Q.(a) In the following pair of compounds, which compound undergoes $S_N2$ reaction faster and why ? 1-iodopropane ($CH_3CH_2CH_2I$)

Showing the 12 most recent of 16 on this concept.

1. CBSE 2026Set 56/3/11 markMCQ
   Q.Which of the following reagent is used to distinguish between $(C_2H_5)_2NH$ and $(C_2H_5)_3N$ ? (A) $CHCl_3 + KOH$ (B) $C_6H_5SO_2Cl$ (C) Conc. $HCl + ZnCl_2$ (D) $NaOH + I_2$

   ›Reveal solutionSolution

   Hinsberg's reagent ($C_6H_5SO_2Cl$) is used to distinguish between secondary and tertiary amines because secondary amines react to form an alkali-insoluble sulfonamide, while tertiary amines do not react. The correct option is (B).

   Amines are organic compounds derived from ammonia ($NH_3$) where one or more hydrogen atoms are replaced by alkyl or aryl groups. They are classified as primary ($1^\circ$), secondary ($2^\circ$), or tertiary ($3^\circ$) based on the number of alkyl/aryl groups attached to the nitrogen atom. This structural difference, specifically the number of hydrogen atoms directly bonded to the nitrogen, dictates their chemical reactivity and forms the basis for distinguishing them.

   In this problem, we need to differentiate between $(C_2H_5)_2NH$ and $(C_2H_5)_3N$.

   • $(C_2H_5)_2NH$ is diethylamine, a secondary amine, as the nitrogen atom is bonded to two ethyl groups and one hydrogen atom.
   • $(C_2H_5)_3N$ is triethylamine, a tertiary amine, as the nitrogen atom is bonded to three ethyl groups and no hydrogen atoms.

   The core idea for distinguishing these two lies in finding a reagent that reacts with the N-H bond present in the secondary amine but cannot react with the tertiary amine due to the absence of such a bond.

   1. Analyze the given compounds:

      • $(C_2H_5)_2NH$ is a secondary amine. It has one hydrogen atom directly attached to the nitrogen.
      • $(C_2H_5)_3N$ is a tertiary amine. It has no hydrogen atoms directly attached to the nitrogen.

   2. Evaluate option (A): $CHCl_3 + KOH$ (Carbylamine reaction)

      • The carbylamine reaction (also known as isocyanide test) is a characteristic reaction for primary amines (both aliphatic and aromatic).
      • In this reaction, a primary amine reacts with chloroform ($CHCl_3$) and alcoholic potassium hydroxide ($KOH$) to form an isocyanide (carbylamine), which has a highly unpleasant odor.
      • Example: $R-NH_2 + CHCl_3 + 3KOH \xrightarrow{\Delta} R-NC + 3KCl + 3H_2O$
      • Secondary and tertiary amines do not give this test.
      • Therefore, this reagent cannot distinguish between a secondary amine and a tertiary amine, as neither will give a positive test.

   3. Evaluate option (B): $C_6H_5SO_2Cl$ (Hinsberg's reagent)

      • $C_6H_5SO_2Cl$ is benzenesulfonyl chloride, commonly known as Hinsberg's reagent. This reagent is specifically used to distinguish between primary, secondary, and tertiary amines.
      • Reaction with secondary amines: A secondary amine reacts with Hinsberg's reagent to form an N,N-dialkylbenzenesulfonamide.

   $(C_2H_5)_2NH + C_6H_5SO_2Cl \longrightarrow (C_2H_5)_2N-SO_2C_6H_5 + HCl$

       The product, N,N-diethylbenzenesulfonamide, does not have any acidic hydrogen attached to the nitrogen atom. Therefore, it is insoluble in alkali (like $KOH$ or $NaOH$).
   *   **Reaction with tertiary amines:** Tertiary amines do not have any hydrogen atoms attached to the nitrogen. Thus, they cannot undergo nucleophilic substitution with Hinsberg's reagent. They simply act as bases and may form a salt with the reagent if it's acidic, but no sulfonamide is formed.
   

