Chapter 9: Amines Long Questions | chemistry Class 12 CBSE

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Chapter 9: Amines Long Questions | chemistry Class 12 CBSE | ByteNotes.in

Long Answer

Hard difficulty • Structured explanation

Hard

Question 1

Long Form

Compare and contrast the basicity of aliphatic and aromatic amines with respect to ammonia, considering inductive effect, resonance, solvation, and steric hindrance.

Aliphatic amines are stronger bases than ammonia because alkyl groups exert a +I effect, pushing electron density onto nitrogen and stabilising the ammonium cation by charge dispersal; pKb values of alkylamines are 3–4.22 versus 4.75 for NH3.
In aqueous solution, solvation of the ammonium cation by water molecules also stabilises it; the primary ammonium cation is most solvated (more N–H bonds for H-bonding), making the order in water: secondary > primary > tertiary > NH3 for methyl series.
Aromatic amines are weaker bases than ammonia (aniline pKb = 9.38) because the lone pair on N is delocalised into the benzene ring through resonance, making it less available for protonation.
Aniline has five resonance structures while the anilinium ion has only two; greater resonance stabilisation of the free base relative to its conjugate acid means aniline is less basic.
Electron-releasing groups (–CH3, –OCH3) on the ring increase the basic strength of aniline derivatives, while electron-withdrawing groups (–NO2, –COOH) decrease it by further depleting the lone pair density.

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Long Answer

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Hard

Question 2

Long Form

Describe six methods of preparation of amines, highlighting the type of amine obtained and any important limitation in each method.

Reduction of nitro compounds (H2/Ni, Pd, Pt or Fe/HCl): gives primary amines; Fe/HCl is preferred as FeCl2 hydrolyses to regenerate HCl; applicable to both aromatic (ArNO2 → ArNH2) and aliphatic nitro compounds.
Ammonolysis of alkyl halides (ethanolic NH3, 373 K, sealed tube): yields a mixture of 1°, 2°, 3° amines and quaternary salt; primary amine is major product when excess NH3 is used; order of reactivity RI > RBr > RCl.
Reduction of nitriles (LiAlH4 or H2/Ni): gives primary amines containing one more carbon than the starting amine (ascent of amine series); e.g., R–C≡N → R–CH2–NH2.
Reduction of amides (LiAlH4): gives the corresponding amine; nature of amine (1°, 2°, 3°) depends on the amide used.
Gabriel phthalimide synthesis: phthalimide + KOH → potassium salt; alkylation with RX → N-alkylphthalimide; alkaline hydrolysis → pure primary amine; cannot prepare aromatic primary amines.
Hoffmann bromamide degradation (Br2/aq.NaOH): amide → primary amine with one less carbon; alkyl/aryl group migrates from C to N; reaction: RCONH2 + Br2 + 4NaOH → RNH2 + Na2CO3 + 2NaBr + 2H2O.

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Long Answer

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Hard

Question 3

Long Form

Discuss the electrophilic substitution reactions of aniline, explaining why it undergoes tribromination and how acetylation helps in controlling the reaction to give monosubstituted products.

Aniline's –NH2 group is ortho and para directing because the lone pair on nitrogen delocalises into the benzene ring, creating high electron density at ortho and para positions.
Bromination with Br2/H2O gives 2,4,6-tribromoaniline directly as a white precipitate, because the ring is so activated that all three ortho/para positions react without a Lewis acid catalyst.
To obtain monosubstituted products (e.g., 4-bromoaniline), the –NH2 group is first protected by acetylation with acetic anhydride/pyridine to give acetanilide (N-phenylethanamide).
In acetanilide, the lone pair on nitrogen is delocalised towards the oxygen of the carbonyl by resonance, reducing the activating effect; bromination now gives predominantly the para product.
Finally, the acetyl group is removed by acid or base hydrolysis to restore the –NH2 group, yielding 4-bromoaniline as the major product.

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Long Answer

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Hard

Question 4

Long Form

Explain the chemical reactions of diazonium salts involving displacement of the diazonium group, giving equations and naming each reaction where applicable.

