The reaction $Ar{-}\overset{+}{N}_2Cl^- \xrightarrow{Cu/HCl} ArCl + N_2 + CuCl$ is named as ____. (i) Sandmeyer reaction (ii) Gatterman reaction (iii) Claisen reaction (iv) Carbylamine reaction

The reaction Ar-overset+N₂Cl^- xrightarrowCu/HCl ArCl + N₂ + CuCl is named as __. (i) Sandmeyer reaction (ii)

NCERT Exemplar · Q20

Ar{-}\overset{+}{N}_2Cl^- \xrightarrow{Cu/HCl} ArCl + N_2 + CuCl

(i) Sandmeyer reaction

(ii) Gatterman reaction

(iii) Claisen reaction

(iv) Carbylamine reaction

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Concept understanding — Nucleophilic Substitution Reactions

Nucleophilic Addition – From Intuition to Precision

Imagine you have a molecule with a carbon–oxygen double bond — a carbonyl group ( $\ce{C=O}$ ). That oxygen is greedy for electrons; it pulls them away from carbon, leaving the carbon slightly positive ( $\delta^+$ ) and the oxygen slightly negative ( $\delta^-$ ). Now, if you bring a species that is rich in electrons (a nucleophile, meaning "nucleus-loving"), it will naturally be attracted to that electron-deficient carbon. The nucleophile attacks the carbon, the $\pi$ bond breaks, and the oxygen picks up a proton (or some other electrophile) to become stable. That, in a nutshell, is nucleophilic addition.

Note

The key idea: a nucleophile adds across a polar multiple bond (usually $\ce{C=O}$ or $\ce{C≡N}$ ), breaking the $\pi$ bond and forming two new sigma bonds.

The Precise Statement

Nucleophilic addition is a reaction in which a nucleophile (an electron-rich species) forms a sigma bond with an electrophilic carbon atom of a polar multiple bond (typically a carbonyl group, $\ce{C=O}$ , or a nitrile, $\ce{C≡N}$ ), while the $\pi$ bond breaks. The resulting intermediate then captures a proton (or another electrophile) to give a neutral product.

In general form:

\ce{R2C=O + Nu^- ->[H^+] R2C(OH)Nu}

The nucleophile ( $\ce{Nu^-}$ ) attacks the carbonyl carbon; the oxygen becomes negatively charged; then a proton ( $\ce{H^+}$ ) from the medium attaches to the oxygen, yielding an alcohol.

Why It Happens – The Driving Force

The carbonyl carbon is electrophilic because:

Oxygen is more electronegative than carbon, so the $\ce{C=O}$ bond is polarised: $\ce{C^{\delta+}=O^{\delta-}}$ .
The $\pi$ bond is weaker than a $\sigma$ bond, so it can break relatively easily.

A nucleophile (like $\ce{OH^-}$ , $\ce{CN^-}$ , or $\ce{NH3}$ ) has a lone pair or a negative charge. It seeks positive centres. The attack forms a new $\sigma$ bond, and the $\pi$ electrons move entirely to oxygen, creating an alkoxide ion ( $\ce{R2C-O^-}$ ). This intermediate is then quenched by a proton.

Watch out

A common mistake: thinking the nucleophile attacks the oxygen. No — oxygen is already electron-rich; the nucleophile goes to the carbon because it is electron-deficient.

A Concrete Example – Addition of HCN to a Ketone

Take acetone ( $\ce{CH3COCH3}$ ) and hydrogen cyanide ( $\ce{HCN}$ ). In the presence of a base, $\ce{CN^-}$ (the nucleophile) attacks the carbonyl carbon:

\ce{CH3COCH3 + CN^- -> CH3C(O^-)(CN)CH3}

The alkoxide intermediate then picks up a proton from $\ce{HCN}$ (or from water) to give a cyanohydrin:

\ce{CH3C(O^-)(CN)CH3 + H^+ -> CH3C(OH)(CN)CH3}

The product is acetone cyanohydrin. Notice: two new sigma bonds formed ( $\ce{C-CN}$ and $\ce{O-H}$ ), and the $\pi$ bond is gone.

What Makes a Good Nucleophile?

Strong nucleophiles are usually negatively charged or have lone pairs:

$\ce{OH^-}$ , $\ce{CN^-}$ , $\ce{NH2^-}$ , $\ce{CH3O^-}$ , $\ce{H^-}$ (from hydride reagents like $\ce{NaBH4}$ or $\ce{LiAlH4}$ )
Neutral but polarisable: $\ce{NH3}$ , $\ce{H2O}$ (weaker, but can add under acidic conditions) …

Why this formula?

