Explain the following reactions: (a) Carbylamine reaction (b) Gattermann reaction (c) HVZ (Hell-Volhard-Zelinsky) reaction (d) Aldol condensation

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> [!TLDR]
> Four named organic reactions: carbylamine (test for 1° amines), Gattermann (diazonium salt → aryl halide), HVZ (α-halogenation of acids), and aldol condensation (self-addition of carbonyl compounds with α-H).
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> **(a) Carbylamine reaction (isocyanide test):**
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> When a **primary amine** (aliphatic or aromatic) is heated with **chloroform (CHCl3) and alcoholic KOH**, it forms an **alkyl/aryl isocyanide (carbylamine)**, which has an extremely unpleasant, offensive smell:
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> $$R	ext{-}NH_2 + CHCl_3 + 3KOH \xrightarrow{\Delta} R	ext{-}NC + 3KCl + 3H_2O$$
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> This reaction is **specific to primary amines** (both 1° aliphatic and 1° aromatic amines respond); secondary and tertiary amines do not give this test. It is therefore used as a **qualitative test to distinguish primary amines** from secondary and tertiary amines.
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> **(b) Gattermann reaction:**
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> An **aromatic diazonium salt**, when treated with **dry copper powder** in the presence of the corresponding halogen acid (Cu/HCl or Cu/HBr), decomposes to give the **aryl halide**, releasing nitrogen gas:
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> $$Ar	ext{-}N_2^+Cl^- \xrightarrow{Cu/HCl} Ar	ext{-}Cl + N_2\uparrow$$
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> This is essentially a variant of the **Sandmeyer reaction**, using copper metal powder directly instead of cuprous salts (Cu2Cl2/Cu2Br2). It is a key method for introducing a halogen atom onto a benzene ring at a specific position (via the amine → diazonium → halide route), which cannot always be achieved by direct halogenation.
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> **(c) HVZ (Hell–Volhard–Zelinsky) reaction:**
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> Carboxylic acids possessing an **α-hydrogen atom** react with **chlorine or bromine (X2) in the presence of a small amount of red phosphorus** to give the corresponding **α-halo carboxylic acid**:
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> $$R	ext{-}CH_2	ext{-}COOH \xrightarrow[X_2]{red\ P} R	ext{-}CHX	ext{-}COOH + HX$$
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> Mechanism outline: red phosphorus first reacts with X2 to form PX3, which converts the carboxylic acid to the corresponding acid halide; the acid halide

Q.Explain the following reactions:

Concept understanding — Carbylamine Reaction

The Carbylamine Reaction: A Stink Test for Primary Amines

The Precise Statement

Why Does It Happen? (The Mechanism)

Q.Explain the following reactions:

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