Class 12 Amines Guide 2027 — Basicity Order, Diazonium Salts, Gabriel Synthesis

In short: Amines (Chapter 13) carries 5 marks in CBSE Class 12 Chemistry 2027, covering the chemistry of nitrogen-containing organic compounds. The basicity order of amines — why aliphatic amines are stronger bases than aromatic amines, and the curious 2° > 1° > 3° order in aqueous solution — is the most frequently tested concept. Diazonium salt reactions (Sandmeyer, coupling) and distinguishing tests between 1°, 2°, and 3° amines appear almost every year.

Amines is Chapter 13 in the NCERT textbook and completes the organic nitrogen chemistry section alongside Biomolecules. It carries approximately 5 marks on the board paper, typically split between one 3-mark question on basicity comparison or diazonium reactions and one 2-mark question on preparation methods, named reactions, or distinguishing tests. The basicity order of amines is one of the most conceptually rich topics in the entire organic chemistry syllabus — once you understand the reasoning, the logic applies to acid-base questions across the subject.

For the complete marks breakdown, see the Class 12 Chemistry chapter-wise weightage. For all reactions and formulas, refer to the complete reactions and formulas list. For the organic reactions flowchart connecting amines to other chapters, see the organic reactions map.

Classification of Amines

Amines are derivatives of ammonia (NH₃) where one or more hydrogen atoms are replaced by alkyl or aryl groups.

Based on number of hydrogen atoms replaced:

Based on nature of the group attached:

• Aliphatic amines: Alkyl group attached to nitrogen (CH₃NH₂, C₂H₅NH₂)
• Aromatic amines: Aryl group attached to nitrogen (C₆H₅NH₂ — aniline)
• Arylalkyl amines: Both alkyl and aryl groups (C₆H₅NHCH₃ — N-methylaniline)

IUPAC naming: Replace the terminal 'e' of the parent alkane with 'amine'. For secondary and tertiary amines, use the prefix 'N-' before the substituent names. Example: (CH₃)₂NH = N-methylmethanamine.

Memory trick — "Count the carbons on nitrogen, not the hydrogens." Primary = 1 carbon on N. Secondary = 2 carbons on N. Tertiary = 3 carbons on N. The number of H atoms on nitrogen decreases accordingly (2, 1, 0).

Basicity of Amines — The Core Concept

This is the single most important and most tested topic in the Amines chapter. Understanding basicity order requires understanding three competing effects.

Why Are Amines Basic?

Amines are basic because the lone pair on nitrogen can accept a proton (H⁺). The stronger the lone pair's availability, the stronger the base.

Base strength is measured by Kb or pKb:

• Higher Kb = stronger base
• Lower pKb = stronger base

Aliphatic Amines Are Stronger Bases Than Aromatic Amines

Reason: In aniline (C₆H₅NH₂), the lone pair on nitrogen is delocalised into the benzene ring through resonance. This makes the lone pair less available for accepting a proton. In aliphatic amines (CH₃NH₂), no such resonance exists — the lone pair is fully available.

Order: CH₃NH₂ (pKb = 3.38) > C₆H₅NH₂ (pKb = 9.38)

Aliphatic amines are about 10⁶ times stronger bases than aniline. This is one of the clearest examples of resonance reducing basicity.

Memory trick — "Aniline's lone pair is busy with the ring — too busy to accept a proton."

The Aqueous Solution Basicity Order: 2° > 1° > 3°

This is the tricky part that confuses most students. In the gas phase, the order is straightforward: 3° > 2° > 1° (more alkyl groups = more +I effect = more electron density on nitrogen = stronger base).

But in aqueous solution, three factors compete:

1. Inductive effect (+I): Alkyl groups push electron density toward nitrogen. More alkyl groups = stronger base. Favours 3° > 2° > 1°.

2. Solvation/Hydration: The conjugate acid (ammonium ion R₃NH⁺) must be stabilised by water molecules through hydrogen bonding. More H atoms on nitrogen = more hydrogen bonds = better stabilisation. Favours 1° > 2° > 3° (because 1° has more N-H bonds).

3. Steric effect: Bulky alkyl groups around nitrogen hinder the approach of the proton and reduce solvation. Favours 1° > 2° > 3°.

Result in water for methylamines:

(CH₃)₂NH > CH₃NH₂ > (CH₃)₃N > NH₃

Translation: Secondary > Primary > Tertiary > Ammonia

The secondary amine hits the sweet spot — enough +I effect from two methyl groups, but still has one N-H bond for decent solvation. The tertiary amine has the most +I effect but the worst solvation (no N-H bonds on the conjugate acid to hydrogen-bond with water).