Nucleophilic Substitution Reactions: Why the Key Formulas Hold

Nucleophilic substitution reactions are a cornerstone of organic chemistry. Instead of just memorizing the rate laws, let's understand why they arise from the molecular events.

1. The Two Main Mechanisms: A Tale of Timing

The key formulas (rate laws) for nucleophilic substitution come directly from how many molecules are involved in the rate-determining step (RDS) — the slowest step that controls the overall reaction speed.

SN1: Unimolecular — The Leaving Group Goes First

The Rate Law:

\text{Rate} = k[\text{RX}]

Why?

The reaction happens in two steps:

Slow step (RDS): The C–X bond breaks spontaneously, forming a carbocation intermediate. Only the substrate ( $\text{RX}$ ) is involved.

\text{RX} \xrightarrow{\text{slow}} \text{R}^+ + \text{X}^-

Fast step: The nucleophile ( $\text{Nu}^-$ ) attacks the carbocation.

\text{R}^+ + \text{Nu}^- \xrightarrow{\text{fast}} \text{RNu}

Since the slow step depends only on the concentration of $\text{RX}$ , the rate law has no dependence on $[\text{Nu}^-]$ . The nucleophile arrives after the carbocation is formed — it cannot affect the speed of the first step.

Key insight: The rate is determined by how easily the leaving group leaves, not by how fast the nucleophile attacks.

SN2: Bimolecular — Simultaneous Attack and Departure

The Rate Law:

\text{Rate} = k[\text{RX}][\text{Nu}^-]

Why?

The reaction occurs in one concerted step:

The nucleophile attacks the carbon from one side at the same time as the leaving group departs from the opposite side.
Both $\text{RX}$ and $\text{Nu}^-$ must collide with the correct orientation and sufficient energy.

The rate depends on the frequency of productive collisions between the two molecules. This is directly proportional to the product of their concentrations:

\text{Rate} \propto [\text{RX}] \times [\text{Nu}^-]

Key insight: Both partners are involved in the transition state simultaneously — if either is missing, the reaction cannot proceed.

2. The Transition State: Why the Formulas Are Not Just "Given"

For SN2, the transition state has a pentavalent carbon (five bonds partially formed/broken). The energy barrier depends on steric hindrance — bulkier groups around carbon make it harder for the nucleophile to approach, which is why SN2 is favored at primary carbons. …

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The reaction of an aryl diazonium chloride with Cu/HCl to form aryl chloride is the Sandmeyer reaction, where copper(I) chloride acts as a catalyst to replace the diazonium group with chlorine.

This is a classic naming reaction from organic chemistry, specifically from the chapter on amines and diazonium salts. The key is to recognise the reagent and the transformation.

1. What the reaction shows

You start with an aryl diazonium chloride ( $Ar{-}\overset{+}{N}_2Cl^-$ ) — a compound where a nitrogen triple-bonded to another nitrogen is attached to an aromatic ring, with a chloride counterion. When you treat it with copper(I) chloride in the presence of hydrochloric acid (written as Cu/HCl), you get an aryl chloride ( $ArCl$ ), nitrogen gas ( $N_2$ ), and copper(I) chloride is regenerated.

The copper(I) chloride here is a catalyst — it participates in the reaction but is re-formed at the end.

2. Why this is the Sandmeyer reaction

The Sandmeyer reaction is specifically the substitution of a diazonium group by a halogen (or other groups like CN, OH, etc.) using a copper(I) halide as the catalyst. The general form is:

ArN_2^+ X^- \xrightarrow{CuX} ArX + N_2

where $X$ is the halogen. In your question, $X = Cl$ , and the reagent is CuCl/HCl — exactly the Sandmeyer conditions.

Tip

A quick way to remember: Sandmeyer uses copper(I) salts (CuCl, CuBr, CuCN). Gatterman uses copper powder and HX — a different, less efficient method.

3. Distinguishing from the other options

Let's check each option:

(i) Sandmeyer reaction — Correct, as explained.
(ii) Gatterman reaction — This also converts diazonium salts to aryl halides, but uses copper powder (not CuCl) and HX. The reaction is $ArN_2^+Cl^- \xrightarrow{Cu, HCl} ArCl$ . It's a variation, but the reagent given is Cu/HCl, which is ambiguous — however, the standard name for CuCl/HCl is Sandmeyer.
(iii) Claisen reaction — This refers to the Claisen condensation (ester condensation) or the Claisen rearrangement (allyl phenyl ether rearrangement). Nothing to do with diazonium salts.
(iv) Carbylamine reaction — This is the test for primary amines: $RNH_2 + CHCl_3 + KOH \rightarrow RNC$ (isocyanide). Again, unrelated. …

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Concept: Diazonium Salt Substitution Reactions

This question tests your understanding of replacement reactions of diazonium salts — specifically, how the aryl diazonium group ( $-N_2^+$ ) can be replaced by a halogen using different copper-based reagents.

