Q.The reaction Ar−N+2Cl−Cu/HClArCl+N2+CuCl is named as ____.
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🔒 Start your 14-day free trial to unlock the full solution →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 (C=O). That oxygen is greedy for electrons; it pulls them away from carbon, leaving the carbon slightly positive (δ+) and the oxygen slightly negative (δ−). 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 π bond breaks, and the oxygen picks up a proton (or some other electrophile) to become stable. That, in a nutshell, is nucleophilic addition.
The key idea: a nucleophile adds across a polar multiple bond (usually C=O or C≡N), breaking the π 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, C=O, or a nitrile, C≡N), while the π bond breaks. The resulting intermediate then captures a proton (or another electrophile) to give a neutral product.
In general form:
RX2C=O+NuX−HX+RX2C(OH)Nu
The nucleophile (NuX−) attacks the carbonyl carbon; the oxygen becomes negatively charged; then a proton (HX+) 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 C=O bond is polarised: CXδ+=OXδ−.
- The π bond is weaker than a σ bond, so it can break relatively easily.
A nucleophile (like OHX−, CNX−, or NHX3) has a lone pair or a negative charge. It seeks positive centres. The attack forms a new σ bond, and the π electrons move entirely to oxygen, creating an alkoxide ion (RX2C−OX−). This intermediate is then quenched by a proton.
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 (CHX3COCHX3) and hydrogen cyanide (HCN). In the presence of a base, CNX− (the nucleophile) attacks the carbonyl carbon:
CHX3COCHX3+CNX−CHX3C(OX−)(CN)CHX3
The alkoxide intermediate then picks up a proton from HCN (or from water) to give a cyanohydrin:
CHX3C(OX−)(CN)CHX3+HX+CHX3C(OH)(CN)CHX3
The product is acetone cyanohydrin. Notice: two new sigma bonds formed (C−CN and O−H), and the π bond is gone.
What Makes a Good Nucleophile?
Strong nucleophiles are usually negatively charged or have lone pairs:
- OHX−, CNX−, NHX2X−, CHX3OX−, HX− (from hydride reagents like NaBHX4 or LiAlHX4)
- Neutral but polarisable: NHX3, HX2O (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:
Rate=k[RX]
Why?
The reaction happens in two steps:
- Slow step (RDS): The C–X bond breaks spontaneously, forming a carbocation intermediate. Only the substrate (RX) is involved.
RXslowR++X−
- Fast step: The nucleophile (Nu−) attacks the carbocation.
R++Nu−fastRNu
Since the slow step depends only on the concentration of RX, the rate law has no dependence on [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:
Rate=k[RX][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 RX and 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:
Rate∝[RX]×[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. …
The key idea is Nucleophilic Substitution Reactions in aromatic diazonium salts. The reaction replaces the diazonium group (−N2+) with a chlorine atom using a copper(I) catalyst.
Step 1: The diazonium salt (ArN2+Cl−) is formed from an aromatic amine.
Step 2: In the presence of Cu/HCl, the N2 group is displaced by Cl−, with copper(I) acting as a catalyst to facilitate the radical or polar mechanism. …
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−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 (N2), 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:
ArN2+X−CuXArX+N2
where X is the halogen. In your question, X=Cl, and the reagent is CuCl/HCl — exactly the Sandmeyer conditions.
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 ArN2+Cl−Cu,HClArCl. 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: RNH2+CHCl3+KOH→RNC (isocyanide). Again, unrelated. …
Concept: Diazonium Salt Substitution Reactions
This question tests your understanding of replacement reactions of diazonium salts — specifically, how the aryl diazonium group (−N2+) can be replaced by a halogen using different copper-based reagents.