Memory trick — "In water, 2° wins the Goldilocks prize — not too few alkyl groups, not too many."

Effect of Electron-Withdrawing Groups

Groups like -NO₂ on the ring of aromatic amines decrease basicity further (withdraw electron density from nitrogen). Groups like -CH₃ on the ring increase basicity (+I effect pushes electrons toward nitrogen).

Order for substituted anilines: p-CH₃C₆H₄NH₂ > C₆H₅NH₂ > p-NO₂C₆H₄NH₂

Preparation Methods

1. Reduction of Nitro Compounds

C₆H₅NO₂ + 3(H₂) →(Ni or Pt catalyst)→ C₆H₅NH₂ + 2H₂O

• Also: Sn/HCl followed by NaOH
• Most common method for preparing aniline

2. Ammonolysis of Alkyl Halides

R-X + 2NH₃ → R-NH₂ + NH₄X

• Problem: gives a MIXTURE of 1°, 2°, 3° amines and quaternary salt
• Not useful for preparing a pure amine

3. Gabriel Phthalimide Synthesis (Important Named Reaction)

• Step 1: Phthalimide + KOH → Potassium phthalimide
• Step 2: Potassium phthalimide + R-X → N-alkylphthalimide
• Step 3: N-alkylphthalimide + NaOH/H₂O (hydrolysis) → Primary amine + Phthalic acid

Key point: Gabriel synthesis gives ONLY primary amines (1°). It does NOT work for aromatic primary amines (aryl halides do not undergo nucleophilic substitution easily).

Memory trick — "Gabriel = Gives 1° amines only. Gabriel is pure-hearted (pure product)."

4. Hofmann Bromamide Degradation (Important Named Reaction)

R-CO-NH₂ + Br₂ + 4NaOH → R-NH₂ + Na₂CO₃ + 2NaBr + 2H₂O

• Converts a primary amide to a primary amine with ONE LESS CARBON
• The amine product has one carbon less than the starting amide

Memory trick — "Hofmann = Hacks off one carbon. The product is shorter."

Diazonium Salts — The Versatile Intermediate

Diazonium salts are formed by treating aromatic primary amines with nitrous acid (NaNO₂ + HCl) at 0-5°C. This reaction is called diazotisation.

C₆H₅NH₂ + NaNO₂ + 2HCl →(0-5°C)→ C₆H₅N₂⁺Cl⁻ + NaCl + 2H₂O

Why 0-5°C? Diazonium salts are unstable above 5°C and decompose with loss of N₂. The low temperature is essential.

Reactions of Diazonium Salts

Reactions involving loss of N₂ (replacement reactions):

Memory trick — "Sandmeyer = Copper halide. Gattermann = Copper powder. Both give ArX, but Sandmeyer uses Cu salt."

Reactions involving retention of N₂ (coupling reactions):

C₆H₅N₂⁺Cl⁻ + C₆H₅OH →(NaOH, 0-5°C)→ p-HOC₆H₄N=NC₆H₅ + HCl (benzene diazonium chloride + phenol → p-hydroxyazobenzene — an azo dye)

C₆H₅N₂⁺Cl⁻ + C₆H₅NH₂ →(0-5°C)→ p-aminoazobenzene (yellow dye)

Key point: Coupling occurs at the para position of the activated ring (phenol or amine). The -N=N- linkage is called an azo group, and the products are azo dyes — this has industrial importance.

Distinguishing Tests for 1°, 2°, 3° Amines

Hinsberg's Test (using benzenesulfonyl chloride, C₆H₅SO₂Cl)

Memory trick — "1° dissolves, 2° doesn't dissolve, 3° doesn't even react."

Carbylamine Test (Isocyanide Test)

R-NH₂ + CHCl₃ + 3KOH → R-NC + 3KCl + 3H₂O (foul-smelling isocyanide)

• This test is specific to primary amines ONLY (both aliphatic and aromatic)
• The foul smell of isocyanide is the positive test
• Secondary and tertiary amines do NOT respond to this test

Nitrous Acid Test (HNO₂ = NaNO₂ + HCl)

Board Exam Question Types

Type 1: Basicity Comparison (3 marks)

"Arrange the following in increasing order of basic strength: Aniline, Methylamine, Dimethylamine, Ammonia. Give reasons."