Method: Reagent-Product Matching

Steps:

Identify the reactant and product
- Reactant: Aryl diazonium chloride ( $Ar{-}\overset{+}{N}_2Cl^-$ )
- Product: Aryl chloride ( $ArCl$ ) + $N_2$ gas + $CuCl$
Recall the two key copper-catalyzed reactions that replace $-N_2^+$ with $-Cl$ :

Reaction Reagent Byproduct
Sandmeyer $CuCl$ (catalytic) + $HCl$ $CuCl$ is regenerated
Gatterman $Cu$ powder + $HCl$ $CuCl$ is formed as a product
Match the given equation
- The equation shows $Cu/HCl$ as reagent and $CuCl$ as a product (not regenerated). …

Reaction	Reagent	Byproduct
Sandmeyer	$CuCl$ (catalytic) + $HCl$	$CuCl$ is regenerated
Gatterman	$Cu$ powder + $HCl$	$CuCl$ is formed as a product

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The question tests your knowledge of named reactions in organic chemistry, specifically the conversion of diazonium salts to aryl halides.

Common Mistakes & How to Avoid Them

1. Confusing Sandmeyer and Gattermann Reactions

The Mistake: Choosing (A) Sandmeyer reaction because both use copper salts.
Why it happens: Students memorize "diazonium + Cu → aryl halide" without noting the catalyst form.
How to avoid:
- Sandmeyer: Uses CuCl or CuBr (stoichiometric copper(I) salt).
- Gattermann: Uses Cu powder (catalytic) + HCl (or HBr).
- Key difference: The given reaction uses Cu/HCl (copper metal + acid), not CuCl directly. This is the Gattermann reaction.

2. Misreading the Reagent

The Mistake: Overlooking the slash in Cu/HCl and assuming it means CuCl.
Why it happens: Slashes in reagents can be ambiguous (e.g., Cu/HCl means copper metal in hydrochloric acid).
How to avoid:
- Always parse the reagent carefully:
  - Cu/HCl = copper metal + HCl (Gattermann)
  - CuCl or CuBr = copper(I) salt (Sandmeyer)
- Write the reagent clearly in your notes.

3. Confusing with Claisen or Carbylamine Reactions

The Mistake: Selecting (C) or (D) because they sound similar or involve nitrogen.
Why it happens: Rote memorization without understanding reaction scope.
How to avoid:
- Claisen reaction: Condensation of esters or aldehydes (e.g., Claisen condensation).
- Carbylamine reaction: Primary amine + chloroform + base → isocyanide (foul smell).
- Neither involves diazonium salts or aryl halides.
- Memory aid:
  - Sandmeyer/Gattermann = diazonium → aryl halide …

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

AP EAPCET 2026Set eng-2026-05-12-FN1 markMCQ
$\mathrm{CH_3CH_2CH_2Cl}$
›Reveal solutionSolution
Two independent routes correctly build n-propylamine (nitro reduction of 1-chloropropane, and nitrile chain-extension + reduction of chloroethane); the Hofmann degradation route instead shortens the chain by one carbon, giving ethylamine.

Concept and Intuition

Three very different strategies for making primary amines are being tested, and the key discipline is to count carbons through each transformation, since some routes preserve chain length and one (Hofmann bromamide degradation) deliberately removes a carbon.

$\mathrm{AgNO_2}$ vs $\mathrm{NaNO_2}$ : nitrite is an ambident nucleophile (can attack through O or N). With the softer, more covalent silver salt $\mathrm{AgNO_2}$ , alkylation occurs preferentially through nitrogen, giving the nitroalkane as the major product (with $\mathrm{NaNO_2}$ , the ionic character favours O-alkylation, giving the alkyl nitrite instead). Reducing a nitroalkane ( $\mathrm{Fe/HCl}$ ) gives the primary amine with the same carbon skeleton.