Sandmeyer reaction: Cl–, Br–, or CN– is introduced using Cu(I) salts (CuCl/HCl, CuBr/HBr, CuCN/KCN) with displacement of N2 gas; yield is better than in Gattermann reaction.
Gattermann reaction: Cl– or Br– is introduced by treating the diazonium salt with copper powder and the halogen acid (Cu/HCl or Cu/HBr); N2 is released.
Replacement by I–: treatment with aqueous KI gives iodobenzene; ArN2+Cl– + KI → ArI + KCl + N2; iodine cannot be introduced by direct halogenation.
Replacement by F– (Balz-Schiemann): diazonium chloride is treated with HBF4 to precipitate arene diazonium fluoroborate, which on heating gives aryl fluoride, BF3, and N2.
Replacement by H (reductive deamination): mild reducing agents like hypophosphorous acid or ethanol reduce the diazonium salt to arene; ArN2+Cl– + H3PO2 + H2O → ArH + N2 + H3PO3 + HCl.

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Long Answer

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Medium

Question 5

Long Form

Analyse how physical properties of amines—odour, state, solubility, and boiling point—are related to their molecular structure and hydrogen bonding ability.

Lower aliphatic amines are gases with fishy odour; primary amines with three or more carbons are liquids; still higher amines are solids; aniline and arylamines are colourless liquids that turn brown on storage due to atmospheric oxidation.
Lower amines are soluble in water due to hydrogen bonding with water molecules (N has EN = 3.0); solubility decreases with increasing molar mass as the hydrophobic alkyl part grows.
Amines are also soluble in organic solvents like alcohol, ether, and benzene.
Primary amines have two N–H hydrogens for intermolecular hydrogen bonding, giving them the highest boiling points; secondary amines have one N–H, giving intermediate boiling points; tertiary amines cannot form N–H…N hydrogen bonds and have the lowest boiling points.
Compared to alcohols of similar molar mass, amines have lower boiling points because N–H bonds are less polar than O–H bonds; e.g., n-C4H9OH (bp 390.3 K) > n-C4H9NH2 (bp 350.8 K).

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Long Answer

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Hard

Question 6

Long Form

Describe Gabriel phthalimide synthesis and Hoffmann bromamide degradation as methods for preparing primary amines. Compare their utility and limitations.

Gabriel synthesis: phthalimide is treated with ethanolic KOH to give its potassium salt; this is heated with an alkyl halide (RX) to give N-alkylphthalimide; alkaline hydrolysis then liberates the primary amine (RNH2) and disodium phthalate.
Limitation of Gabriel: aromatic primary amines cannot be prepared because aryl halides resist nucleophilic substitution with the phthalimide anion; only aliphatic primary amines are accessible.
Hoffmann degradation: an amide (RCONH2) reacts with Br2 and aqueous NaOH; an alkyl or aryl group migrates from carbonyl carbon to nitrogen to give a primary amine (RNH2) with one carbon less than the amide.
Limitation of Hoffmann: the method is a descending reaction—it reduces the carbon chain by one; the amide must be available; the reaction conditions must be carefully controlled.
Comparative utility: Gabriel synthesis gives pure primary amines without a mixture of products (unlike ammonolysis); Hoffmann degradation is preferred when an amide is available and a one-carbon-shorter primary amine is needed.

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Long Answer

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Medium

Question 7

Long Form

Explain the coupling reactions of diazonium salts, the structure of azo products, and their importance as dyes.

Coupling reactions occur between an arenediazonium salt and an electron-rich aromatic compound (phenol or arylamine) by electrophilic substitution, predominantly at the para position.
Benzenediazonium chloride couples with phenol in alkaline medium at the para position to form p-hydroxyazobenzene (orange dye): C6H5N2+Cl– + C6H5OH → C6H5–N=N–C6H4–OH + HCl.
With aniline in acidic medium it forms p-aminoazobenzene (yellow dye): C6H5N2+Cl– + C6H5NH2 → C6H5–N=N–C6H4–NH2 + HCl.
Azo products contain an extended conjugated system with both aromatic rings joined through the –N=N– (azo) bond; this extended conjugation is responsible for the intense colour.
These azo compounds are used as azo dyes in the textile and food industries; the retention of the diazonium/azo group (no N2 loss) distinguishes coupling from displacement reactions of diazonium salts.

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Long Answer

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Hard

Question 8

Long Form

Discuss the reaction of amines with nitrous acid, contrasting the behaviour of primary, secondary, and tertiary aliphatic and aromatic amines.