Approach: Identify aliphatic vs aromatic. Aromatic amines are weakest due to resonance. Among aliphatic amines in water, apply the 2° > 1° > 3° rule. Write the specific reason for each step (resonance for aniline, +I effect and solvation for the rest).

Type 2: Named Reactions (2-3 marks)

"Write the equations for (a) Gabriel phthalimide synthesis, (b) Hofmann bromamide degradation."

Approach: Write complete balanced equations with reagents above the arrow and conditions below. Mention the key feature (Gabriel gives 1° amine only; Hofmann reduces by one carbon). Structures should be clear — draw the phthalimide ring properly.

Type 3: Diazonium Salt Reactions (3 marks)

"How would you convert aniline to: (a) chlorobenzene, (b) phenol, (c) fluorobenzene?"

Approach: All three go through the diazonium salt intermediate. Step 1 is always diazotisation (NaNO₂ + HCl, 0-5°C). Step 2 varies: (a) CuCl/HCl (Sandmeyer), (b) warm with water, (c) HBF₄ then heat (Balz-Schiemann). Always mention the 0-5°C temperature for diazotisation.

Type 4: Distinguishing Tests (2 marks)

"How would you distinguish between: (a) methylamine and dimethylamine, (b) aniline and N-methylaniline?"

Approach: Use the carbylamine test for 1° vs 2° (only 1° gives foul-smelling isocyanide). Alternatively, Hinsberg's test works for all three types. Mention the reagent, observation, and conclusion.

Type 5: Conversions (2 marks)

"Convert: (a) nitrobenzene to aniline, (b) aniline to p-bromoaniline."

Approach: For nitrobenzene → aniline, use Sn/HCl then NaOH (or H₂/Ni). For aniline → p-bromoaniline, first protect the -NH₂ group by acetylation (CH₃COCl), then brominate (Br₂/CH₃COOH), then hydrolyse the amide back to the amine. Direct bromination of aniline gives 2,4,6-tribromoaniline (all three positions), not para only.

Common Mistakes That Cost Marks

1. Applying gas-phase basicity order in aqueous solution. In the gas phase, the order is 3° > 2° > 1° (pure +I effect). In water, it changes to 2° > 1° > 3° because of solvation and steric effects. Board exams always ask about aqueous solution — write the aqueous order.

2. Claiming Gabriel synthesis works for aromatic amines. Gabriel synthesis works only for primary aliphatic amines. Aryl halides do not undergo nucleophilic substitution with potassium phthalimide. Stating this limitation earns extra marks in board exams.

3. Forgetting the 0-5°C condition for diazotisation. Diazonium salts decompose above 5°C. If you forget to write this temperature, the examiner may deduct a mark. Always write "0-5°C" or "273-278 K" next to the arrow.

4. Confusing Sandmeyer with Gattermann reaction. Sandmeyer uses CuCl/HCl (copper salt). Gattermann uses Cu powder/HCl (copper metal). Both convert diazonium salts to aryl halides, but the reagents differ. Board exams may specifically ask for Sandmeyer — don't write Gattermann instead.

5. Writing direct bromination for mono-substituted product of aniline. Direct bromination of aniline with Br₂ water gives 2,4,6-tribromoaniline (because -NH₂ is a powerful activating group). To get p-bromoaniline, you must first acetylate the amino group, then brominate, then hydrolyse. This protection-deprotection strategy is a favourite board question.

6. Not explaining WHY in basicity order questions. Simply writing the order without the reasoning gets only 1 mark out of 3. You must explain the role of +I effect, resonance (for aniline), solvation, and steric effects to get full marks.

4-Week Practice Plan

Week 1: Classification, Naming, Basicity

• Read NCERT Chapter 13, Sections 13.1-13.4
• Practice IUPAC naming of 15 amines (1°, 2°, 3°, aromatic)
• Study basicity order in detail — write out the three competing effects
• Solve all NCERT in-text questions on basicity comparisons
• Reference: NCERT Exercise Q1, Q2, Q5

Week 2: Preparation Methods and Named Reactions

• Learn all preparation methods (reduction, ammonolysis, Gabriel, Hofmann)
• Write Gabriel synthesis and Hofmann bromamide from memory 3 times
• Practice 10 conversion problems involving amines
• Reference: NCERT Exercise Q3, Q4, Q8, Q10

Week 3: Diazonium Salts and Distinguishing Tests

• Memorise all diazonium salt reactions (make a table and fill from memory)
• Practice 10 conversions through diazonium salt intermediate
• Learn Hinsberg's test, carbylamine test, and nitrous acid test thoroughly
• Reference: NCERT Exercise Q6, Q7, Q9, Q11-Q14