   $(C_2H_5)_3N + C_6H_5SO_2Cl \longrightarrow \text{No reaction (no sulfonamide formed)}$

       The tertiary amine remains unreacted and is insoluble in alkali.
   *   **Distinction:** When $(C_2H_5)_2NH$ is treated with Hinsberg's reagent, an insoluble product (N,N-diethylbenzenesulfonamide) is formed. When $(C_2H_5)_3N$ is treated with Hinsberg's reagent, no reaction occurs, and the tertiary amine itself is insoluble in the aqueous layer. However, the key is the *formation of a new product* in the case of the secondary amine. The difference in reactivity (reaction vs. no reaction) allows for distinction. …
   

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2. CBSE 2026Set ANNUAL1 mark
   Q.True/False: Carboxylic acids are weaker acids than alcohols.

   ›Reveal solutionSolution

   False. Carboxylic acids are considerably stronger acids than alcohols because their conjugate base (carboxylate ion) is resonance-stabilised.

   When a carboxylic acid, $R-COOH$, loses a proton, the resulting carboxylate ion, $R-COO^-$, has its negative charge delocalised equally over both oxygen atoms by resonance, making the ion much more stable. An alkoxide ion, $R-O^-$, formed from an alcohol has no such resonance stabilisation — the negative charge stays localised on a single oxygen. Because a more stable conjugate base means a stronger acid, carboxylic …

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3. CBSE 2026Set ANNUAL1 markMCQ
   Q.Assertion (A) : The alpha-hydrogen atom in carbonyl compound is less acidic. Reason (R) : The anion formed after the loss of alpha-hydrogen atom is resonance stabilised.

   (a) Both (A) and (R) are true and (R) is the correct explanation of (A).

   (b) Both (A) and (R) are true but (R) is not the correct explanation of (A).

   (c) (A) is true but (R) is false.

   (d) (A) is false but (R) is true.

   ›Reveal solutionSolution

   The α-hydrogen of a carbonyl compound is actually MORE acidic than an ordinary alkane C–H (not less), precisely because the anion (enolate) left behind after its removal is resonance stabilised by the adjacent C=O group.

   Assertion: "The alpha-hydrogen atom in carbonyl compounds is less acidic" — this is false. In reality, α-hydrogens of carbonyl compounds are unusually acidic compared to ordinary C–H bonds (their pKa is around 20, far lower/more acidic than a typical alkane C–H at ~50).

   Reason: "The anion formed after the loss of the α-hydrogen atom is resonance stabilised" — this is true. When the α-H is removed (by a base), the resulting carbanion is stabilised by delocalisation of the negative charge onto the electronegative oxygen of the carbonyl group (forming the enolate ion):

   …

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4. CBSE 2025Set 56/6/11 markMCQ
   Q.In the Hinsberg's method for separation of primary, secondary and tertiary amines, the reagent used is : (A) Nitrous acid (B) $CHCl_3$ + aq. NaOH (C) $C_6H_5SO_2Cl$ (benzenesulphonyl chloride) (D) $HCl/ZnCl_2$

   ›Reveal solutionSolution

   Hinsberg's method separates amines based on their reactivity with benzenesulphonyl chloride ($C_6H_5SO_2Cl$). Primary amines form a soluble salt, secondary amines form an insoluble solid, and tertiary amines do not react. The correct reagent is (C).

   Why Hinsberg’s method works — the concept

   The key idea is that amines differ in how many hydrogen atoms are attached to the nitrogen. A primary amine ($RNH_2$) has two hydrogens, a secondary amine ($R_2NH$) has one, and a tertiary amine ($R_3N$) has none. Benzenesulphonyl chloride ($C_6H_5SO_2Cl$) reacts with the N–H bond, replacing the hydrogen with a sulphonyl group. The product’s solubility in alkali depends on whether there is still an N–H hydrogen left to be removed by base.

