Preparation Methods of Aldehydes and Ketones
Aldehydes and ketones are key carbonyl compounds widely used in organic chemistry and industry. They can be prepared by oxidation, dehydrogenation, hydration of alkynes, ozonolysis, Rosenmund reduction, and Friedel–Crafts acylation.
1️⃣ Oxidation of Alcohols
- Primary alcohols → aldehydes
- Secondary alcohols → ketones
- Oxidizing agents: PCC, KMnO₄, CrO₃, Jones reagent
- Avoid strong oxidizers to prevent aldehydes → acids
Example: CH₃CH₂OH → CH₃CHO (with PCC)
2️⃣ Dehydrogenation of Alcohols
- Alcohol vapors over heated Cu at ~300°C
- Primary → aldehyde | Secondary → ketone
Example: CH₃CH(OH)CH₃ → CH₃COCH₃ + H₂
3️⃣ Hydration of Alkynes
- Alkynes + H₂O (H₂SO₄ + HgSO₄ catalyst)
- Terminal alkynes → aldehydes | Internal → ketones
- Involves keto–enol tautomerism
Example: CH≡CH + H₂O → CH₃CHO
4️⃣ Ozonolysis of Alkenes
- Alkene + O₃ → cleavage of double bond
- Reductive workup (Zn/H₂O)
Example: CH₂=CH₂ + O₃ → 2HCHO
5️⃣ Rosenmund Reduction (Aldehydes only)
- Acid chlorides + H₂ (Pd/BaSO₄ catalyst)
- Stops at aldehyde stage
Example: RCOCl + H₂ → RCHO
6️⃣ Friedel–Crafts Acylation (Ketones)
- Benzene + acid chloride (AlCl₃ catalyst)
- Forms aromatic ketones
Example: C₆H₆ + CH₃COCl → C₆H₅COCH₃
Practice Quiz: Preparation of Aldehydes & Ketones
MCQ
1. Which reagent is used in Rosenmund reduction?
- a) KMnO₄
- b) Pd/BaSO₄
- c) Zn/H₂O
- d) AlCl₃
Answer: b) Pd/BaSO₄
True / False
1. Secondary alcohols oxidize to give aldehydes. → ❌ False (They give ketones)
2. Friedel–Crafts acylation is used to prepare aromatic ketones. → ✅ True
Fill in the blanks
1. In hydration of alkynes, _______ tautomerism occurs. → (Keto–enol)
2. Oxidation of ethanol with PCC gives _______. → (Acetaldehyde)
📖 Related reading: Nomenclature of Aldehydes & Ketones
🔗 Learn more: Named Organic Reactions
