Quick answer: In aqueous solution, aliphatic amine basicity follows the order 2° > 1° > 3° > NH3 — not the simple 3° > 2° > 1° order the +I effect alone would suggest — because steric hindrance to solvation and fewer available N-H bonds for hydrogen bonding both work against the bulkier tertiary amine. Aniline, by contrast, is a much weaker base than ammonia, because its nitrogen lone pair is delocalised into the aromatic ring by resonance and is far less available to accept a proton.
This chapter closes the oxygen-and-nitrogen loop of Class 12 Organic Chemistry, and it repeats a pattern you've already seen: an "obvious" electronic argument that gets overturned by a second, less obvious effect once you move from theory to actual aqueous solution — exactly like the alcohol acidity order. Examiners love this chapter because it rewards students who understand why the naive answer is wrong, not just those who memorised a sequence of letters.
This guide covers classification and nomenclature, the key preparation routes including Gabriel synthesis and Hofmann bromamide degradation, the two basicity comparisons examiners test every year, the reactions that distinguish 1°/2°/3° amines, diazonium salt chemistry, the 8 traps examiners set most often, and a short FAQ.
Amines typically contributes 2–3 questions in JEE Main and 2–3 questions in NEET, and the basicity-order question (aliphatic 1°/2°/3° ranking, or aniline vs ammonia) is one of the most repeated single questions in the entire Organic Chemistry syllabus — on par with the alcohol acidity order.
Classification and Nomenclature
Amines are classified by how many hydrogens on ammonia's nitrogen have been replaced by alkyl or aryl groups — a different classification logic from alcohols, and a common source of confusion right at the start:
- Primary (1°) amine, R-NH2: one alkyl/aryl group on nitrogen.
- Secondary (2°) amine, R2NH: two alkyl/aryl groups on nitrogen.
- Tertiary (3°) amine, R3N: three alkyl/aryl groups on nitrogen — note this is unlike alcohols, where classification depends on the carbon skeleton, not on the number of substituents on oxygen.
- Quaternary ammonium salt, R4N+X-: four alkyl/aryl groups on a positively charged nitrogen — not a base at all, since nitrogen has no lone pair left.
Key Methods of Preparation
Basicity Order Still Blurring Together?
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Book Free DemoBasicity: The Aqueous Order and Aniline vs Ammonia
This is the single most examined idea in the chapter, and just like alcohol acidity, it has a naive answer and a real one.
Aliphatic amines in aqueous solution: the order is 2° > 1° > 3° > NH3. The pure inductive (+I) argument would predict 3° > 2° > 1° > NH3, since more alkyl groups should push more electron density onto nitrogen. But in solution, two more effects matter: steric hindrance blocks water from solvating the bulky protonated ammonium ion, and fewer N-H bonds on the tertiary amine mean less hydrogen bonding to stabilise its conjugate acid. These effects combine so that the secondary amine — with the best balance of electron donation and manageable steric bulk — ends up most basic, not the tertiary.
Aniline vs ammonia: aniline is markedly less basic than ammonia (and less basic than any aliphatic amine), because the nitrogen lone pair delocalises into the benzene ring by resonance, making it much less available to accept a proton. This is the exact same resonance logic that makes phenol more acidic than alcohol — the lone pair that would otherwise be free is instead partly "spent" stabilising the ring.
Substituent Effects on Aniline's Basicity
- Electron-donating groups (-CH3, -OCH3) at ortho/para increase aniline's basicity — they push extra electron density toward nitrogen, making the lone pair more available.
- Electron-withdrawing groups (-NO2, -Cl) decrease aniline's basicity — they pull electron density away from nitrogen. This is the exact OPPOSITE sense to how these same groups affect phenol's acidity (EWG increases phenol's acidity but decreases aniline's basicity) — a very common point of confusion.
Reactions That Distinguish 1°, 2° and 3° Amines
Diazonium Salts
Treating a primary amine with nitrous acid (NaNO2 + HCl/H2SO4, generated in situ) behaves completely differently depending on whether the amine is aliphatic or aromatic:
Once formed, the stable aryl diazonium salt is one of the most versatile intermediates in Organic Chemistry — it feeds directly into the Sandmeyer and Gattermann reactions covered in the Haloalkanes & Haloarenes guide (to make aryl halides), hydrolyses to phenol, reduces to benzene with H3PO2, and couples with phenols or amines to form azo dyes.
The 8 Traps Examiners Set Every Year
Applying the Pure Inductive Order to Aqueous Amine Basicity
The naive +I argument gives 3° > 2° > 1°, but the real aqueous order is 2° > 1° > 3° > NH3 once steric hindrance to solvation and hydrogen bonding are accounted for. Stopping at the inductive effect alone is the single most common error in this chapter.
