JEE Chem — Amines Deep Dive

Chapter Overview & Weightage

Amines are a high-yield organic chapter for JEE. The chapter offers 1–2 direct questions in Main and frequently appears in Advanced through reaction sequences and identification problems. Combined with diazonium chemistry, it’s almost guaranteed marks for any well-prepared student.

JEE Main Weightage (Year-by-Year)

Year	Questions	Marks	Topics
2024	2	8	Hofmann reaction, basicity comparison
2023	1	4	Diazonium coupling
2022	2	8	Carbylamine test, separation of 1°/2°/3° amines
2021	1	4	Reduction of nitro to amine

Key Concepts You Must Know

Classification: Primary (1°), secondary (2°), tertiary (3°), and quaternary ammonium salts. Methods of preparation differ for each.

Preparation methods:

Reduction of nitro compounds (Sn/HCl, Fe/HCl, $H_2$ /Pt) — gives 1° amines
Ammonolysis of alkyl halides — gives mixture (3° + 4°)
Gabriel phthalimide synthesis — pure 1° amines (no 2°/3° contamination)
Hofmann bromamide reaction — converts amide to 1° amine with one fewer carbon
Reduction of nitriles ( $LiAlH_4$ or $H_2$ /Ni) and amides ( $LiAlH_4$ )

Basicity: Aliphatic > $NH_3$ > Aromatic. In gas phase: $3° > 2° > 1°$ . In water (solvation effects): $2° > 1° > 3°$ for methylamines.

Reactions:

Hinsberg’s test (separation of 1°/2°/3° using benzenesulfonyl chloride)
Carbylamine test (1° amines + $CHCl_3$ + KOH → bad smell of isocyanide)
Diazotisation (1° aromatic amines only at 0–5°C)
Nitrous acid test (1°→ alcohol+ $N_2$ , 2°→ N-nitrosamine, 3°→ no reaction with primary alkyls)
Acetylation (acid chloride or anhydride)

Important Reactions

$RCONH_2 + Br_2 + 4NaOH \to RNH_2 + Na_2CO_3 + 2NaBr + 2H_2O$

Note the carbon count: RCONH₂ has $n+1$ carbons, RNH₂ has $n$ . One carbon is lost as carbonate.

Phthalimide + KOH → potassium phthalimide → (alkyl halide, $S_N2$ ) → N-alkyl phthalimide → hydrolysis (or hydrazinolysis) → 1° amine.

Restriction: works only for primary alkyl halides (no aryl halides, no 2°/3° halides easily).

$ArNH_2 \xrightarrow{NaNO_2/HCl, 0-5°C} ArN_2^+Cl^-$

Then with various nucleophiles:

$CuCl/HCl \to ArCl$
$CuBr/HBr \to ArBr$
$CuCN/KCN \to ArCN$
$H_2O$ , warm $\to ArOH$
$H_3PO_2 \to ArH$ (deamination)
$KI \to ArI$ (just KI, no Cu needed)

Solved Previous Year Questions

PYQ 1 — JEE Main 2024

Arrange in increasing order of basicity (in aqueous solution): $NH_3$ , $CH_3NH_2$ , $(CH_3)_2NH$ , $(CH_3)_3N$ .

In aqueous solution, both inductive effect (boosts basicity) and solvation (boosts basicity through hydrogen-bonding stabilisation of cation) matter. Tertiary amines have only inductive but worst solvation, so they fall.

Order: $NH_3 < (CH_3)_3N < CH_3NH_2 < (CH_3)_2NH$ .

PYQ 2 — JEE Main 2023

Identify the major product when aniline is treated with $NaNO_2/HCl$ at 0°C, then heated with $H_3PO_2$ .

Aniline → benzenediazonium chloride (at 0°C). Then $H_3PO_2$ replaces $-N_2^+$ with $-H$ , giving benzene ( $C_6H_6$ ). This is reductive deamination — JEE Main staple.

PYQ 3 — JEE Advanced 2022

Distinguish between methylamine, dimethylamine, and trimethylamine using a chemical test.

Use Hinsberg’s test with benzenesulfonyl chloride ( $C_6H_5SO_2Cl$ ):

1° amine ( $CH_3NH_2$ ) → soluble in NaOH (acidic N–H proton in sulfonamide)
2° amine ( $(CH_3)_2NH$ ) → insoluble in NaOH (no acidic H)
3° amine ( $(CH_3)_3N$ ) → no reaction at all (no N–H)

This is the standard JEE separation method, asked nearly every other year.

Difficulty Distribution

Difficulty	%	Sub-topics
Easy	30%	Definitions, basic preparations
Medium	50%	Reactions, distinguishing tests, basicity
Hard	20%	Multi-step syntheses, mechanism-based, identification

Expert Strategy

Week 1 — Master preparations. Reduce nitro, Gabriel, Hofmann bromamide. Each method has a specific use case — know which to use for which target amine.

Week 2 — Reactions and tests. Hinsberg, carbylamine, nitrous acid. Practice flowcharts for distinguishing 1°/2°/3° amines.

Week 3 — Diazonium chemistry. This adjacent topic tests almost every paper. Memorise the table of conversions ( $-N_2^+$ to halides, OH, H, CN, etc.).

Topper’s mnemonic for basicity in water: “2° beats 1° beats 3° beats $NH_3$ ” for methylamines. For ethyl, the order is similar but $1°$ and $2°$ swap. Always state “in water” or “in gas phase” when answering basicity questions.

Common Traps

Trap 1: Wrong product in Hofmann bromamide due to carbon count.

Hofmann reduces an amide $RCONH_2$ to $RNH_2$ — losing one carbon. If starting amide has 4 carbons, the product amine has 3.

Trap 2: Applying Gabriel to aryl halides.

Gabriel works via $S_N2$ on alkyl halides. Aryl halides don’t undergo $S_N2$ . So aniline cannot be made via Gabriel.

Trap 3: Forgetting diazonium temperature limit.

Aliphatic primary amines + $HNO_2$ give carbocations + $N_2$ (immediate decomposition). Only aromatic primary amines give stable diazonium salts, and only at 0–5°C.

Trap 4: Confusing primary aromatic vs aliphatic basicity.

Aniline ( $pK_b \sim 9.4$ ) is much weaker base than methylamine ( $pK_b \sim 3.4$ ) because the lone pair on N is delocalised into the ring. Don’t lump them together.

Trap 5: Using ammonolysis for selective synthesis.

$RX + NH_3$ gives a mixture: 1°, 2°, 3°, and 4°. For pure 1° amine, use Gabriel synthesis (no over-alkylation). Examiners ask “best method to prepare a pure primary amine” — answer is always Gabriel.