This chapter's central figure is the amine's nitrogen lone pair — it's directly responsible for amines being basic, being nucleophilic, and being able to form the diazonium salts that make this chapter's second half such a versatile synthetic toolkit.
Amine basicity depends on how available that nitrogen lone pair actually is, and this depends on a genuine tug-of-war between electron-donating alkyl groups (which increase basicity) and steric hindrance plus solvation effects (which can decrease it) — which is exactly why the basicity order among primary, secondary and tertiary amines isn't always the simple pattern a first glance might suggest. Diazonium salts, formed from primary aromatic amines, are this chapter's most practically important synthetic tool, since they can be converted into an enormous range of other functional groups, making aromatic amines a genuine gateway to many other aromatic compound classes.
Key amine and diazonium reactions
| Reaction | Product / use |
|---|---|
| Basicity order (gas phase / aqueous, alkyl amines) | 3° > 2° > 1° > NH₃ (gas phase, roughly) |
| Hofmann bromamide degradation | amide → primary amine with one fewer carbon |
| Gabriel phthalimide synthesis | gives pure primary amine, avoiding over-alkylation |
| Diazotisation | primary aromatic amine + NaNO₂/HCl (0–5°C) → diazonium salt |
| Sandmeyer reaction | ArN₂⁺ + CuCl/CuBr/CuCN → Ar-Cl / Ar-Br / Ar-CN |
| Balz-Schiemann reaction | ArN₂⁺ + HBF₄ → Ar-F |
| Coupling reaction | ArN₂⁺ + phenol/aniline → azo dye |
Topics covered
- Classification and nomenclature of amines — primary, secondary and tertiary, and the specific naming conventions used
- Preparation of amines — from haloalkanes, nitro compounds, nitriles and amides
- Basicity of amines — the competing electronic and steric factors, and why the simple 3°>2°>1° pattern doesn't always hold
- Distinguishing primary, secondary and tertiary amines — specific chemical tests including the Hinsberg test
- Gabriel phthalimide synthesis — and why it specifically avoids the over-alkylation problem of direct amine alkylation
- Hofmann bromamide degradation — and its specific use for converting an amide to a primary amine with one fewer carbon
- Diazonium salts — preparation via diazotisation, and their characteristic instability
- Reactions of diazonium salts — substitution reactions replacing the diazonium group, and coupling reactions forming azo dyes
Why this chapter matters
In NEET and JEE, amine basicity comparison questions and diazonium salt reaction identification (particularly Sandmeyer and Balz-Schiemann reactions) are among the most consistently repeated question types in this section, since diazonium chemistry offers such a versatile way to test synthetic reasoning. Distinguishing amine tests (Hinsberg test) are a reliable secondary favourite.
For Class 12 boards, expect an amine basicity ranking question with justification and a diazonium salt reaction sequence question.
Mistakes students make in this chapter
- Assuming amine basicity always follows the simple order 3° > 2° > 1° > NH₃. In aqueous solution, steric hindrance and solvation effects can reverse this expected order for certain amines, since a bulkier, more substituted amine cation is harder to stabilise through hydrogen bonding with water — the actual aqueous order can differ from the simple gas-phase electronic-effects-only prediction.
- Forgetting diazonium salts are only stable at low temperature. They must be prepared and generally used immediately at 0–5°C, since they decompose readily at higher temperatures, releasing nitrogen gas.
- Confusing which copper halide is needed for which Sandmeyer product. CuCl gives the aryl chloride, CuBr gives the aryl bromide, and CuCN gives the aryl nitrile — using the wrong copper salt in a synthesis question gives the wrong final product.
- Believing direct alkylation of ammonia gives a clean, pure primary amine. Direct alkylation tends to over-alkylate, producing a mixture of primary, secondary, tertiary amines and even a quaternary ammonium salt — the Gabriel synthesis exists specifically to avoid this problem and give a pure primary amine.
- Forgetting azo coupling requires an activated (electron-rich) aromatic ring. The diazonium salt couples with phenols or aromatic amines specifically because these rings are sufficiently electron-rich to act as a nucleophile toward the diazonium's electrophilic terminal nitrogen — an unactivated benzene ring won't undergo this coupling.
Worked example: why trimethylamine is not the strongest base
- Identify what makes an amine basic. The nitrogen lone pair must be available to accept a proton. Anything that increases electron density on nitrogen raises basicity; anything that ties up or blocks the lone pair lowers it.
- Deal with aniline first — it is easily the weakest. The nitrogen lone pair is delocalised into the benzene ring through resonance, so it is no longer freely available to bind a proton:aniline : lone pair in resonance with ring → least basic
- Write the gas phase order for the aliphatic amines. With no solvent involved, only the +I effect of the methyl groups matters, and more methyl groups means more electron density on nitrogen:gas phase : NH₃ < CH₃NH₂ < (CH₃)₂NH < (CH₃)₃N
- Now add the two effects water introduces. A protonated amine is stabilised by hydrogen bonding to water, and the more N–H bonds the cation has, the better it is solvated. Bulky methyl groups also physically block water and the incoming proton:(CH₃)₃NH⁺ : only 1 N–H → poorly solvated(CH₃)₂NH₂⁺ : 2 N–H → well solvated
- Combine all three effects. The secondary amine hits the best compromise between electron donation and solvation, so it comes out on top, while the tertiary amine drops below both primary and secondary:aqueous : C₆H₅NH₂ < NH₃ < (CH₃)₃N < CH₃NH₂ < (CH₃)₂NH
What the notes contain
The PDF explains amine basicity with both the electronic and steric/solvation factors worked through for specific comparative examples, covers the Gabriel and Hofmann methods with full mechanisms, and works through diazonium salt chemistry including Sandmeyer, Balz-Schiemann and coupling reactions with the product predicted for each specific reagent.
How to use this: build the concept from these notes first, then do one quick NCERT pass before the exam for exact wording and diagram labels, and spend whatever time is left on previous year questions.
Questions
Why doesn't amine basicity always follow the expected 3° > 2° > 1° order in water?
In the gas phase, more alkyl substitution genuinely increases basicity through greater electron donation. In aqueous solution, however, a more substituted (bulkier) ammonium cation is harder for water molecules to surround and stabilise through hydrogen bonding, which can partially or fully offset the electronic advantage — the PDF explains this competing-factors reasoning with specific comparative examples.
Why is the Gabriel synthesis preferred over direct alkylation for making primary amines?
Direct alkylation of ammonia with a haloalkane tends to keep reacting further, since the resulting primary amine is itself nucleophilic and can react with more haloalkane, producing a mixture of amines rather than a clean single product. The Gabriel synthesis uses phthalimide specifically to prevent this over-alkylation, releasing a pure primary amine only at the final hydrolysis step.
What determines which product forms from a diazonium salt reaction?
It depends entirely on the specific reagent used — different copper salts (Sandmeyer reaction) give different aryl halides or the aryl nitrile, fluoroboric acid (Balz-Schiemann reaction) gives the aryl fluoride, and water gives a phenol. The PDF includes a reference table connecting each reagent to its specific product.
Does this cover the mechanism of the Hofmann bromamide reaction?
Yes, including why the resulting primary amine ends up with exactly one fewer carbon atom than the starting amide, a distinctive and frequently tested feature of this particular reaction.
All other chapters
Every chapter of Class 11 and Class 12 Physics, Chemistry and Biology — each with its own notes and PDF.