   This gives a clean, visual separation: one fraction dissolves in NaOH, another precipitates, and the third stays as an oily layer that doesn’t react at all.

   ---

   Step-by-step reasoning

   1. What does Hinsberg’s reagent do? Benzenesulphonyl chloride ($C_6H_5SO_2Cl$) is an electrophile. The nitrogen lone pair attacks the sulphur atom, displacing chloride. The product is a sulphonamide. The reaction is:

   $RNH_2 + C_6H_5SO_2Cl \rightarrow C_6H_5SO_2NHR + HCl$

   2. Primary amine — two N–H hydrogens The initial product $C_6H_5SO_2NHR$ still has one N–H hydrogen. This hydrogen is acidic enough to be removed by aqueous NaOH, forming a water-soluble sodium salt:

   $C_6H_5SO_2NHR + NaOH \rightarrow C_6H_5SO_2N(R)Na^+ + H_2O$

   So the primary amine ends up dissolved in the alkaline layer.

   3. Secondary amine — one N–H hydrogen

      The product $C_6H_5SO_2NR_2$ has no N–H hydrogen left (both are replaced by R groups). It cannot be deprotonated by NaOH, so it remains as an insoluble solid or oil that can be filtered off.

   4. Tertiary amine — no N–H hydrogen at all

      Tertiary amines have no hydrogen on nitrogen. They cannot undergo the substitution reaction with $C_6H_5SO_2Cl$ at all (no N–H bond to attack). The amine remains unreacted and can be extracted as a separate layer.

   5. Why not the other options? …

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5. CBSE 2025Set X11 markMCQ
   Q.Given below are two statements : Statement I : Ammonolysis of alkyl halides has the disadvantage of yielding a mixture of primary, secondary, tertiary amines and quaternary ammonium salt. Statement II : Tertiary amine is obtained as a major product by taking large excess of ammonia in ammonolysis of alkyl halides. In the light of the above statements, choose the appropriate answer from the options given below :

   (a) Statement I is incorrect but Statement II is correct

   (b) Both Statement I and Statement II are correct

   (c) Both Statement I and Statement II are incorrect

   (d) Statement I is correct but Statement II is incorrect

   ›Reveal solutionSolution

   Ammonolysis genuinely gives a mixture (I correct), but a large excess of ammonia favours the PRIMARY amine (not tertiary), so II is incorrect → option (d).

   Statement I — Ammonolysis of an alkyl halide with ammonia is a nucleophilic substitution in which the primary amine formed is itself a nucleophile and reacts further, giving a mixture of 1°, 2°, 3° amines and finally the quaternary ammonium salt. This is a well-known drawback of the method → correct.

   $\text{R-X} \xrightarrow{\text{NH}_3} \text{RNH}_2 \xrightarrow{\text{R-X}} \text{R}_2\text{NH} \xrightarrow{\text{R-X}} \text{R}_3\text{N} \xrightarrow{\text{R-X}} \text{R}_4\text{N}^+\text{X}^-$ …

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6. CBSE 2024Set 56/3/11 markMCQ
   Q.Which of the following compounds on treatment with benzene sulphonyl chloride forms an alkali-soluble precipitate ? (A) $CH_3CONH_2$ (B) $(CH_3)_3N$ (C) $(CH_3)_2NH$ (D) $CH_3CH_2NH_2$

   ›Reveal solutionSolution

   The Hinsberg test distinguishes amines by their reaction with benzene sulphonyl chloride: only primary amines form N-alkyl sulphonamides that are acidic enough to dissolve in alkali. The answer is (D) $CH_3CH_2NH_2$.

   The question tests the Hinsberg test, a classic method to distinguish between primary, secondary, and tertiary amines using benzene sulphonyl chloride ($C_6H_5SO_2Cl$). The key insight is that different classes of amines react differently, and only one product has the right acidity to dissolve in base after initially precipitating.