Nitrous acid (HNO2) is prepared in situ from NaNO2 and a mineral acid; it reacts differently with each class of amine.
Primary aliphatic amines form alkyldiazonium salts that are highly unstable; they immediately decompose to give nitrogen gas quantitatively and alcohols; quantitative N2 evolution is used in amino acid estimation.
Primary aromatic amines (e.g., aniline) react with NaNO2/HCl at 273–278 K to form stable arenediazonium salts (e.g., benzenediazonium chloride); these are used as important synthetic intermediates.
Secondary amines (aliphatic or aromatic) react with nitrous acid to form N-nitrosamines, which are yellow oily compounds and generally carcinogenic.
Tertiary amines react with nitrous acid in a different manner without forming diazonium or nitroso products; this differential reactivity is used to distinguish between amine classes.

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Long Answer

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Hard

Question 9

Long Form

Examine how the basic strength of methylamines and ethylamines in aqueous solution deviates from the expected trend based solely on the inductive effect, and explain the factors responsible.

Expected order based on +I effect alone: tertiary > secondary > primary > NH3; however, in aqueous solution this order is not followed.
Observed order for methylamines: (CH3)2NH > CH3NH2 > (CH3)3N > NH3; for ethylamines: (C2H5)2NH > (C2H5)3N > C2H5NH2 > NH3.
In aqueous solution, the ammonium cation is stabilised not only by +I effect but also by solvation; primary ammonium ions have three N–H bonds and are most effectively solvated by water.
As the size of alkyl group increases (especially beyond CH3), steric hindrance reduces solvation; bulkier tertiary ammonium ions are less solvated and hence less stabilised despite the maximum +I effect.
The interplay of inductive effect (increases with alkylation), solvation effect (decreases with increasing size), and steric hindrance determines the actual basic strength in aqueous solution.

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Long Answer

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Question 10

Long Form

Summarise the importance of diazonium salts as synthetic intermediates for preparing substituted aromatic compounds that cannot be made by direct substitution.

Diazonium salts allow introduction of –F, –Cl, –Br, –I, –CN, –OH, –NO2, and –H into the aromatic ring via replacement of the diazonium group; nitrogen gas is released in all displacement reactions.
Aryl fluorides (fluorobenzene) cannot be prepared by direct halogenation but are made via the Balz-Schiemann reaction (diazonium chloride + HBF4, then heat).
Aryl iodides (iodobenzene) are also not accessible by direct halogenation but are obtained by treating the diazonium salt with aqueous KI.
Cyanobenzene cannot be made by nucleophilic substitution of chlorobenzene (aryl ring resists SN reactions) but is easily obtained from benzenediazonium salt via Sandmeyer reaction with CuCN/KCN.
Coupling reactions with phenols and arylamines produce azo dyes; thus diazonium salts bridge arylamines and a vast range of aromatic derivatives, making them indispensable in aromatic synthesis.

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Chapter 9: Amines | Long Questions

Compare and contrast the basicity of aliphatic and aromatic amines with respect to ammonia, considering inductive effect, resonance, solvation, and steric hindrance.

Describe six methods of preparation of amines, highlighting the type of amine obtained and any important limitation in each method.

Discuss the electrophilic substitution reactions of aniline, explaining why it undergoes tribromination and how acetylation helps in controlling the reaction to give monosubstituted products.

Explain the chemical reactions of diazonium salts involving displacement of the diazonium group, giving equations and naming each reaction where applicable.

Analyse how physical properties of amines—odour, state, solubility, and boiling point—are related to their molecular structure and hydrogen bonding ability.

Describe Gabriel phthalimide synthesis and Hoffmann bromamide degradation as methods for preparing primary amines. Compare their utility and limitations.

Explain the coupling reactions of diazonium salts, the structure of azo products, and their importance as dyes.

Discuss the reaction of amines with nitrous acid, contrasting the behaviour of primary, secondary, and tertiary aliphatic and aromatic amines.

Examine how the basic strength of methylamines and ethylamines in aqueous solution deviates from the expected trend based solely on the inductive effect, and explain the factors responsible.

Summarise the importance of diazonium salts as synthetic intermediates for preparing substituted aromatic compounds that cannot be made by direct substitution.

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