Course Topics
Introduction to Carbonyl Compounds
Carbonyl Compounds: Organic compounds containing the carbonyl functional group (C=O) are called carbonyl compounds. They include both aldehydes and ketones.
Aldehydes
- Functional Group: -CHO (carbonyl group bonded to at least one H atom)
- General Formula: CₙH₂ₙO
- Structure: R-CHO (occurs at end of chain)
- Examples: Formaldehyde (HCHO), Acetaldehyde (CH₃CHO), Benzaldehyde (C₆H₅CHO)
- Occurrence: Present in most sugars, essential oils, fragrances
Ketones
- Functional Group: >C=O (carbonyl group bonded to two carbon atoms)
- General Formula: CₙH₂ₙO
- Structure: R-CO-R’ (occurs within chain)
- Examples: Acetone (CH₃COCH₃), Butanone (CH₃COCH₂CH₃)
- Occurrence: Present in camphor, fructose, hormones
Aldehyde = Aldehyde group at the END (A for At End), Ketone = Ketone group in the MIDDLE (K for Keep in Middle)
Preparation of Aldehydes and Ketones
Key Preparation Methods:
| Method | Aldehydes | Ketones |
|---|---|---|
| Oxidation of Alcohols | Primary alcohols → Aldehydes (using K₂Cr₂O₇/H₂SO₄) | Secondary alcohols → Ketones |
| Ozonolysis of Alkenes | Alkenes + O₃ → Aldehydes/Ketones (C=C → 2 C=O) | |
| Hydration of Alkynes | Terminal alkynes → Aldehydes | Internal alkynes → Ketones |
| Friedel-Crafts Acylation | – | Aromatic ketones from benzene + acyl chloride |
| From Calcium Salts | Calcium formate + calcium acetate → Acetaldehyde | – |
- Industrial method: Methanol + Air over Fe₂O₃-MoO₃ or Ag catalyst at 500°C
- Laboratory method: Methanol + Air over platinized asbestos or Cu/Ag at 300°C
- Formalin: 40% formaldehyde + 8% methanol + 52% water
Remember: Primary alcohol → Aldehyde, Secondary alcohol → Ketone. Use Primary → Aldehyde, Secondary → Ketone (P-A, S-K)!
Reactivity of Carbonyl Group
Why Carbonyl Group is Reactive:
- Both carbon and oxygen are sp² hybridized
- Polarity: δ⁺ on carbon, δ⁻ on oxygen (electronegativity difference = 1.0)
- Presence of π-bond allows nucleophilic addition reactions
Why Ketones are Less Reactive than Aldehydes:
- Electronic effects: Alkyl groups in ketones are electron-donating, reducing δ⁺ on carbonyl carbon
- Steric hindrance: Two alkyl groups create more steric hindrance in ketones
- Inductive effect: +I effect of alkyl groups decreases electrophilicity
Ketones are less reactive because they have TWO alkyl groups that donate electrons and block approach (steric hindrance)!
Nucleophilic Addition Reactions
Characteristic Reaction: Carbonyl compounds undergo nucleophilic addition reactions.
Base-Catalyzed Addition
- With strong nucleophiles
- Base generates nucleophile
- Examples: HCN, Grignard reagents, NaHSO₃
Acid-Catalyzed Addition
- With weak nucleophiles
- Acid protonates carbonyl oxygen
- Increases electrophilicity of carbon
- Examples: Alcohol addition, polymerization
Important Nucleophilic Addition Reactions:
| Reagent | Product | Importance |
|---|---|---|
| HCN | Cyanohydrin | Forms α-hydroxy acids on hydrolysis |
| Grignard (RMgX) | Alcohols (1°, 2°, 3°) | Formaldehyde → 1° alcohols, Aldehydes → 2° alcohols, Ketones → 3° alcohols |
| NaHSO₃ | Bisulphite adduct (white ppt) | Used for purification and separation |
| Ammonia derivatives | Imines, hydrazones, etc. | Used for identification and characterization |
Nucleophilic Addition: Nucleophile attacks the δ⁺ carbon of carbonyl group! Strong nucleophile = Base catalysis, Weak nucleophile = Acid catalysis!