   When benzene sulphonyl chloride reacts with an amine, it acts as an electrophile. The nitrogen's lone pair attacks the sulphur, displacing chloride. But what happens next depends entirely on whether the nitrogen still has a hydrogen attached.

   Why acidity matters

   A sulphonamide with an N–H bond is surprisingly acidic (p$K_a$ ~ 10) because the negative charge on nitrogen, after deprotonation, is stabilized by resonance with the adjacent $SO_2$ group. The sulphonyl group is strongly electron-withdrawing, delocalizing the negative charge onto the oxygens. This makes the conjugate base stable enough that aqueous alkali (NaOH) can deprotonate it, converting the precipitate into a soluble sodium salt.

   Step-by-step analysis

   1. Option (A): $CH_3CONH_2$ (acetamide)

      This is an amide, not an amine. Amides are extremely weak nucleophiles because the lone pair on nitrogen is delocalized into the carbonyl $\pi^*$ orbital. Benzene sulphonyl chloride won't react with it under normal Hinsberg conditions. No precipitate forms at all.

   2. Option (B): $(CH_3)_3N$ (trimethylamine, tertiary)

      Tertiary amines have no N–H bond. They can form an unstable ionic complex with the sulphonyl chloride, but they cannot form a stable sulphonamide (no hydrogen to lose as HCl). The product, if any, remains in solution or decomposes. No precipitate.

   3. Option (C): $(CH_3)_2NH$ (dimethylamine, secondary)

      Secondary amines react to form N,N-dialkyl sulphonamides:

      $(CH_3)_2NH + C_6H_5SO_2Cl \longrightarrow C_6H_5SO_2N(CH_3)_2 + HCl$ …

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7. CBSE 2024Set 56/2/11 markMCQ
   Q.Anisole reacts with HI to give : (A) Phenol + $CH_3-I$ (B) Iodobenzene + $CH_3-OH$ (C) Benzyl alcohol + $CH_3-I$ (D) Benzyl iodide + $CH_3-OH$

   ›Reveal solutionSolution

   Anisole undergoes nucleophilic substitution with HI, where iodide ion attacks the less hindered methyl carbon (not the aromatic ring), cleaving the $\ce{C-O}$ bond to yield phenol and methyl iodide.

   Understanding Ether Cleavage with Hydrogen Halides

   Anisole is methoxybenzene, $\ce{C6H5-O-CH3}$, an aromatic ether. When ethers react with strong acids like HI, they undergo cleavage through nucleophilic substitution. The key is understanding where the bond breaks and why.

   Hydrogen iodide is both a strong acid and an excellent nucleophile (iodide ion). The reaction proceeds in two conceptual stages: protonation followed by nucleophilic attack.

   Step-by-Step Mechanism

   1. Protonation of the ether oxygen The lone pair on oxygen accepts a proton from HI, converting the ether into an oxonium ion:

   $\ce{C6H5-O-CH3 + HI -> C6H5-\overset{+}{O}H-CH3 + I^-}$

   This protonation is crucial because it transforms oxygen from a poor leaving group ($\ce{O^-}$ would be terrible) into a good one ($\ce{OH}$, a neutral molecule).

   2. Nucleophilic attack by iodide

      Now the iodide ion must attack. But where? Two carbons are bonded to oxygen: the aromatic ring carbon and the methyl carbon. The iodide attacks the methyl carbon because:

      • It's less sterically hindered (primary vs. aromatic)
      • $\mathrm{S_N2}$ displacement at an $sp^3$ carbon is facile
      • Attack at the aromatic carbon would require breaking aromaticity, which is energetically prohibitive

   3. Bond cleavage and product formation

      The $\ce{C-O}$ bond between methyl and oxygen breaks as iodide displaces the phenol:

   $\ce{C6H5-\overset{+}{O}H-CH3 + I^- -> C6H5-OH + CH3-I}$

   The products are phenol ($\ce{C6H5OH}$) and methyl iodide ($\ce{CH3I}$). …

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8. CBSE 2024Set 56/2/11 markMCQ
   Q.Ethanol on heating with conc. $H_2SO_4$ at 413 K gives : (A) $C_2H_5OSO_3H$ (B) $C_2H_5-O-CH_3$ (C) $C_2H_5-O-C_2H_5$ (D) $CH_2=CH_2$

   ›Reveal solutionSolution

   At 413 K, concentrated sulfuric acid dehydrates ethanol to form diethyl ether via an intermolecular dehydration mechanism. The correct product is diethyl ether, option (C).

