//M0//QN1//CS//DL0

Amines are nitrogen-containing organic compounds categorized based on the number of alkyl or aryl groups attached to the nitrogen atom. Primary amines (R-NH2) have one alkyl/aryl group, secondary amines (R2-NH) have two, and tertiary amines (R3-N) have three. Amines are further classified as alkyl or aryl amines depending on whether the substituent is an alkyl group (e.g., CH3-NH2) or an aromatic ring

(e.g., C6H5-NH2). Their structural diversity makes the key intermediates in organic synthesis.

(a) Explain the difference between primary and secondary amines using examples.(b) Why are amines considered nucleophilic in nature?(c) What type of amine is aniline, and why?(d) How does the electronic effect of aryl groups affect the basicity of amines?

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(a) Primary amines have one alkyl/aryl group
e.g., (CH3-NH2), while secondary amines have two (e.g., (CH3)2-NH).
(b) Due to the lone pair of electrons on the nitrogen atom, which can donate electrons.
(c) Aniline (C6H5-NH2) is a primary aromatic amine as it has one aromatic group attached to nitrogen.
(d) The electron-withdrawing resonance effect in aryl groups decreases the availability of the lone pair on nitrogen, reducing basicity.

//M0//QN2//CS//DL0//EQ

Amines can be prepared through various methods, such as the reduction of nitro compounds, alkylation of ammonia, and the Gabriel phthalimide synthesis. For example, nitrobenzene (C6H5-NO2) can be reduced to aniline (C6H5-NH2) using Sn/HCl or catalytic hydrogenation. The Gabriel phthalimide synthesis is specifically used for preparing primary amines using potassium phthalimide as an intermediate.
(a) What is the role of Sn/HCl in the preparation of aniline from nitrobenzene?(b) Why is the Gabriel phthalimide synthesis suitable only for primary amines?(c) Write the chemical reaction for the reduction of nitrobenzene using Sn/HCl.(d) How does catalytic hydrogenation differ from Sn/HCl reduction in terms of application?

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(a) It acts as a reducing agent to convert nitrobenzene into aniline.
(b) The reaction mechanism ensures the production of a single alkyl group attached to nitrogen.
(c) C6H5NO2+6[H] C6H5NH2+2H2O.
(d) Catalytic hydrogenation is preferred for industrial-scale reactions due to its efficiency and cleaner by-products.

//M0//QN3//CS//DL0

Amines are basic in nature, and their basic strength is expressed through the dissociation constant Kb or its logarithmic form pKb. The reaction can be represented as:
RNH2 + H2O ↔ RNH3+ + OH–
Kb = [RNH+3][OH–] / [RNH2] and pKb = –logKb
A higher Kb value or a smaller pKb value indicates stronger basicity. Aliphatic amines are more basic than aryl amines like aniline because the lone pair of electrons on the nitrogen atom in aniline participates in resonance with the benzene ring, reducing its availability for protonation. Substituents also influence the basicity of aniline. Electron-releasing groups increase basic strength, while electron-withdrawing groups decrease it. The effect of substituents is more pronounced at the para (p) position than at the meta (m) position. Additionally, ortho (o) substituted anilines are less basic due to the combined effects of electronic and steric hindrance, known as the ortho effect.
(a) Why are aliphatic amines more basic than aromatic amines like aniline?(b) What is the relationship between Kb, pKb and basic strength of amines?(c) How do substituents at the para position affect the basicity of aniline derivatives?(d) What causes the ortho effect in o-substituted anilines?

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(a) The lone pair on nitrogen in aniline participates in resonance with the benzene ring, reducing its availability for protonation.
(b) Higher Kb or lower pKb indicates stronger basicity.
(c) Electron-releasing groups increase basicity, while electron-withdrawing groups decrease it, with a stronger effect at the para position.
(d) The ortho effect arises from a combination of electronic effects and steric hindrance, decreasing the basicity.

//M0//QN4//CS//DL0//EQ

Amines undergo diverse chemical reactions, including alkylation, acylation, and diazotization. In diazotization, primary aromatic amines react with nitrous acid to form diazonium salts, which are key intermediates in azo dye synthesis. The Carbylamine test is used to identify primary amines by producing isocyanides with a characteristic foul odour.
(a) Why is diazotization specific to primary aromatic amines?(b) Write the chemical equation for the Carbylamine test.(c) What product is formed when aniline reacts with bromine water?(d) How can diazonium salts be used in the synthesis of azo dyes?

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(a) Only primary aromatic amines can form stable diazonium salts under reaction conditions.
(b) R–NH2+CHCl3 + 3KOH R–N≡C + 3KCl
+ 3H2O
(c) 2,4,6-Tribromoaniline is formed due to electrophilic substitution at ortho and para positions.
(d) They react with phenols or aromatic amines to form azo compounds through electrophilic coupling.

//M0//QN5//CS//DL0

A pharmaceutical lab synthesizes several aromatic and aliphatic amines used in drug intermediates. To optimise reaction yields, chemist study:

Basicity of amines.
Aromatic Vs aliphatic reactivity.
Electrophilic substitution on aniline.
Acylation and alkylation reaction.
Distinguishing tests for amines.
They perform the following experiments: 1. Aniline reacts with nitrous acid at 273K to give diazonium salt. 2. Ethylamine reacts with nitrous acid to give ethanol (clear solution) 3. Aniline reacts with bromine water to give 2, 4, 6 - tribromoaniline (white ppt) 4. Basicity order observed (in aqueous solution) Ethylamine > Ammonia > Aniline 5. Acetanilide is less reactive than aniline towards bromination(a) Explain why aniline is less basic than ethylamine in aqueous solution.(b) Why does aniline react with nitrous acid to give diazonium salt, but ethylaminc give alcohol?(c) Why does aniline undergo bromination very quickly with Br2 water?(d) Why is acetanilide less reactive than aniline toward electrophilic substitution?