Condensation Reactions
Definition: Reactions where two molecules combine to form a new compound with elimination of small molecules like H₂O or NH₃.
Important Condensation Reactions:
| Reaction | Requirements | Products | Significance |
|---|---|---|---|
| Aldol Condensation | α-hydrogen atoms present | Aldol (β-hydroxy carbonyl) | Forms C-C bonds, important in synthesis |
| Cannizzaro Reaction | No α-hydrogen atoms | Alcohol + Carboxylic acid salt | Disproportionation (self redox) |
| With NH₂OH | Aldehydes/Ketones | Oximes | Identification, purification |
| With 2,4-DNPH | Aldehydes/Ketones | 2,4-DNP hydrazones | Yellow/orange ppt for identification |
Mechanism: Disproportionation reaction where one aldehyde molecule is oxidized to carboxylic acid salt and another is reduced to alcohol.
(Formaldehyde → Methanol + Sodium formate)
Only aldehydes without α-hydrogen undergo this reaction (e.g., formaldehyde, benzaldehyde).
Aldol condensation needs α-H, Cannizzaro needs NO α-H! Aldol = Addition, Cannizzaro = Disproportionation!
Identification Tests
Distinguishing Tests for Aldehydes and Ketones:
| Test | Aldehydes | Ketones | Observation |
|---|---|---|---|
| Tollen’s Test | ✓ Positive | ✗ Negative | Silver mirror formed |
| Fehling’s Test | ✓ Positive | ✗ Negative | Brick red ppt of Cu₂O |
| Benedict’s Test | ✓ Positive | ✗ Negative | Brick red ppt of Cu₂O |
| 2,4-DNPH Test | ✓ Positive | ✓ Positive | Yellow/orange ppt |
| NaHSO₃ Test | ✓ Positive | Only methyl ketones | White crystalline ppt |
| Sodium Nitroprusside | ✗ Negative | ✓ Positive | Wine red/orange red color |
| Iodoform Test | Only acetaldehyde | Only methyl ketones | Yellow ppt of CHI₃ |
Reagent: Ammoniacal silver nitrate [Ag(NH₃)₂]⁺ OH⁻
Observation: Silver mirror on test tube walls. This test distinguishes aldehydes from ketones as ketones don’t reduce Tollen’s reagent.
Tollen’s, Fehling’s, Benedict’s = ALDEHYDES ONLY (They reduce these reagents, ketones don’t)!
Reduction Reactions
Reduction of Carbonyl Compounds:
Aldehydes
- Reduced to Primary alcohols
- Using: NaBH₄, LiAlH₄, H₂/Pd, etc.
- Example: CH₃CHO → CH₃CH₂OH
Ketones
- Reduced to Secondary alcohols
- Using: NaBH₄, LiAlH₄, H₂/Pd, etc.
- Example: CH₃COCH₃ → CH₃CH(OH)CH₃
Reducing Agents:
- Catalytic Hydrogenation: H₂ with Pd, Pt, or Ni catalyst
- Sodium Borohydride (NaBH₄): Mild, selective reducing agent
- Lithium Aluminium Hydride (LiAlH₄): Strong reducing agent
- Clemmensen Reduction: Zn-Hg/HCl → CH₂ from C=O
- Wolff-Kishner Reduction: NH₂NH₂/KOH → CH₂ from C=O
R-CO-R’ + 2[H] → R-CH(OH)-R’ (Secondary alcohol)
Reduction: Aldehydes → 1° alcohols, Ketones → 2° alcohols. Count the carbons attached: Aldehyde = 1C + 1H → 1°, Ketone = 2C → 2°!
Oxidation Reactions
Key Difference: Aldehydes are easily oxidized, ketones are resistant to oxidation.
Aldehydes Oxidation
- Mild oxidizing agents: Tollen’s, Fehling’s, Benedict’s
- Strong oxidizing agents: K₂Cr₂O₇/H₂SO₄, KMnO₄/H₂SO₄, HNO₃
- Products: Carboxylic acids with same number of carbons
- R-CHO → R-COOH
Ketones Oxidation
- Mild oxidizing agents: No reaction
- Strong oxidizing agents: Break C-C bond (requires harsh conditions)
- Products: Mixture of carboxylic acids with fewer carbons
- Symmetrical: R-CO-R → 2R-COOH
- Unsymmetrical: R-CO-R’ → R-COOH + R’-COOH
The hydrogen atom attached to carbonyl carbon in aldehydes is relatively easy to oxidize to -OH group, forming carboxylic acids. Ketones lack this hydrogen and require breaking of strong C-C bonds.
