JEE Mains Chemistry · Amines

Structure, Physical Properties and Basicity

Amines are pyramidal nitrogen bases; hydrogen bonding sets their boiling points, and the availability of the nitrogen lone pair sets their basic strength.

All Amines notes JEE Mains Chemistry strategy

Why this matters

Seventeen PYQs, one numerical, two from 2026. Seven rank aliphatic amines, amides and caged amines by basic strength in water; six rank aryl amines and nitrogen rings; four test boiling points, the colour of aniline on storage and other physical facts.

Concept 1 of 3: Structure and physical properties of amines

An amine is ammonia with one, two or three hydrogens replaced by carbon groups. The nitrogen keeps its lone pair, so the molecule is pyramidal. Every N–H left on the nitrogen can hydrogen-bond to a neighbour, and that single fact explains most of the physical properties.

Definition

Primary (1°) $\mathrm{RNH_2}$ , secondary (2°) $\mathrm{R_2NH}$ , tertiary (3°) $\mathrm{R_3N}$ . The class counts carbons on nitrogen, not on the carbon next to it.
Nitrogen is $sp^3$ with the lone pair in the fourth orbital; the C–N–C angle in trimethylamine is about 108°.
Boiling point at similar molar mass: alcohol > 1° amine > 2° amine > 3° amine ≈ alkane. O–H is more polar than N–H, and a 3° amine has no N–H at all.
Lower aliphatic amines dissolve in water by hydrogen bonding; solubility falls as the carbon part grows.
Pure aniline is colourless. On storage it darkens because air oxidises it.

Property	What is observed	Reason
Shape at nitrogen	Pyramidal, C–N–C about 108° in $\mathrm{(CH_3)_3N}$	$sp^3$ nitrogen with one lone pair
Physical state	Lower aliphatic amines are gases with a fishy smell; 1° amines with three or more carbons are liquids	Molar mass and hydrogen bonding rise together
Boiling point of isomers	1° > 2° > 3°	Two N–H, one N–H, then no N–H for intermolecular hydrogen bonds
Amine against alcohol	Alcohol boils higher at similar molar mass	O–H is more polar than N–H, so its hydrogen bonds are stronger
Solubility in water	Lower amines dissolve; higher amines and aniline barely dissolve	Hydrogen bonds to water, outweighed by a large hydrophobic part
Aniline on storage	Colourless when pure, turns brown on standing	Atmospheric oxidation of the activated ring

Count the N–H bonds first: they decide hydrogen bonding, and hydrogen bonding decides boiling point.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2026 · 6 Apr 2026 Shift 1 · Q43Moderate

Example 1 · Amines · Structure, Physical Properties and Basicity

Arrange the following compounds according to increasing order of boiling points.

n -C_{4}H_{9}OH(A),n -C_{4}H_{9}NH_{2}(\text{ }B)

n -C_{4}H_{10}(C)

and

C_{2}H_{5}NHC_{2}H_{5}(D)

Primary amines associate more than secondary amines

A primary amine has two N–H bonds and a secondary amine has one, so primary amines form more intermolecular hydrogen bonds and boil higher than their secondary isomers.

Aniline darkens by oxidation, not reduction

The electron-rich ring of an aryl amine is oxidised by air, giving coloured products. A statement that arylamines colour on storage by atmospheric reduction is false.

Drill 3 more on structure and physical properties of amines

Concept 2 of 3: Basic strength of aliphatic amines and amides

An amine is a base because its lone pair takes a proton. Alkyl groups push electrons onto nitrogen, so in the gas phase every extra alkyl group makes the amine more basic. In water two more effects join in: the protonated ion is stabilised by hydrogen bonds to water, which needs N–H bonds, and bulky groups crowd the nitrogen. The order in water is a compromise.

Definition

Gas phase (inductive effect only): 3° > 2° > 1° > $\mathrm{NH_3}$ .
Water, methyl series: $\mathrm{(CH_3)_2NH > CH_3NH_2 > (CH_3)_3N > NH_3}$ .
Water, ethyl series: $\mathrm{(C_2H_5)_2NH > (C_2H_5)_3N > C_2H_5NH_2 > NH_3}$ . The two series differ, so learn both.
$pK_b$ values (NCERT): $\mathrm{NH_3}$ 4.75; $\mathrm{C_2H_5NH_2}$ 3.29; $\mathrm{(C_2H_5)_2NH}$ 3.00; $\mathrm{(C_2H_5)_3N}$ 3.25; aniline 9.38. Smaller $pK_b$ , stronger base.
Hydrazine $\mathrm{H_2N{-}NH_2}$ is weaker than ammonia: the second nitrogen withdraws electrons.
Amides and imides are barely basic: the lone pair is delocalised onto C=O. Acetamide is weaker than aniline, and $\mathrm{(CH_3CO)_2NH}$ is weaker still.
Tying the alkyl groups back in a cage (quinuclidine) removes the crowding, so it is more basic than triethylamine.

Base dissociation of an amine and its pKb

\mathrm{RNH_2 + H_2O \rightleftharpoons RNH_3^+ + OH^-} \qquad K_b = \dfrac{[\mathrm{RNH_3^+}][\mathrm{OH^-}]}{[\mathrm{RNH_2}]} \qquad pK_b = -\log K_b

Worked example

Arrange in decreasing basic strength in water:

\mathrm{NH_3}

\mathrm{C_2H_5NH_2}

\mathrm{(C_2H_5)_2NH}

\mathrm{(C_2H_5)_3N}

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2023 · 25 Jan 2023 · Q40Moderate

Example 2 · Amines · Structure, Physical Properties and Basicity

The correct order in aqueous medium of basic strength in case of methyl substituted amines is:

Tertiary is not the strongest base in water

In the gas phase a tertiary amine is the strongest, but in water it loses out on solvation and crowding. Trimethylamine is weaker than methylamine in water; triethylamine is weaker than diethylamine.