   The Concept: Nucleophilic Substitution in Alcohol Dehydration

   When ethanol is heated with concentrated sulfuric acid, the acid acts as both a catalyst and a dehydrating agent. The key is temperature control — the same reactants give different products at different temperatures. At 413 K (≈140 °C), the reaction favours intermolecular dehydration (between two ethanol molecules), producing an ether. At a higher temperature (443 K, ≈170 °C), intramolecular dehydration (within one molecule) dominates, giving ethene.

   The mechanism is a classic nucleophilic substitution (SN2-like) where one ethanol molecule acts as the nucleophile and another, after protonation, becomes the electrophile.

   Step-by-Step Reasoning

   1. Protonation of ethanol Concentrated $H_2SO_4$ donates a proton to the hydroxyl group of ethanol:

   $C_2H_5OH + H^+ \rightleftharpoons C_2H_5OH_2^+$

   This converts the poor leaving group ($-OH$) into a good one ($-OH_2^+$).

   2. Nucleophilic attack by a second ethanol molecule A second ethanol molecule (the nucleophile) attacks the electron-deficient carbon attached to the protonated hydroxyl:

   $C_2H_5OH + C_2H_5OH_2^+ \rightarrow [C_2H_5-O(H)-C_2H_5]^+ + H_2O$

   This is an SN2-like step — the oxygen lone pair of the attacking ethanol displaces water.

   3. Deprotonation to form the ether The oxonium ion intermediate loses a proton to a base (e.g., $HSO_4^-$ or water):

   $[C_2H_5-O(H)-C_2H_5]^+ \rightarrow C_2H_5-O-C_2H_5 + H^+$

   The proton is recycled, regenerating the acid catalyst. …

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9. CBSE 2024Set 56/2/11 markMCQ
   Q.Assertion (A) : Aliphatic primary amines can be prepared by Gabriel phthalimide synthesis. Reason (R) : Alkyl halides undergo nucleophilic substitution with anion formed by phthalimide. Select the correct answer from the codes given below : (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Assertion (A) is false, but Reason (R) is true.

   ›Reveal solutionSolution

   Gabriel phthalimide synthesis is a highly effective method for preparing pure primary aliphatic amines because the phthalimide anion undergoes nucleophilic substitution with alkyl halides, and the subsequent hydrolysis yields only primary amines, preventing overalkylation. Both Assertion (A) and Reason (R) are true, and Reason (R) correctly explains Assertion (A).

   Gabriel phthalimide synthesis is a classic and very important reaction in organic chemistry, specifically designed for the preparation of primary amines. The key challenge in synthesizing primary amines directly from ammonia and alkyl halides is that the primary amine formed can act as a nucleophile itself, reacting further to produce secondary and tertiary amines, and even quaternary ammonium salts. This leads to a mixture of products that is difficult to separate. Gabriel synthesis elegantly bypasses this problem.

   The underlying principle of Gabriel synthesis relies on using a protected form of ammonia (phthalimide) that can only be alkylated once, followed by a reaction that releases the primary amine.

   Here's a step-by-step breakdown of the process and the reasoning behind it:

   1. Formation of the Phthalimide Anion:

      Phthalimide is an imide, meaning it has an $-\text{NH}-$ group flanked by two carbonyl groups. The hydrogen atom attached to the nitrogen is acidic because the resulting anion (phthalimide anion) is resonance-stabilized by the two adjacent carbonyl groups.

