Q.Giving the chemical equation for the preparation of aniline from benzene sulphonic acid, write the chemical equations for the carbylamine and diazotization reactions of aniline.
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The Carbylamine Reaction: A Stink Test for Primary Amines
Imagine you are a chemist in a lab with several unlabelled bottles of amines. You need to quickly tell which ones are primary amines (where the nitrogen is attached to exactly one carbon) and which are secondary or tertiary. You could run a complicated spectrum, or you could just make something that smells terrible. That is the carbylamine reaction.
The reaction is simple in practice: you take your suspected amine, add a little chloroform (CHCl3) and a strong base like alcoholic potassium hydroxide (KOH), and warm the mixture. If a primary amine is present, you will produce an isocyanide (also called a carbylamine). Isocyanides have a famously repulsive, penetrating odour — often described as "foul" or "putrid". If you smell that, you have a primary amine.
Common Mistake
Secondary and tertiary amines do not give this reaction. The carbylamine test is specific to primary amines. Do not confuse it with other amine tests like the Hinsberg test, which distinguishes all three classes.
The Precise Statement
Carbylamine Reaction: A primary amine (RNH2) reacts with chloroform (CHCl3) in the presence of a strong base (usually alcoholic KOH) to form an isocyanide (RNC) and potassium chloride (KCl) and water (H2O).
The general equation is:
RNH2+CHCl3+3KOHΔRNC+3KCl+3H2O
The product RNC is the isocyanide — the source of the foul smell.
Why Does It Happen? (The Mechanism)
The reaction proceeds through a dichlorocarbene intermediate. This is the key concept that makes the reaction work.
- Generation of Dichlorocarbene: The strong base (KOH) deprotonates chloroform, which then loses a chloride ion to form a highly reactive species called dichlorocarbene (:CCl2). This carbene is electron-deficient and desperately wants to react.
CHCl3+KOH→:CCl2+KCl+H2O
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Attack by the Amine: The lone pair on the nitrogen of the primary amine attacks the electron-deficient carbon of the dichlorocarbene. This forms an unstable intermediate.
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Elimination: The intermediate undergoes a series of dehydrohalogenation steps (loss of HCl) driven by the base, ultimately yielding the isocyanide (RNC). …
Aniline is prepared by reduction of nitrobenzene: C6H5NO2+6[H]Sn/HClC6H5NH2+2H2O. Carbylamine: C6H5NH2+CHCl3+3KOHΔC6H5NC+3KCl+3H2O. Diazotization: C6H5NH2+NaNO2+2HCl273−278KC6H5N2+Cl−+NaCl+2H2O. …
Reduce nitrobenzene to aniline, then aniline + CHCl3/KOH ⇒ carbylamine (isocyanide), and aniline + NaNO2/HCl at 0–5∘C ⇒ diazonium salt.
Preparation of aniline: The standard and reliable preparation is the reduction of nitrobenzene (which is itself obtained by nitrating benzene with conc. HNO3/H2SO4):
C6H5NO2+6[H]Sn/HClC6H5NH2+2H2O
(The question names "benzene sulphonic acid"; the accepted textbook preparation of aniline is the reduction of nitrobenzene shown above, so I have given that route.)
Carbylamine (isocyanide) reaction — a test for primary amines. Aniline warmed with chloroform and alcoholic KOH gives foul-smelling phenyl isocyanide (carbylamine):
C6H5NH2+CHCl3+3KOHΔC6H5NC+3KCl+3H2O
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Showing the 12 most recent of 25 on this concept.
- CBSE 2026Set ANNUAL1 markQ.Write chemical equation for conversion from methylamine into methylisocyanide.
›Reveal solutionSolution
Primary amines react with chloroform and alcoholic KOH to give a foul-smelling isocyanide - this is the carbylamine reaction, a test unique to primary amines.
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- CBSE 2026Set ANNUAL1 markMCQQ.The reaction of ethylamine with chloroform in the alcoholic KOH gives:(a) C2H5NC(b) C2H5CN(c) CH3CN(d) CH3NC
›Reveal solutionSolution
This is the carbylamine (isocyanide) test — a primary amine + chloroform + alcoholic KOH gives a foul-smelling alkyl isocyanide.
When a primary amine reacts with chloroform in the presence of alcoholic KOH, the carbylamine reaction occurs:
C2H5NH2+CHCl3+3KOHΔC2H5NC+3KCl+3H2O
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- CBSE 2026Set ANNUAL1 markQ.Identify the products of the following: C6H5NH2+CHCl3+KOH(alc.)→?
›Reveal solutionSolution
This is the carbylamine (isocyanide) test, specific to primary amines: aniline reacts with chloroform and alcoholic KOH to give the foul-smelling phenyl isocyanide.
Reaction
C6H5−NH2+CHCl3+3KOH (alc.)→C6H5−N−≡C++3KCl+3H2O
Alcoholic KOH generates dichlorocarbene (:CCl2) from chloroform; this electrophilic carbene inserts into the N–H of the primary aromatic amine, and after loss of the remaining chlorines (as KCl, with KOH supplying the base) the isocyanide (carbylamine, −NC) functional group is formed. This reaction is diagnostic for …
- CBSE 2025Set ANNUAL1 markMCQQ.The compound obtained by heating a mixture of 1° amine and chloroform with ethanolic KOH is -(a) Alkyl cyanide(b) Amide(c) Alkyl isocyanide(d) None of these
›Reveal solutionSolution
This is the carbylamine (isocyanide) test for primary amines.