Week 4: Board Paper Practice and Revision

• Solve Amines questions from the last 5 years of CBSE board papers
• Time yourself: 3-mark basicity questions in 7 minutes
• Write all named reactions from memory in 5 minutes
• Final drill: conversion chains (nitrobenzene → aniline → diazonium salt → products)
• Practice distinguishing test questions: write reagent, observation, conclusion

Quick Revision Card for Exam Day

• 1° amine: R-NH₂ | 2°: R₂NH | 3°: R₃N
• Basicity (aqueous): 2° > 1° > 3° > aromatic (for methylamines)
• Aliphatic > Aromatic because resonance in aniline delocalises lone pair
• Gabriel: Potassium phthalimide + RX → 1° amine only (not aromatic)
• Hofmann: Amide + Br₂ + 4NaOH → Amine (one carbon less)
• Diazotisation: ArNH₂ + NaNO₂ + 2HCl → ArN₂⁺Cl⁻ (0-5°C)
• Sandmeyer: ArN₂⁺ + CuX → ArX + N₂ (X = Cl, Br, CN)
• Coupling: ArN₂⁺ + ArOH → Azo dye (at para position)
• Carbylamine test: 1° amine only → foul-smelling isocyanide
• Hinsberg: 1° soluble in NaOH, 2° insoluble, 3° no reaction

For guided practice on amines, basicity, and diazonium salt reactions, try Padhaao's AI tutor.

Frequently Asked Questions

How many marks does Amines carry in CBSE Class 12 Chemistry 2027?

Amines carries approximately 5 marks in the CBSE Class 12 Chemistry 2027 board paper. You can expect one 3-mark question on basicity comparison or diazonium salt reactions and one 2-mark question on preparation methods, named reactions, or distinguishing tests. The basicity order comparison is one of the most frequently repeated questions across board papers.

Why is dimethylamine a stronger base than trimethylamine in water?

In aqueous solution, the conjugate acid of dimethylamine ((CH₃)₂NH₂⁺) has two N-H bonds that can form hydrogen bonds with water molecules, stabilising it effectively. Trimethylamine's conjugate acid ((CH₃)₃NH⁺) has only one N-H bond and is surrounded by bulky methyl groups that hinder solvation. Although trimethylamine has more +I effect, the poor solvation of its conjugate acid makes it a weaker base than dimethylamine in water.

What is Gabriel phthalimide synthesis and why does it fail for aromatic amines?

Gabriel synthesis uses potassium phthalimide reacting with an alkyl halide to form N-alkylphthalimide, which on hydrolysis gives a pure primary amine. It fails for aromatic amines because aryl halides do not undergo nucleophilic substitution with potassium phthalimide under normal conditions — the C-X bond in aryl halides is too strong due to resonance and the sp² nature of the carbon.

How do you convert aniline to p-bromoaniline?

You cannot directly brominate aniline because the -NH₂ group is so strongly activating that Br₂ water gives 2,4,6-tribromoaniline. Instead, first acetylate aniline using acetyl chloride (CH₃COCl) to form acetanilide, then brominate with Br₂ in acetic acid to get p-bromoacetanilide, and finally hydrolyse with dilute acid to get p-bromoaniline. The acetyl group reduces the activating power of nitrogen, allowing mono-substitution.

What is the difference between Sandmeyer and Gattermann reactions?

Both reactions convert diazonium salts to aryl halides, but they use different forms of copper. Sandmeyer reaction uses cuprous halide salts (CuCl/HCl or CuBr/HBr or CuCN/HCN), while Gattermann reaction uses copper powder with HCl or HBr. Sandmeyer gives better yields and is more commonly asked in board exams. Both reactions proceed through a radical mechanism involving the copper catalyst.

How does the carbylamine test work and which amines give a positive test?

The carbylamine test involves heating an amine with chloroform (CHCl₃) and alcoholic KOH. Only primary amines (both aliphatic and aromatic) give a positive test — they form isocyanides (R-NC) which have a characteristic foul, offensive smell. Secondary and tertiary amines do not react. The test is the most reliable way to confirm a primary amine in qualitative organic analysis.

Amines bridges organic and inorganic nitrogen chemistry and is deeply connected to the diazonium salt reactions used in the Haloalkanes chapter and the nitrogen cycle in biochemistry. Master the basicity reasoning, learn the named reactions with equations, and practise distinguishing tests — those 5 marks reward understanding over memorisation.