structure and reactivity? | |
Ans. Ketones also contain a carbonyl group (C=O), but unlike aldehydes, the carbonyl carbon in ketones is bonded to two other carbon atoms. This structural difference makes ketones less reactive than aldehydes, as they do not have a hydrogen atom directly attached to the carbonyl carbon. Ketones are often used as solvents and in the production of various chemicals due to their stability and relatively low reactivity.
| 3. What are carboxylic acids, and how do they form? | |
Ans. Carboxylic acids are organic compounds that contain a carboxyl group (-COOH). They can be formed through the oxidation of aldehydes or primary alcohols. The presence of the hydroxyl group (-OH) in carboxylic acids contributes to their acidity, as they can donate a proton (H⁺) in solution. Carboxylic acids are commonly found in nature and are used in the production of food additives, preservatives, and pharmaceuticals.
| 4. What are some common reactions involving aldehydes, ketones, and carboxylic acids? | |
Ans. Aldehydes can undergo oxidation to form carboxylic acids, while ketones are generally resistant to oxidation. Both aldehydes and ketones can participate in nucleophilic addition reactions, where nucleophiles attack the carbonyl carbon. Carboxylic acids can participate in esterification reactions, where they react with alcohols to form esters, and in neutralisation reactions with bases to form salts and water.
| 5. What is the significance of the functional groups in aldehydes, ketones, and carboxylic acids? | |
Ans. The functional groups in these compounds dictate their chemical behaviour and reactivity. The carbonyl group in aldehydes and ketones defines their ability to undergo nucleophilic addition and oxidation reactions, while the carboxyl group in carboxylic acids is responsible for their acidic properties. Understanding these functional groups is crucial in organic chemistry, as they influence the properties and applications of these compounds in various industries.
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