The cyanide route ( $\mathrm{NaCN}$ , then reduction) is a classic chain-extension: the nucleophilic $\mathrm{CN^-}$ displaces the halide, adding one carbon to the chain as a nitrile, and reducing the nitrile ( $\mathrm{H_2/Ni}$ or $\mathrm{LiAlH_4}$ ) gives a primary amine that is one carbon longer than the starting halide.

The Hofmann bromamide degradation ( $\mathrm{Br_2/OH^-}$ on an amide) is the opposite: it removes the carbonyl carbon (as it leaves via an isocyanate/carbamate intermediate that loses $\mathrm{CO_2}$ ), giving a primary amine with one carbon fewer than the starting amide.

Step-by-Step Solution

I: $\mathrm{CH_3CH_2CH_2Cl}$ (3 carbons) + $\mathrm{AgNO_2}$ → $\mathrm{CH_3CH_2CH_2NO_2}$ (1-nitropropane, N-alkylation favoured by the covalent Ag–N bond). + $\mathrm{Fe/HCl}$ (reduction) → $\mathrm{CH_3CH_2CH_2NH_2}$ = n-propylamine. Matches. …
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AP EAPCET 2025Set eng-2025-05-22-AN1 markMCQ
$\mathrm{HCl + ZnCl_2}$
›Reveal solutionSolution
Tests why thionyl chloride, not the phosphorus halides, is the preferred reagent purely on the grounds of product purity.

Concept and Intuition

Several reagents convert $R-OH \to R-Cl$ , but "preferred for pure product" is a purity argument, not a yield argument. The reagent of choice is the one whose by-products leave the flask on their own (as gases), so no work-up/purification is needed to remove them from the alkyl chloride.

Step-by-Step Solution

$R-OH + HCl/ZnCl_2 \to R-Cl + H_2O$ (Lucas reagent) — works well only for tertiary/some secondary alcohols and leaves $ZnCl_2$ /water behind.

$R-OH + PCl_5 \to R-Cl + POCl_3 + HCl$ — the by-product $POCl_3$ is a liquid that must be removed by distillation, contaminating the crude product.

$R-OH + PCl_3 \to R-Cl + H_3PO_3$ — leaves phosphorous acid behind, again needing separation.

$3R-OH + SOCl_2 \to R-Cl + SO_2\uparrow + HCl\uparrow$ — both by-products are gases that simply bubble out, so the alkyl chloride is obtained directly in pure form with essentially no work-up. …
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AP EAPCET 2025Set eng-2025-05-23-FN1 markMCQ
$Q. The reaction of benzene diazonium chloride with Cu and HCl is known as (A) Sandmeyer reaction (B) Etard reaction (C) Finkelstein reaction (D) Gattermann reaction$
›Reveal solutionSolution
This distinguishes the Sandmeyer reaction (uses cuprous salts, e.g. $Cu_2Cl_2/Cu_2Br_2$ ) from the closely related Gattermann reaction (uses copper powder directly with HCl/HBr) for converting a diazonium salt to an aryl halide.

Concept and Intuition

Both reactions replace the diazonium group ( $-N_2^+$ ) with a halogen via a radical/copper-mediated mechanism, and both give the same type of product (aryl halide + $N_2$ ). The distinguishing feature examiners test is the reagent form: Sandmeyer's reaction historically uses a cuprous halide salt (e.g. $Cu_2Cl_2$ ) generated in situ or added directly, while the Gattermann reaction is the simplified variant that uses copper powder (metallic Cu) together with the corresponding hydrohalic acid (HCl/HBr) — no cuprous salt needed.

Step-by-Step Solution

The reaction described is $ArN_2^+Cl^-$ treated with Cu (metal powder) and HCl.

Because the copper source is elemental copper powder (not a cuprous salt like $Cu_2Cl_2$ ), this specific reagent combination is named the Gattermann reaction, distinguishing it from the Sandmeyer reaction (which specifically uses cuprous chloride/cuprous bromide). …
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AP EAPCET 2023Set ap-2023-05-23-AN1 markMCQ
$\mathrm{(CH_3)_2CHCH_2NH_2 + HCOOH}$
›Reveal solutionSolution
Isonitrile hydrolysis always gives the corresponding primary amine (with the SAME alkyl group as the isonitrile) plus formic acid — the alkyl group itself is never altered.

Concept and Intuition

An isocyanide, $R-N\equiv C$ , is hydrolysed under acidic aqueous conditions by nucleophilic attack of water at the terminal carbon, ultimately cleaving the C–N bond to release the alkyl group AS AN AMINE (with its nitrogen intact, unchanged connectivity to R) while the isocyanide carbon (originally bonded only to N and nothing else besides the lone pair/triple bond) ends up as formic acid, HCOOH. This is a key distinguishing test reaction of isocyanides vs their isomeric nitriles (which hydrolyse differently, to carboxylic acids + ammonia/amine on the OTHER side).