Method: Reagent-Product Matching
Steps:
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Identify the reactant and product
- Reactant: Aryl diazonium chloride (Ar−N+2Cl−)
- Product: Aryl chloride (ArCl) + N2 gas + CuCl
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Recall the two key copper-catalyzed reactions that replace −N2+ 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). …
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/HCland assuming it means CuCl. - Why it happens: Slashes in reagents can be ambiguous (e.g.,
Cu/HClmeans copper metal in hydrochloric acid). - How to avoid:
- Always parse the reagent carefully:
Cu/HCl= copper metal + HCl (Gattermann)CuClorCuBr= copper(I) salt (Sandmeyer)
- Write the reagent clearly in your notes.
- Always parse the reagent carefully:
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 …
- TG EAPCET 2026Set eng-2026-05-10-FN1 markMCQQ.The reagent which is used to distinguish C6H5N(CH3)2 and (C2H5)2NH is (anhy = anhydrous, conc = concentrated, alc = alcoholic) (A) C6H5SO2Cl (B) Anhy. ZnCl2 ∣ conc. HCl (C) CrO3 ∣ H2SO4 (D) CHCl3 ∣ alc. KOH
›Reveal solutionSolution
The pair is a tertiary amine and a secondary amine. Only Hinsberg's reagent, C6H5SO2Cl (option A), separates them: the secondary amine gives an alkali-insoluble sulphonamide, the tertiary amine does not react.
The concept first
Before choosing a reagent, classify the two compounds:
- C6H5N(CH3)2 (N,N-dimethylaniline): the nitrogen carries three carbon groups → tertiary amine, no N–H.
- (C2H5)2NH (diethylamine): the nitrogen carries two carbon groups and one H → secondary amine, one N–H.
So you need a test that distinguishes 2∘ from 3∘ — i.e. a test that keys off the presence of an N–H bond, not off basicity.
Hinsberg's test does exactly that. Benzenesulphonyl chloride acylates the nitrogen, but only if there is an N–H available:
- 1∘ amine → C6H5SO2NHR. The N–H left over is made acidic by the two electron-withdrawing S=O groups → the sulphonamide dissolves in KOH/NaOH.
- 2∘ amine → C6H5SO2NR2. No N–H remains, so the sulphonamide is a solid that is insoluble in alkali.
- 3∘ amine → no reaction (no N–H to substitute); the amine remains as an oily layer, dissolving only on adding acid.
Step-by-step
- Test option (D), CHCl3 + alc. KOH (carbylamine). It gives the foul-smelling isocyanide only with primary amines. Both compounds here are non-primary → both give a negative test → cannot distinguish. ✗
- Test option (B), anhy. ZnCl2 + conc. HCl (Lucas reagent). This is a test for alcohols (distinguishing 1∘/2∘/3∘ ROH by turbidity). Amines are not alcohols. ✗ …
- TG EAPCET 2024Set ap-2024-05-08-FN1 markMCQQ.An organic compound C7H9N on reduction with reagent X gave Y. Reaction of Y with p-toluene sulphonyl chloride gave Z which is insoluble in alkali. X and Y respectively are (A) LiAlH4 , C6H5−CH2−NH2 (benzylamine) (B) NaBH4 , C6H5−NHCH3 (N-methylaniline) (C) H2∣Ni , C6H5−CH2−NH2 (benzylamine) (D) H2∣Ni , C6H5−NHCH3 (N-methylaniline)
›Reveal solutionSolution
The alkali-insoluble benzenesulphonamide tells us Y must be a secondary amine, which fixes Y=C6H5NHCH3; a secondary amine of this shape arises from catalytic reduction of an isocyanide, so X=H2/Ni. Option (D).
The concept: why alkali-solubility identifies the class of amine
When an amine reacts with a sulphonyl chloride (Hinsberg's reagent, here p-toluenesulphonyl chloride):
- 1° amine RNH2 gives RNH−SO2Ar. The N–H left on nitrogen sits between an electron-withdrawing SO2 group and the ring, so it is acidic — the sulphonamide dissolves in NaOH forming a salt.
- 2° amine R2NH gives R2N−SO2Ar. Nitrogen now has no hydrogen at all, nothing to ionise, so the product is insoluble in alkali.
- 3° amine does not react at all.