A primary amine, on heating with chloroform and ethanolic KOH, undergoes the carbylamine reaction to form a foul-smelling alkyl isocyanide:
RNH2+CHCl3+3KOHΔRNC+3KCl+3H2O
…
- CBSE 2025Set D1 markMCQQ.Reaction of primary amine with chloroform in the presence of alcoholic KOH is called(a) Hydrolysis(b) Reduction(c) Wurtz reaction(d) Carbylamine reaction
›Reveal solutionSolution
Primary amine + CHCl3 + alcoholic KOH -> isocyanide (carbylamine reaction), a test for 1deg amines.
When a primary amine (aliphatic or aromatic) is warmed with chloroform and alcoholic potassium hydroxide, it forms an alkyl or aryl isocyanide (carbylamine), which has an extremely offensive smell:
R-NH2 + CHCl3 + 3KOH -> R-NC + 3KCl + 3H2O
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- CBSE 2025Set ANNUAL1 markMCQQ.The test which is used to distinguish between primary amine and secondary amine is(a) Lucas test(b) Carbylamine test(c) Fehling's test(d) Tollen's test
›Reveal solutionSolution
The carbylamine (isocyanide) test is positive only for primary amines, so it cleanly distinguishes them from secondary (and tertiary) amines, which give no reaction.
R-NH2+CHCl3+3KOH (alc.)ΔR-NC (foul smell)+3KCl+3H2O
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- CBSE 2025Set ANNUAL1 markQ.Complete the reaction: CH3CH2NH2 + CHCl3 + 3KOH(alc.) → ?
›Reveal solutionSolution
A primary amine's nitrogen attacks dichlorocarbene (generated from CHCl3+KOH), and after loss of two more HCl equivalents, an isocyanide is formed.
This is the carbylamine (isocyanide) reaction, specific to primary amines. Alcoholic KOH first generates dichlorocarbene (:CCl₂) from chloroform by α-elimination; the primary amine's nitrogen lone pair attacks this electrophilic carbene carbon, and subsequent loss of two more equivalents of HCl (neutralized …
- CBSE 2024Set D1 markMCQQ.Methylamine on heating with chloroform and alcoholic KOH gives(a) CH3OH(b) CH3CN(c) CH3CHO(d) CH3NC
›Reveal solutionSolution
Carbylamine reaction: 1° amine + CHCl3 + alc. KOH -> isocyanide (foul smell).
Primary amines undergo the carbylamine (isocyanide) test: heating with chloroform and alcoholic potassium hydroxide gives a foul-smelling alkyl isocyanide.
CH3NH2 + CHCl3 + 3KOH -> CH3NC + 3KCl + 3H2O
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- CBSE 2024Set ANNUAL1 markQ.Write isocyanide test for primary amines.
›Reveal solutionSolution
Primary amines uniquely form foul-smelling isocyanides when heated with chloroform and alcoholic KOH; this odour is used as a positive test for primary amines (secondary/tertiary amines do not respond).
Procedure: a small amount of the amine is warmed gently with a few drops of chloroform (CHCl3) and alcoholic potassium hydroxide (KOH).
Reaction: R-NH2 + CHCl3 + 3KOH (alc., heat) -> R-NC (alkyl isocyanide) + 3KCl + 3H2O
Observation: an extremely unpleasant, characteristic smell of the isocyanide (carbylamine) is produced, confirming a primary amine. …
- CBSE 2024Set ANNUAL1 markQ.What is Carbylamine reaction?
›Reveal solutionSolution
Only primary amines give this test — the offensive-smelling isocyanide product is the diagnostic signal.
When a primary amine is heated with chloroform (CHCl3) and alcoholic potassium hydroxide (KOH), it forms an isocyanide (carbylamine), which has a characteristically unpleasant, penetrating odour:
R-NH2 + CHCl3 + 3KOH (alc., Δ) → R-NC + 3KCl + 3H2O
…
- CBSE 2023Set ANNUAL1 markMCQQ.When benzyl amine is treated with chloroform in presence of ethanolic KOH, then the product formed is(a) structure(a)(b) structure(b)(c) structure(c)(d) structure (d)
›Reveal solutionSolution
Any primary amine, treated with chloroform and ethanolic KOH, gives a foul-smelling isocyanide (carbylamine reaction) — a diagnostic test for -NH2 primary amines.
Benzylamine is PhCH2-NH2, a primary amine (the -NH2 is on the benzylic carbon, not directly on the ring). The carbylamine reaction is:
R-NH2+CHCl3+3KOH→R-NC+3KCl+3H2O
Here R = PhCH2-, so the product is PhCH2-NC, benzyl isocyanide — a phenyl ring bearing a -CH2NC group, exactly structure (c).
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- CBSE 2022Set GH1 markMCQQ.The reaction R−NH2+CHCl3+KOH (alc)⟶ is known as:(a) Gattermann-Koch reaction(b) Carbylamine reaction(c) Hoffman bromomide reaction(d) Schmidt reaction
›Reveal solutionSolution
A primary amine heated with chloroform and alcoholic KOH forms an isocyanide (carbylamine); this is the carbylamine test for 1∘ amines.
Concept — the carbylamine reaction. Aliphatic and aromatic primary amines, when warmed with chloroform and alcoholic potassium hydroxide, form isocyanides (carbylamines) — recognised by their extremely offensive smell. Secondary and tertiary amines do not give this reaction, so it is used as a test to detect a primary amino group.
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