Step-by-Step Solution

Isobutyl isonitrile: $(CH_3)_2CHCH_2-N\equiv C$ — the isobutyl group ( $(CH_3)_2CHCH_2-$ ) is attached to the isocyanide nitrogen.

Acid-catalysed hydrolysis proceeds: $R-NC + 2H_2O \xrightarrow{H^+} R-NH_2 + HCOOH$ .

Applying this to isobutyl isonitrile: the alkyl group R = $(CH_3)_2CHCH_2-$ ends up as the primary amine $(CH_3)_2CHCH_2NH_2$ (isobutylamine), retaining the exact same carbon skeleton and nitrogen attachment. …
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AP EAPCET 2022Set eng-2022-07-04-FN1 markMCQ
$\mathrm{CH_3CH_2CH_2CH_2OH}$
›Reveal solutionSolution
Aqueous KOH gives substitution (1-butanol); alcoholic KOH gives elimination, and since only C2 has beta-hydrogens, the only possible alkene is but-1-ene.

Concept and Intuition

Alkyl halides react differently depending on the solvent/base used: aqueous KOH (with water as solvent) favours $S_N2$ nucleophilic substitution because $\mathrm{OH^-}$ acts predominantly as a nucleophile in the polar protic medium, while alcoholic KOH (KOH dissolved in ethanol, with much less free water) favours $E2$ elimination, as the base character of $\mathrm{OH^-}$ /ethoxide dominates, abstracting a beta-hydrogen to form an alkene.

Step-by-Step Solution

Substrate: 1-chlorobutane, $\mathrm{CH_3CH_2CH_2CH_2Cl}$ -- a primary alkyl halide, Cl on C1.

Aqueous KOH: favours $S_N2$ substitution (primary halides are ideal $S_N2$ substrates -- unhindered backside attack). $\mathrm{OH^-}$ displaces $\mathrm{Cl^-}$ directly: product P = $\mathrm{CH_3CH_2CH_2CH_2OH}$ (1-butanol).

Alcoholic KOH: favours E2 elimination. The base removes a beta-hydrogen (a hydrogen on the carbon adjacent to the one bearing Cl).

In 1-chlorobutane, C1 bears Cl; the only carbon adjacent to C1 is C2 -- so the only beta-hydrogens available are those on C2. There is no other beta-carbon (C1 is a terminal/primary carbon), so there is only one possible elimination direction. …
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AP EAPCET 2021Set eng-2021-08-25-AN1 markMCQ
$CH_3Cl \xrightarrow{KCN} (A) \xrightarrow{H^+/H_2O} (B)$
›Reveal solutionSolution
$CH_3Cl$ with $KCN$ forms methyl cyanide, and acidic hydrolysis of that nitrile gives acetic acid.

Concept and Intuition

Potassium cyanide, being an ionic (predominantly carbon-nucleophilic) source of $CN^-$ , substitutes alkyl halides via their carbon end to give alkyl cyanides (nitriles), $R$ – $C\equiv N$ , rather than isocyanides. Nitriles are the functional equivalent of a "masked carboxylic acid" — acidic (or basic) hydrolysis converts the $C\equiv N$ group all the way to $-COOH$ (via an amide intermediate).

Step-by-Step Solution

$CH_3Cl + KCN \rightarrow CH_3CN + KCl$ — this is $(A)$ , methyl cyanide (acetonitrile).

Acidic hydrolysis of a nitrile: $CH_3CN + 2H_2O \xrightarrow{H^+} CH_3COOH + NH_3$ (via the amide $CH_3CONH_2$ intermediate).

So $(B) = CH_3COOH$ (acetic acid).

Common Mistakes …
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Q.The reaction Ar−N+2Cl−→Cu/HClArCl+N2+CuClAr{-}\overset{+}{N}_2Cl^- \xrightarrow{Cu/HCl} ArCl + N_2 + CuClAr−N+2​Cl−Cu/HCl​ArCl+N2​+CuCl is named as ____.

1. What the reaction shows

2. Why this is the Sandmeyer reaction

3. Distinguishing from the other options

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Q.The reaction $Ar{-}\overset{+}{N}_2Cl^- \xrightarrow{Cu/HCl} ArCl + N_2 + CuCl$ is named as ____.

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