Aldehydes, Ketones, and Carboxylic Acids – Structure, Preparation & Reactions
Comprehensive guide to aldehydes, ketones, and carboxylic acids: carbonyl structure, Rosenmund reduction, nucleophilic addition, and oxidation reactions.
What are Aldehydes, Ketones, and Carboxylic Acids?
Aldehydes, Ketones, and Carboxylic Acids are carbonyl compounds which contain a carbon-oxygen double bond (>C=O). These organic compounds play a central role in biochemical metabolism and industrial chemical manufacturing.
What are Aldehydes?
Aldehydes are carbonyl compounds in which the carbonyl carbon is bonded to at least one hydrogen atom and an alkyl or aryl group. The general molecular formula is R-CHO.
- Methanal (Formaldehyde): H-CHO (used as formalin solution for preserving biological specimens)
- Ethanal (Acetaldehyde): CH₃-CHO (raw material in vinegar and polymer production)
- Benzaldehyde: C₆H₅-CHO (oil of bitter almonds, used in perfumes and flavoring)
Preparation of Aldehydes
1. Rosenmund Reduction (Acid Chlorides): Catalytic hydrogenation of acyl chloride over palladium supported on barium sulfate (Pd/BaSO₄) partially deactivated (poisoned) with sulfur or quinoline yields aldehydes selectively.
Rosenmund Reduction SchemeR-CO-Cl + H₂ ──[ Pd / BaSO₄ , Quinoline (Poison) ]──> R-CHO + HCl C₆H₅-CO-Cl (Benzoyl chloride) + H₂ ──[ Pd/BaSO₄ ]──> C₆H₅-CHO (Benzaldehyde) + HCl
2. Controlled Oxidation of Primary Alcohols: Primary alcohols (1°) undergo mild oxidation with Pyridinium Chlorochromate (PCC) in CH₂Cl₂ to stop cleanly at the aldehyde stage without over-oxidizing to carboxylic acid.
PCC Oxidation of 1° AlcoholR-CH₂-OH ──[ PCC / CH₂Cl₂ ]──> R-CHO CH₃-CH₂-OH (Ethanol) ──[ PCC ]──> CH₃-CHO (Ethanal)
What are Ketones?
Ketones are carbonyl compounds in which the carbonyl carbon is attached to two carbon-containing hydrocarbon groups (alkyl or aryl). The general formula is R-CO-R'.
- Propanone (Acetone): CH₃-CO-CH₃ (simplest symmetrical ketone, used as laboratory solvent)
- Acetophenone: C₆H₅-CO-CH₃ (mixed aromatic-aliphatic ketone)
- Benzophenone: C₆H₅-CO-C₆H₅ (diaryl symmetrical ketone)
Ketone Carbonyl StructureO ║ R ─── C ─── R' (R, R' may be identical or different alkyl/aryl groups)
Preparation of Ketones
1. Oxidation of Secondary Alcohols (2°): Secondary alcohols on treatment with chromic anhydride (CrO₃) or acidified potassium dichromate (K₂Cr₂O₇) give corresponding ketones:
2° Alcohol OxidationCH₃-CH(OH)-CH₃ (Propan-2-ol) ──[ CrO₃ / H₂SO₄ ]──> CH₃-CO-CH₃ (Propanone) + H₂O
2. Friedel-Crafts Acylation of Aromatic Rings:
Friedel-Crafts AcylationC₆H₆ (Benzene) + CH₃-CO-Cl ──[ Anhydrous AlCl₃ ]──> C₆H₅-CO-CH₃ (Acetophenone) + HCl
Carboxylic Acids & Functional Properties
Carboxylic acids contain a carboxyl group (-COOH) consisting of a carbonyl group attached directly to a hydroxyl group. Because of hydrogen bonding between molecules, carboxylic acids have higher boiling points than aldehydes and ketones of comparable molecular weight.
Distinction Tests (Aldehydes vs Ketones)
| Test | Aldehydes (R-CHO) | Ketones (R-CO-R') |
|---|---|---|
| Tollens' Test (AgNO₃ + NH₄OH) | Forms bright silver mirror on test tube walls (Ag⁺ reduced to Ag) | No silver mirror formed (Ketones do not react) |
| Fehling's Solution (A + B) | Gives red-brown precipitate of Cuprous oxide (Cu₂O) | No red precipitate with aliphatic/aromatic ketones |
| Schiff's Reagent | Restores pink/magenta color | No color change with ketones |
Summary of Functional Groups
| Compound Class | General Formula | Functional Group | IUPAC Suffix | Example |
|---|---|---|---|---|
| Aldehydes | R-CHO | -CHO | -al | Ethanal (CH₃CHO) |
| Ketones | R-CO-R' | >C=O | -one | Propanone (CH₃COCH₃) |
| Carboxylic Acids | R-COOH | -COOH | -oic acid | Ethanoic Acid (CH₃COOH) |
Indexed Topics & Examination Keywords
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