(Aldehyde → Carboxylic acid with same C number)
Aldehydes oxidize easily because they have a “H” on carbonyl carbon that can be replaced by “OH”! Ketones have no such H!
Special Reactions
Important Special Reactions:
| Reaction | Reactants | Products | Significance |
|---|---|---|---|
| Haloform Reaction | Methyl ketones/CH₃CH₂OH with X₂/NaOH | Haloform (CHX₃) + Carboxylate | Test for methyl ketones/ethanol |
| Polymerization | Formaldehyde/Acetaldehyde + dil. H₂SO₄ | Metaformaldehyde/Paraldehyde | Trimer formation |
| Addition of Alcohols | Aldehydes + Alcohols (dry HCl) | Acetals (Aldehyde protection) | Protects aldehyde group |
| Wittig Reaction | Aldehydes/Ketones + Phosphorus ylide | Alkenes | C=C bond formation |
Compounds that give positive test:
- Acetaldehyde (only aldehyde)
- Methyl ketones (CH₃-CO-R)
- Ethanol (only primary alcohol)
- 2-Alkanols (secondary alcohols with CH₃CH(OH)- group)
(Yellow ppt of CHI₃ with iodine)
Haloform reaction tests for compounds with CH₃-CO- group (methyl ketones) or those that can form it (ethanol, acetaldehyde)!
Applications & Importance
Practical Applications of Aldehydes and Ketones:
| Compound | Applications |
|---|---|
| Formaldehyde (HCHO) | Disinfectant, preservative (formalin), resins (Bakelite, urea-formaldehyde), embalming fluid |
| Acetaldehyde (CH₃CHO) | Production of acetic acid, plastics, dyes, perfumes, drugs |
| Acetone (CH₃COCH₃) | Solvent for paints, varnishes, plastics, nail polish remover, production of polymers |
| Benzaldehyde (C₆H₅CHO) | Perfumes, flavoring agent (almond flavor), dye intermediate, pharmaceuticals |
| Formalin | Biological specimen preservation, disinfectant, tissue fixative |
| Vanillin | Food flavoring, perfumes |
| Citral | Perfumes, flavoring agent (lemon scent), synthesis of vitamin A |
Aldehydes and ketones are crucial intermediates in organic synthesis. They are used in:
- Pharmaceuticals: Synthesis of drugs, vitamins, hormones
- Polymers: Production of plastics, resins, fibers
- Perfumes and Flavors: Many natural fragrances contain carbonyl groups
- Agrochemicals: Pesticides, herbicides
- Solvents: Acetone, methyl ethyl ketone as industrial solvents
Formaldehyde = Preservative, Acetone = Solvent, Benzaldehyde = Almond flavor. Each has unique industrial uses based on its structure!
Comparison Summary
Comprehensive Comparison Table:
| Property/Reaction | Aldehydes | Ketones |
|---|---|---|
| General Formula | R-CHO | R-CO-R’ |
| Carbonyl Position | End of chain | Within chain |
| Reactivity | More reactive | Less reactive |
| Oxidation | Easy (to carboxylic acids) | Difficult (requires C-C break) |
| Tollen’s Test | Positive (Silver mirror) | Negative |
| Fehling’s/Benedict’s | Positive (Red ppt) | Negative |
| 2,4-DNPH Test | Positive (Yellow/Orange ppt) | Positive (Yellow/Orange ppt) |
| Cannizzaro Reaction | Only without α-H | No reaction |
| Aldol Condensation | With α-H | With α-H |
| Reduction Product | Primary alcohols | Secondary alcohols |
| Iodoform Test | Only acetaldehyde | Only methyl ketones |
| Polymerization | Forms trimers | Generally doesn’t polymerize |
| Acetal Formation | Forms acetals | No reaction |
Key differences: Aldehydes are reducing agents (Tollen’s/Fehling’s positive), Ketones are NOT reducing agents! This is the most important distinction!