So "Z is insoluble in alkali" is a direct statement that Y is a secondary amine.
Step-by-step
- Use the Hinsberg clue. Z insoluble in alkali ⇒ Y has no N–H after sulphonylation ⇒ Y is 2°.
- Screen the options. C6H5CH2NH2 (benzylamine) is a primary amine — its tosylamide C6H5CH2NH−SO2C6H4CH3 still has an acidic N–H and would dissolve in KOH. So options (A) and (C) are out. Y must be C6H5NHCH3.
- Now identify the reduction. A secondary amine bearing an N−CH3 group is the hallmark product of reducing an isocyanide (carbylamine): C6H5−NC+4[H] H2/Ni C6H5−NH−CH3 …
- TG EAPCET 2024Set eng-2024-05-10-AN1 markMCQQ.Identify the set, in which X and Y are correctly matched (A) NH2OH, Hydrazone (B) NH2NH2, Semicarbazone (C) C6H5NH2, Schiff base (D) RNH2, Oxime
›Reveal solutionSolution
The question asks which pair of reagent (X) and product (Y) is correctly matched. The correct match is aniline (C6H5NH2) with Schiff base, so option (C) is correct.
Concept & Intuition
This is a classic organic chemistry matching problem about carbonyl derivatives. Each reagent (X) reacts with a carbonyl compound (aldehyde or ketone) to give a specific nitrogen-containing derivative. The key is to recall the functional group of the product formed:
- Oximes come from hydroxylamine (NH2OH).
- Hydrazones come from hydrazine (NH2NH2).
- Semicarbazones come from semicarbazide (NH2NHCONH2).
- Schiff bases (imines) come from primary amines (RNH2), especially aromatic ones like aniline.
Let’s check each option step by step.
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Option (A): NH2OH → Hydrazone
Hydroxylamine (NH2OH) reacts with a carbonyl to form an oxime (with a C=NOH group), not a hydrazone. Hydrazones come from hydrazine. So this is incorrect.
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Option (B): NH2NH2 → Semicarbazone
Hydrazine (NH2NH2) gives a hydrazone (C=NNH2). Semicarbazones require semicarbazide (NH2NHCONH2). So this is incorrect.
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Option (C): C6H5NH2 → Schiff base …
- TG EAPCET 2023Set eng-2023-05-12-AN1 markMCQQ.Identify what is Y in the following reaction sequence?
[!FORMULA] CH3−CO−NH2Br2NaOH (aq)X(i) NaNO2+HCl(ii) H2OY
(A) CH3NH2 (B) CH3CONHBr (C) CH3OH (D) BrCH2CONH2›Reveal solutionSolution
Acetamide undergoes Hofmann bromamide degradation to methylamine (X); nitrous acid converts this primary aliphatic amine into an unstable diazonium ion that immediately loses NX2 and picks up water, giving methanol — option (C).
The concept first: why aliphatic diazonium salts die instantly
Diazotisation (NaNOX2+HCl) makes the same −NX2X+ group from any primary amine. The difference is stability. In an aryl diazonium salt the −NX2X+ is conjugated with the ring, so at 273–278 K it survives long enough to be used in coupling/Sandmeyer reactions. In an alkyl diazonium ion there is no such delocalisation, and NX2 is an outstanding leaving group — so the ion falls apart the moment it forms, giving a carbocation that the solvent (water) captures. That is why primary aliphatic amines simply effervesce and give alcohols with nitrous acid.
Step-by-step
Step 1 — Identify X. Acetamide with bromine in aqueous alkali is the textbook Hofmann bromamide degradation:
CHX3CONHX2+BrX2+4NaOHCHX3NHX2+NaX2COX3+2NaBr+2HX2O …
- TG EAPCET 2023Set eng-2023-05-13-AN1 markMCQQ.An amine (X) reacts with p-toluene sulphonyl chloride to give the product Y, which is insoluble in alkali. The product of X with benzoyl chloride is (A) CH3CH2CH(NH2)−COC6H5 (B) CH3CH2CH2N(CH3)COCH2C6H5 (C) CH3CH2NHCOC6H5 (D) CH3CH2N(CH3)COC6H5
›Reveal solutionSolution
A sulphonamide that is insoluble in alkali can only come from a secondary amine, so X is CHX3CHX2NHCHX3; benzoyl chloride then acylates its nitrogen to give CHX3CHX2N(CHX3)COCX6HX5 — option (D).
