Write the chemical equation involved in the following reactions: (a) Carbylamine reaction (b) Gabriel phthalim

Question

Q. Write the chemical equation involved in the following reactions:

(a) Carbylamine reaction

(b) Gabriel phthalimide synthesis

CBSECBSE Class XII Board 2023Subjective· 2mImportance★★★★★

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Concept understanding — Carbylamine Reaction

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 ( $CHCl_3$ ) 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.

Watch out

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 ( $RNH_2$ ) reacts with chloroform ( $CHCl_3$ ) in the presence of a strong base (usually alcoholic $KOH$ ) to form an isocyanide ( $RNC$ ) and potassium chloride ( $KCl$ ) and water ( $H_2O$ ).

The general equation is:

RNH_2 + CHCl_3 + 3KOH \xrightarrow{\Delta} RNC + 3KCl + 3H_2O

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 ( $:CCl_2$ ). This carbene is electron-deficient and desperately wants to react.

CHCl_3 + KOH \rightarrow :CCl_2 + KCl + H_2O

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.
Elimination: The intermediate undergoes a series of dehydrohalogenation steps (loss of $HCl$ ) driven by the base, ultimately yielding the isocyanide ( $RNC$ ). …

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Both reactions synthesize primary amines: the carbylamine reaction tests for primary amines using chloroform and alcoholic KOH (producing foul-smelling isocyanides), while Gabriel phthalimide synthesis builds primary amines from alkyl halides via phthalimide.

These are two classic name reactions in amine chemistry, each with a distinct purpose and mechanism. The carbylamine reaction serves as a qualitative test for primary amines, while Gabriel phthalimide synthesis is a preparative method to obtain pure primary amines without contamination by secondary or tertiary amines.

(a) Carbylamine Reaction

This reaction exploits the unique ability of primary amines to react with chloroform in the presence of a strong base. When a primary amine is heated with chloroform ( $\ce{CHCl3}$ ) and alcoholic potassium hydroxide, it produces an isocyanide (carbylamine) — a compound with an unbearably foul odor that makes the test unmistakable.

The mechanism proceeds through the formation of dichlorocarbene ( $\ce{:CCl2}$ ), an extremely reactive intermediate generated when base abstracts a proton from chloroform. This carbene inserts into the N–H bond of the primary amine, and subsequent elimination steps yield the isocyanide.

Chemical equation:

\ce{R-NH2 + CHCl3 + 3KOH ->[heat] R-N≡C + 3KCl + 3H2O}

where $\ce{R-N≡C}$ is the alkyl isocyanide (carbylamine).

For example, with aniline:

\ce{C6H5-NH2 + CHCl3 + 3KOH ->[heat] C6H5-N≡C + 3KCl + 3H2O}

Watch out

Secondary and tertiary amines do not give this reaction because they lack the two N–H bonds required for the mechanism. This specificity makes it an excellent diagnostic test for primary amines.

(b) Gabriel Phthalimide Synthesis

This elegant method converts alkyl halides into primary amines without the risk of over-alkylation that plagues direct alkylation of ammonia. The strategy uses phthalimide as a protected form of ammonia.

The synthesis unfolds in three stages:

Deprotonation: Phthalimide is treated with alcoholic KOH or $\ce{KOH/C2H5OH}$ to form the phthalimide anion, a good nucleophile.

$\ce{C6H4(CO)2NH + KOH -> C6H4(CO)2N^- K^+ + H2O}$ …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

Showing the 12 most recent of 25 on this concept.

CBSE 2026Set ANNUAL1 mark
$Q. Write chemical equation for conversion from methylamine into methylisocyanide.$
›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 markMCQ
$Q. The reaction of ethylamine with chloroform in the alcoholic KOH gives:$
(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:

$C_2H_5NH_2 + CHCl_3 + 3KOH \xrightarrow{\Delta} C_2H_5NC + 3KCl + 3H_2O$

…

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CBSE 2026Set ANNUAL1 mark
$C_6H_5NH_2 + CHCl_3 + KOH(alc.) \rightarrow ?$
›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

$C_6H_5-NH_2 + CHCl_3 + 3KOH\ (alc.) \rightarrow C_6H_5-\overset{-}{N}\equiv \overset{+}{C} + 3KCl + 3H_2O$

Alcoholic KOH generates dichlorocarbene ( $:CCl_2$ ) 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 …

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CBSE 2025Set ANNUAL1 markMCQ
$Q. The compound obtained by heating a mixture of 1° amine and chloroform with ethanolic KOH is -$
(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:

$RNH_2 + CHCl_3 + 3KOH \xrightarrow{\Delta} RNC + 3KCl + 3H_2O$

…

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CBSE 2025Set D1 markMCQ
$Q. Reaction of primary amine with chloroform in the presence of alcoholic KOH is called$
(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 markMCQ
$Q. The test which is used to distinguish between primary amine and secondary amine is$
(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.

$\text{R-NH}_2 + \text{CHCl}_3 + 3\text{KOH (alc.)} \xrightarrow{\Delta} \text{R-NC (foul smell)} + 3\text{KCl} + 3\text{H}_2\text{O}$

…

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CBSE 2025Set ANNUAL1 mark
$Q. Complete the reaction: CH3CH2NH2 + CHCl3 + 3KOH(alc.) \to ?$
›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 …

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CBSE 2024Set D1 markMCQ
$Q. Methylamine on heating with chloroform and alcoholic KOH gives$
(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 mark
$Q. Write isocyanide test for primary amines.$
›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. …

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CBSE 2024Set ANNUAL1 mark
$Q. What is Carbylamine reaction?$
›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

…

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CBSE 2023Set ANNUAL1 markMCQ
$Q. When benzyl amine is treated with chloroform in presence of ethanolic KOH, then the product formed is$
(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:

$R\text{-}NH_2 + CHCl_3 + 3KOH \rightarrow R\text{-}NC + 3KCl + 3H_2O$

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 markMCQ
$R-NH_2 + CHCl_3 + KOH\ (alc) \longrightarrow$
(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 $1^\circ$ 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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Q.Write the chemical equation involved in the following reactions:

(a) Carbylamine Reaction

(b) Gabriel Phthalimide Synthesis

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