      When phthalimide is treated with a strong base, such as potassium hydroxide ($\text{KOH}$) or sodium ethoxide ($\text{NaOEt}$), it loses this acidic proton to form a stable, negatively charged phthalimide anion.

      $\text{Phthalimide} + \text{KOH} \longrightarrow \text{Potassium phthalimide} + \text{H}_2\text{O}$

      The nitrogen atom in this anion carries a negative charge, making it a strong nucleophile.

   2. Nucleophilic Substitution Reaction:

      The potassium phthalimide (or the phthalimide anion) then reacts with an alkyl halide ($R-X$, where $R$ is an aliphatic alkyl group and $X$ is a halogen like $\text{Cl}$, $\text{Br}$, or $\text{I}$). This is a classic $S_N2$ (bimolecular nucleophilic substitution) reaction. The nucleophilic nitrogen of the phthalimide anion attacks the electrophilic carbon atom bearing the halogen in the alkyl halide, displacing the halide ion.

      $\text{Potassium phthalimide} + R-X \longrightarrow N\text{-alkylphthalimide} + \text{KX}$

      This step is precisely what Reason (R) describes: "Alkyl halides undergo nucleophilic substitution with anion formed by phthalimide." This reaction incorporates the desired alkyl group ($R$) onto the nitrogen atom.

      Watch out

      This $S_N2$ reaction works best with primary alkyl halides. Secondary alkyl halides may undergo elimination ($E2$) reactions, and tertiary alkyl halides predominantly undergo elimination. Aryl halides (like bromobenzene) do not undergo this nucleophilic substitution reaction under these conditions because the carbon-halogen bond in aryl halides is much stronger and less susceptible to $S_N2$ attack due to the $sp^2$ hybridization of the carbon and resonance effects. This is why the assertion specifies "aliphatic primary amines."

   3. Hydrolysis to Yield Primary Amine:

      The $N$-alkylphthalimide formed in the previous step is then hydrolyzed. This can be achieved by heating with an aqueous acid (like $\text{HCl}$) or a base (like $\text{NaOH}$), or more commonly and efficiently, by treating it with hydrazine ($\text{N}_2\text{H}_4$).

      • Acidic/Basic Hydrolysis: This breaks the two amide bonds, releasing the primary amine ($R-\text{NH}_2$) and phthalic acid (or its salt). …

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10. CBSE 2024Set ANNUAL1 mark
    Q.Why do amines act as nucleophiles?

    ›Reveal solutionSolution

    Amines act as nucleophiles because the nitrogen atom carries a lone pair of electrons that it can readily donate to an electron-deficient (electrophilic) centre.

    In an amine, $R{-}\overset{\displaystyle ..}{N}H_2$, nitrogen is $sp^3$ hybridized with three bond pairs (to $R$ and two $H$, or the equivalent for secondary/tertiary amines) and one lone pair occupying the fourth $sp^3$ orbital. This lone pair is:

    • Not delocalized/tied up in any $\pi$-system (unlike, say, the nitrogen lone pair in an amide, which is drawn into conjugation with the carbonyl and is far less available).
    • Available for donation — nitrogen's relatively low electronegativity and its non-bonding electron pair together make it a good electron-pair donor. …

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11. CBSE 2023Set 56/1/11 markMCQ
    Q.The synthesis of alkyl fluoride is best obtained from : (A) Free radicals (B) Swartz reaction (C) Sandmeyer reaction (D) Finkelstein reaction

    ›Reveal solutionSolution

    The best method for synthesizing alkyl fluorides is the Swartz reaction, which uses $\text{Hg}_2\text{F}_2$ or $\text{CoF}_2$ to replace chlorine/bromine with fluorine. The correct option is (B).

    Why this question matters

    Alkyl fluorides are the most stable of the alkyl halides due to the strong C–F bond, but they are also the hardest to make by simple nucleophilic substitution. Fluoride ion ($\text{F}^-$) is a poor nucleophile in polar solvents because it is heavily solvated (small, high charge density) and also a strong base — so direct $\text{S}_\text{N}2$ with $\text{F}^-$ often gives elimination instead. This is why special methods exist.