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(a) Basicity order is:
Ethylamine > Ammonia > Aniline
Reason:
In aniline, the lone pair on nitrogen is delocalised into the benzene ring through resonance.
Therefore, it is less available for protonation.
In ethylamine, the +I effect of the alkyl group increases electron density.
(b) Different reaction with nitrous acid:
Aniline (Aromatic amine):
C6H5NH2 + HNO2 → C6H5N+2 Cl–
Aromatic amine forms stable diazonium salt.
Ethylamine (Aliphatic amine):
C2H5NH2 + HNO2 → C2H5 OH + N2 + H2O
Aliphatic diazonium salts are unstable.
So, the decompose → So alcohol form.
(c) In Aniline, –NH2 group is strongly activating,
+M effect.
Electron density increases at ortho and para positions.
Br2 water reacts violently.
(d) In acetanilide, –NHCOCH3 group is less reactive.
Reason:
Less activating
Resonance reduces lone pair availability
Weaker electron - donating than - NH2

//M0//QN6//CS//DL0

Thus, electrophilic substitution slows down. A due manufacturing company uses diazonium salts to prepare azo dyes. To improve the brightness of dye and its stability, they conducted the following experiment:

Diazonium salt formation
Coupling reaction with phenols and anilines.
Stability of diazonium salts.
Reduction reaction
Substitution on aromatic rings via diazonium intermediates.
Results:
(i) Bezenediazonium chloride is stable at 273K, but decomposes above 283K.(ii) It couples with phenol to produce p-hydroxyazobenzene (orange dye)(iii) Reduction with Sn / HCl converts diazonium salt to aniline.(iv) Diazonium salts can be replaced by:
Br (using CuBr)
CN (using CuCN)
H (using hypophosphorous acid)
(v) Electron-donating groups on phenol increases coupling rate(a) Why are diazonium salts stable only at low temperature?(b) Explain why phenol react strongly with diazonium salt in the para position.(c) Give the reaction when benzene diazonium chloride reacts with CuBr.(d) Why does reduction of diazonium salt with

Sn / HCl regenerate aniline?

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(a) Diazonium salt decompose to N2 gas at higher temperature.
Low temperature (273K) prevent decomposition, stabilises N ≡ N+ bond and maintains aromatic diazonium cation.
(b) Phenol has –OH group: So,
Strong electron - donating (+M)
Increases electron density at ortho and para location.
Para position is less sterically hindered → coupling occurs predominantly at para position.
(c) Sandmeyer reaction:
C6H5N+2 Cl– + CuBr → C6H5Br + N2 + CuCl
(d) Sn / HCl reduces diazonium group:
C6H5N+2 Cl– → C6H5NH2
Reason:
Sn / HCl donates Hydrogen
Converts – N+2 into –NH2
Restores aniline.
Amine: Derivative of ammonia where one or more hydrogens are replaced by alkyl or aryl groups.
Primary / Secondary / Tertiary Amine: Amines
containing one, two, or three organic groups attached to nitrogen (RNH2, R2NH, R3N).
Quaternary Ammonium Salt: Nitrogen bonded to four organic groups forming a permanently charged cation (R4N+X–).
Benzylic / Allylic / Vinylic Amine: Benzylic = attached next to benzene; allylic = next to C=C; vinylic = directly on C=C (least reactive).
Basicity (of Amines): Ability of nitrogen to donate its lone pair; influenced by inductive effects, resonance, and solvation.
pKb / Kb: Quantitative measure of amine basic strength; smaller pKb means a stronger base.
Ammonolysis: Reaction of alkyl halides with ammonia to form amines; gives mixtures unless NH3 is in excess.
Gabriel Synthesis: Method to prepare primary aliphatic amines using phthalimide alkylation followed by hydrolysis.
Hofmann Bromamide Reaction: Converts amides to amines with one carbon less using Br2 and base.
Reduction Methods: Conversion of nitro compounds, nitriles, or amides intoamines using reducing agents (H2/Pd, Sn/HCl, LiAlH4, etc.).
Alkylation of Amines: Reaction with alkyl halides forming higher amines; may lead to quaternary salts.
Acylation of Amines: Formation of amides by reacting amines with acyl chlorides or anhydrides.
Hinsberg Test: Differentiates 1°, 2°, and 3° amines based on solubility patterns with benzene sulfonyl chloride.
Carbylamine Test: Test for primary amines producing foul-smelling isocyanides with CHCl3 and KOH.
Diazotization: Formation of diazonium salts when 1° aromatic amines react with nitrous acid in cold conditions.
Diazonium Salt: Highly reactive intermediate (Ar–N+2X–) used for aromatic substitutions and dye formation.
Sandmeyer Reaction: Conversion of diazonium salts to halo-/cyano-arenes using Cu(I) salts.
Azo Coupling: Reaction of diazonium salts with activated aromatics to form colourful azo dyes.
Electrophilic Substitution on Aniline: Aniline strongly activates the benzene ring; protection (acylation) is often required to control substitution.
Solubility & Boiling Points: Low-molecular-weight amines are water-soluble due to H-bonding; boiling points lie between alcohols and alkanes.