Chemical properties of aldehydes

Chemical Properties of Aldehydes | एल्डिहाइड के रासायनिक गुणधर्म

🔹 Introduction | परिचय

Aldehydes are organic compounds containing the functional group –CHO. They are highly reactive due to the presence of the carbonyl group (C=O) attached to a hydrogen atom.

👉 In Hindi: एल्डिहाइड वे यौगिक हैं जिनमें –CHO समूह पाया जाता है। यह समूह अत्यधिक प्रतिक्रियाशील होता है क्योंकि इसमें कार्बोनिल समूह (C=O) और हाइड्रोजन दोनों उपस्थित रहते हैं।

🧪 General Formula

The general formula of aldehydes is R–CHO, where R is an alkyl or aryl group. Examples: Methanal (HCHO), Ethanal (CH₃CHO), Benzaldehyde (C₆H₅CHO)

⚗️ Chemical Properties of Aldehydes | एल्डिहाइड के रासायनिक गुण

1️⃣ Oxidation Reaction (ऑक्सीकरण अभिक्रिया)

Aldehydes are easily oxidized to carboxylic acids when treated with oxidizing agents like potassium dichromate (K₂Cr₂O₇), potassium permanganate (KMnO₄), or Tollens’ reagent.

R–CHO + [O] → R–COOH

Example: CH₃CHO + [O] → CH₃COOH (Ethanal → Ethanoic acid)

2️⃣ Reduction Reaction (अपचयन अभिक्रिया)

Aldehydes are reduced to primary alcohols when treated with reducing agents like NaBH₄ or LiAlH₄.

R–CHO + 2[H] → R–CH₂OH

Example: CH₃CHO → CH₃CH₂OH (Ethanol)

3️⃣ Addition of Hydrogen Cyanide (HCN) | हाइड्रोजन साइनाइड का योग

Aldehydes react with HCN to form cyanohydrins. This is an example of nucleophilic addition reaction.

R–CHO + HCN → R–CH(OH)–CN

Example: CH₃CHO + HCN → CH₃CH(OH)CN (Lactic nitrile)

4️⃣ Reaction with Sodium Bisulfite (NaHSO₃) | सोडियम बाइसल्फाइट से अभिक्रिया

Aldehydes form crystalline addition compounds with sodium bisulfite, which helps in their purification.

R–CHO + NaHSO₃ → R–CH(OH)–SO₃Na

5️⃣ Cannizzaro Reaction (कैनिज़ारो अभिक्रिया)

Aldehydes without α-hydrogen atoms (like formaldehyde, benzaldehyde) undergo disproportionation when treated with concentrated alkali. One molecule is oxidized, and another is reduced.

2R–CHO + NaOH → R–COONa + R–CH₂OH

Example: 2HCHO → HCOONa + CH₃OH

6️⃣ Aldol Condensation (एल्डोल संघनन)

Aldehydes containing α-hydrogen undergo self-condensation in the presence of dilute alkali to form β-hydroxy aldehydes (Aldols).

2CH₃CHO → CH₃CH(OH)CH₂CHO

On heating, the aldol loses water and forms an α,β-unsaturated aldehyde.

7️⃣ Silver Mirror Test (Tollens’ Test) | चाँदी दर्पण परीक्षण

Aldehydes reduce Tollens’ reagent (ammoniacal silver nitrate) to metallic silver, forming a mirror-like deposit on the inner wall of the test tube.

R–CHO + 2[Ag(NH₃)₂]⁺ + 3OH⁻ → R–COO⁻ + 2Ag↓ + 4NH₃ + 2H₂O

8️⃣ Fehling’s Test | फेलिंग विलयन परीक्षण

Aldehydes reduce Fehling’s solution to a red precipitate of cuprous oxide (Cu₂O).
Ketones do not give this test.

R–CHO + 2Cu²⁺ + 5OH⁻ → R–COO⁻ + Cu₂O↓ + 3H₂O

🧠 Summary | सारांश

• Aldehydes are highly reactive due to the –CHO group.
• They undergo oxidation, reduction, and addition reactions easily.
• Important characteristic tests: Tollens’ and Fehling’s.
• Used in perfume, dye, and polymer industries.

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