    Let’s examine each option.

    ---

    1. Free radicals (Option A)

    Free radical halogenation of alkanes with fluorine is violently exothermic and uncontrollable — it typically explodes or gives polyfluorinated products. Even with careful conditions, selectivity is terrible. This is not a practical laboratory synthesis for a specific alkyl fluoride.

    ---

    2. Swartz reaction (Option B)

    This is the classic method. A silver or mercury fluoride (like $\text{AgF}$, $\text{Hg}_2\text{F}_2$, or $\text{CoF}_3$) is used to replace a chlorine or bromine atom with fluorine:

    $\text{R–Cl} + \text{Hg}_2\text{F}_2 \rightarrow \text{R–F} + \text{Hg}_2\text{Cl}_2$

    The driving force is the precipitation of the metal halide (e.g., $\text{Hg}_2\text{Cl}_2$ is insoluble). This works cleanly for alkyl, allyl, and benzyl halides. It is the standard method for making alkyl fluorides in the lab.

    Tip

    Swartz reaction is to alkyl fluorides what the Finkelstein reaction is to alkyl iodides — a specific halide-exchange method that works because the byproduct is insoluble.

    ---

    3. Sandmeyer reaction (Option C)

    This is for converting aryl diazonium salts into aryl halides (Cl, Br, I, CN) using copper(I) salts. It does not give alkyl fluorides, and it does not work for fluorine (the fluoro analogue uses $\text{HBF}_4$ — the Schiemann reaction, not Sandmeyer). So this is irrelevant here.

    ---

    4. Finkelstein reaction (Option D) …

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12. CBSE 2023Set 56/2/11 markMCQ
    Q.Given below are two statements labelled as Assertion (A) and Reason (R). Select the most appropriate answer from the options given below : Assertion (A) : Nucleophilic substitution of iodoethane is easier than chloroethane. Reason (R) : Bond enthalpy of C-I bond is less than that of C-Cl bond. (A) Both (A) and (R) are true and (R) is the correct explanation of (A). (B) Both (A) and (R) are true, but (R) is not the correct explanation of (A). (C) (A) is true, but (R) is false. (D) (A) is false, but (R) is true.

    ›Reveal solutionSolution

    The ease of nucleophilic substitution depends on the leaving group's ability to depart. A weaker C–I bond (lower bond enthalpy) makes iodide a better leaving group than chloride, so both Assertion and Reason are true, and Reason correctly explains Assertion.

    Concept first: what makes a good leaving group in nucleophilic substitution?

    In an $S_N1$ or $S_N2$ reaction, the leaving group (halide ion) must break away from the carbon. The weaker the carbon–halogen bond, the easier it is to break — so the halide leaves more readily. Bond enthalpy (bond dissociation energy) is a direct measure of bond strength: lower bond enthalpy means a weaker bond.

    Iodine is a larger atom than chlorine, so the C–I bond is longer and weaker. The C–I bond enthalpy is about 240 kJ/mol, while the C–Cl bond enthalpy is about 330 kJ/mol. That difference is the key.

    Now let’s check each statement.

    1. Assertion (A): "Nucleophilic substitution of iodoethane is easier than chloroethane."

       This is true. In both $S_N1$ and $S_N2$ mechanisms, the rate-determining step involves breaking the C–X bond (in $S_N1$, it’s the first step; in $S_N2$, it’s the concerted step where the leaving group departs). Since the C–I bond is weaker, iodoethane reacts faster than chloroethane under identical conditions. Iodide is a better leaving group than chloride.

    2. Reason (R): "Bond enthalpy of C–I bond is less than that of C–Cl bond."

       This is also true. Bond enthalpy decreases down the halogen group: C–F > C–Cl > C–Br > C–I. The C–I bond is indeed weaker.

    3. Does (R) correctly explain (A)? …

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