The methyl and ethyl orders are different

In water the methyl series runs 2° > 1° > 3°, while the ethyl series runs 2° > 3° > 1°. Swapping the two is the most common slip in these questions.

An amide is not an amine

Acetamide has a nitrogen with a lone pair, but the lone pair is shared with the carbonyl group. Acetamide is a weaker base than aniline, not a stronger one.

Drill 6 more on basic strength of aliphatic amines and amides

Concept 3 of 3: Basic strength of aryl amines and nitrogen heterocycles

In aniline the nitrogen lone pair spreads into the benzene ring, so it is less free to take a proton. Anything that pushes more electrons toward nitrogen makes the amine more basic; anything that pulls them away makes it weaker. When the lone pair is part of an aromatic ring, as in pyrrole, it is hardly available at all.

Definition

Aniline ( $pK_b$ 9.38) is far weaker than an aliphatic amine. Benzylamine, $\mathrm{C_6H_5CH_2NH_2}$ ( $pK_b$ 4.70), has an $sp^3$ carbon between ring and nitrogen, so it behaves as an aliphatic amine.
In water, N-alkyl anilines are slightly stronger than aniline: $\mathrm{C_6H_5N(CH_3)_2}$ 8.92 < $\mathrm{C_6H_5NHCH_3}$ 9.30 < $\mathrm{C_6H_5NH_2}$ 9.38 ( $pK_b$ ).
Ring substituents: electron donors ( $\mathrm{OCH_3}$ , $\mathrm{CH_3}$ ) at the para position raise basicity; electron acceptors ( $\mathrm{Cl}$ , $\mathrm{CN}$ , $\mathrm{NO_2}$ ) lower it.
A second phenyl group lowers basicity further: diphenylamine is much weaker than aniline.
Nitrogen rings: piperidine and pyrrolidine ( $sp^3$ N) are strong; pyridine ( $sp^2$ N, lone pair outside the ring π system) is weak; pyrrole's lone pair is part of its aromatic sextet, so pyrrole is almost non-basic.
The weakest base gives the strongest conjugate acid.

A base and its conjugate acid at 298 K

pK_a(\mathrm{BH^+}) + pK_b(\mathrm{B}) = 14

Worked example

Arrange in increasing basic strength: 4-chloroaniline, 4-methylaniline, 4-nitroaniline and aniline.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2021 · Paper 26 · Q48Moderate

Example 3 · Amines · Structure, Physical Properties and Basicity

A. Phenyl methanamine B. N, N-Dimethylaniline C. N-Methyl aniline D. Benzenamine Choose the correct order of basic nature of the above amines.

Benzylamine is not an aryl amine for basicity

In benzylamine the nitrogen sits on a

\mathrm{CH_2}

group, not on the ring, so its lone pair is not delocalised. It is about as basic as an aliphatic amine and far stronger than aniline.

Pyridine and pyrrole are not equally basic

Pyridine's lone pair lies in the ring plane, outside the aromatic π system, so it can take a proton. Pyrrole's lone pair is part of the aromatic sextet; protonating it destroys aromaticity, so pyrrole is almost non-basic.

Drill 5 more on basic strength of aryl amines and nitrogen heterocycles

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (2)

Basic strength of aliphatic amines and amides
Base dissociation of an amine and its pKb
$\mathrm{RNH_2 + H_2O \rightleftharpoons RNH_3^+ + OH^-} \qquad K_b = \dfrac{[\mathrm{RNH_3^+}][\mathrm{OH^-}]}{[\mathrm{RNH_2}]} \qquad pK_b = -\log K_b$
Basic strength of aryl amines and nitrogen heterocycles
A base and its conjugate acid at 298 K
$pK_a(\mathrm{BH^+}) + pK_b(\mathrm{B}) = 14$

Reference tables (1)

Structure and physical properties of amines6 rows

Property	What is observed	Reason
Shape at nitrogen	Pyramidal, C–N–C about 108° in $\mathrm{(CH_3)_3N}$	$sp^3$ nitrogen with one lone pair
Physical state	Lower aliphatic amines are gases with a fishy smell; 1° amines with three or more carbons are liquids	Molar mass and hydrogen bonding rise together
Boiling point of isomers	1° > 2° > 3°	Two N–H, one N–H, then no N–H for intermolecular hydrogen bonds
Amine against alcohol	Alcohol boils higher at similar molar mass	O–H is more polar than N–H, so its hydrogen bonds are stronger
Solubility in water	Lower amines dissolve; higher amines and aniline barely dissolve	Hydrogen bonds to water, outweighed by a large hydrophobic part
Aniline on storage	Colourless when pure, turns brown on standing	Atmospheric oxidation of the activated ring

Count the N–H bonds first: they decide hydrogen bonding, and hydrogen bonding decides boiling point.

Watch out for (7)

Primary amines associate more than secondary amines
→ Structure and physical properties of amines
Aniline darkens by oxidation, not reduction
→ Structure and physical properties of amines
Tertiary is not the strongest base in water
→ Basic strength of aliphatic amines and amides
The methyl and ethyl orders are different
→ Basic strength of aliphatic amines and amides
An amide is not an amine
→ Basic strength of aliphatic amines and amides
Benzylamine is not an aryl amine for basicity
→ Basic strength of aryl amines and nitrogen heterocycles
Pyridine and pyrrole are not equally basic
→ Basic strength of aryl amines and nitrogen heterocycles

Test yourself on Amines

20 past JEE Mains questions from this chapter, timed at 48 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

Take the Amines chapter test Drill all 17 questions