Assuming Aniline Is a Stronger Base Than Ammonia
Because aniline is "an amine with extra stuff attached," students often assume it should be at least as basic as ammonia. Resonance delocalisation of the lone pair into the ring makes it noticeably weaker, not stronger.
Miscounting Carbons After Hofmann Bromamide Degradation
The product amine has ONE FEWER carbon than the starting amide, because the carbonyl carbon is lost as CO2. Students who forget this write a product with the same carbon count as the amide.
Using Gabriel Synthesis for Aromatic or Secondary Amines
Gabriel synthesis cleanly gives only primary aliphatic amines, since the key step is an SN2 attack on the alkyl halide — aryl halides never undergo SN2, so aromatic amines can't be made this way.
Reversing the EWG/EDG Effect Between Phenol Acidity and Aniline Basicity
An electron-withdrawing group INCREASES phenol's acidity but DECREASES aniline's basicity — the same substituent, opposite direction of effect, because one stabilises a negative ion and the other destabilises electron availability on nitrogen. Applying one rule to the other topic is a very common slip.
Misreading the Carbylamine Test Result
Only primary amines (aliphatic and aromatic) give the foul-smelling isocyanide with CHCl3/KOH. A "no reaction" result specifically rules out a primary amine — it does not distinguish between secondary and tertiary.
Getting the Hinsberg Test's KOH-Solubility Backwards
The 1° amine's sulfonamide DISSOLVES in excess KOH (it still has an acidic N-H); the 2° amine's sulfonamide stays INSOLUBLE (no N-H left). Swapping which one dissolves is a frequent examiner trap.
Forgetting Aliphatic Diazonium Salts Are Unstable
Only aromatic diazonium salts are stable (and only at 0-5°C). An aliphatic diazonium salt decomposes the instant it forms, releasing N2 gas — treating both classes as equally stable is a common error in synthesis questions.
Frequently Asked Questions
Why is the aqueous basicity order of aliphatic amines 2° > 1° > 3° > NH3?
The +I effect alone would predict 3° highest, but steric hindrance to solvation of the bulky protonated ammonium ion and fewer N-H bonds for hydrogen bonding both work against the tertiary amine in water. These effects balance most favourably for the secondary amine, making it the most basic overall.
Why is aniline a weaker base than ammonia?
Aniline's nitrogen lone pair delocalises into the aromatic ring by resonance, making it far less available to accept a proton than ammonia's fully localised lone pair.
What is the Gabriel phthalimide synthesis and why can't it make aromatic amines?
It reacts potassium phthalimide with a primary alkyl halide via SN2, then hydrolyses to release a pure primary amine. It fails for aromatic amines because aryl halides cannot undergo SN2 substitution.
What is the Hofmann bromamide degradation?
It converts a primary amide into a primary amine with one fewer carbon, using Br2/NaOH via an isocyanate intermediate — the original carbonyl carbon is lost as CO2.
How does the Hinsberg test distinguish primary, secondary, and tertiary amines?
A 1° amine's sulfonamide (from benzenesulfonyl chloride) still has an acidic N-H and dissolves in excess KOH. A 2° amine's sulfonamide has no N-H and stays insoluble. A 3° amine doesn't react at all and is separated using dilute acid.
Your Revision Checklist
- State and justify the real aqueous basicity order of aliphatic amines using solvation and hydrogen bonding, not induction alone.
- Explain why aniline is less basic than ammonia using resonance delocalisation of the lone pair.
- Predict whether an ortho/para substituent increases or decreases aniline's basicity, and contrast that with its effect on phenol's acidity.
- Write the Gabriel phthalimide synthesis and state why it cannot make aromatic amines.
- Write the Hofmann bromamide degradation and correctly track the carbon count.
- Predict the carbylamine test result for a given amine and identify what a "no reaction" outcome rules out.
- Predict the Hinsberg test outcome (solubility in KOH) for 1°, 2°, and 3° amines.
- Distinguish the stability of aromatic vs aliphatic diazonium salts and the conditions each requires.
- Connect a diazonium salt to its downstream products: aryl halide (Sandmeyer), phenol (hydrolysis), arene (H3PO2), azo dye (coupling).
- Distinguish nitrile reduction (adds a carbon) from Hofmann degradation (removes a carbon).
This chapter completes the nitrogen-and-oxygen picture that Haloalkanes/Haloarenes and Alcohols/Phenols/Ethers built — the same resonance argument that protects an aryl-O bond and weakens phenyl cation formation is exactly why aniline is a weak base and why aryl diazonium salts are stable only through ring delocalisation.
For the SN2 mechanism this chapter's Gabriel synthesis leans on, see the Haloalkanes & Haloarenes guide, and for the parallel "naive electronic argument vs real solution behaviour" pattern in acidity, see the Alcohols, Phenols & Ethers guide. If the basicity order or diazonium chemistry are still not sticking, book a free 30-minute demo class and we will work through the exact question types your target exam favours.