The concept first: why alkali-solubility fingerprints the amine class
p-Toluenesulphonyl chloride (a Hinsberg-type reagent) reacts with the amine's lone pair, replacing an N−H hydrogen with the bulky −SOX2Ar group:
- Primary amine R−NHX2 → R−NH−SOX2Ar. One N−H remains. That hydrogen is acidic, because the resulting anion is stabilised by the strongly electron-withdrawing sulphonyl group. Hence the product dissolves in KOH/NaOH.
- Secondary amine RX2NH → RX2N−SOX2Ar. No N−H is left. There is no acidic proton to remove, so the product is insoluble in alkali. ✓
- Tertiary amine RX3N → no reaction (there is no N−H to substitute in the first place), so no product Y at all.
The stem says a product Y forms and is insoluble in alkali. Both facts together force X to be secondary.
Step-by-step
Step 1 — Classify X. Product formed ⇒ not tertiary. Product alkali-insoluble ⇒ not primary. Therefore X is a 2∘ amine.
Step 2 — Benzoylation (Schotten–Baumann). Benzoyl chloride CX6HX5COCl acylates the amine nitrogen:
RX2NH+CX6HX5COClRX2N−CO−CX6HX5+HCl
The amide nitrogen ends up carrying both original alkyl groups plus the benzoyl group — i.e. it is a tertiary (N,N-disubstituted) amide with no N−H.
Step 3 — Test each option against "benzoyl on a 2∘ nitrogen".
- (A) CHX3CHX2CH(NHX2)COCX6HX5 — the benzoyl is on carbon, and a free −NHX2 survives. Wrong on both counts. …
- TG EAPCET 2022Set eng-2022-07-18-FN1 markMCQQ.Lithium nitrate on heating gives (A) Li2O+NO2 (B) Li2O+NO2+O2 (C) LiNO2+O2 (D) Li2O2+NO2+O2
›Reveal solutionSolution
Lithium nitrate decomposes differently from other alkali metal nitrates because of the small size and high polarising power of Li⁺; it gives Li2O, NO2, and O2, so the correct option is (B).
Concept & Intuition
Most alkali metal nitrates (like NaNO₃, KNO₃) decompose on strong heating to give the nitrite and oxygen:
2MNO3→2MNO2+O2.
But lithium is an exception. Because Li⁺ is very small, it has a high charge density and strongly polarises the nitrate ion. This destabilises the nitrate, causing it to break down further — lithium nitrite (LiNO₂) itself is unstable at high temperature and decomposes to lithium oxide, nitrogen dioxide, and oxygen. So the final products are not simply nitrite + O₂, but oxide + NO₂ + O₂.
Step-by-step reasoning
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General trend for alkali metal nitrates
For Na, K, Rb, Cs:
2MNO3Δ2MNO2+O2
The nitrite is stable at the decomposition temperature.
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Lithium’s anomaly
Li⁺ is much smaller than other alkali ions. Its high polarising power weakens the N–O bonds in the nitrate ion, so decomposition occurs at a lower temperature and proceeds further.
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First step – formation of nitrite
Initially, LiNO₃ does form LiNO₂ and O₂:
2LiNO3→2LiNO2+O2
But LiNO₂ is not stable at the temperature needed for decomposition.
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Second step – further decomposition of LiNO₂
Lithium nitrite decomposes to lithium oxide, nitrogen dioxide, and oxygen:
2LiNO2→Li2O+NO2+NO
However, NO reacts immediately with O₂ to give NO₂: …
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