Which of the following is a 3° amine? (i) 1-methylcyclohexylamine (ii) Triethylamine (iii) tert-butylamine (iv

NCERT Exemplar · Q1

Q. Which of the following is a 3° amine?

(i) 1-methylcyclohexylamine

(ii) Triethylamine

(iii) tert-butylamine

(iv) N-methylaniline

Rajasthan RbseMCQ· 1mImportance★★★★★

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Concept understanding — Basicity Order Amines

Basicity of Amines: From Intuition to Precision

Imagine you have a nitrogen atom with a lone pair of electrons — that pair is like a key that can grab a proton ( $H^+$ ). The more willing the nitrogen is to share that pair, the stronger the base. That's the core idea.

The Intuition: What Makes a Nitrogen "Want" a Proton?

Amines are organic derivatives of ammonia ( $NH_3$ ), where one or more hydrogen atoms are replaced by alkyl groups ( $R$ ). Alkyl groups are electron-donating — they push electron density toward the nitrogen. More alkyl groups = more electron density on nitrogen = stronger base.

So you'd expect: tertiary > secondary > primary > ammonia. That's the inductive effect argument.

But reality is more interesting. In water (the usual solvent for basicity measurements), the order is:

Secondary > Primary > Tertiary > Ammonia

Why the reversal for tertiary amines? Because basicity isn't just about the free amine — it's about the stability of the conjugate acid (the ammonium ion $RNH_3^+$ ) once the proton is grabbed.

The Precise Explanation: Three Factors at Play

1. Inductive Effect (pushes electron density)

Alkyl groups donate electrons through sigma bonds. More alkyl groups = more electron density on N = stronger base. This favours: tertiary > secondary > primary > ammonia.

2. Solvation Effect (stabilises the conjugate acid)

Once the amine grabs a proton, the resulting ammonium ion is positively charged. Water molecules surround it, stabilising the charge through hydrogen bonding. The more hydrogens on the nitrogen, the more H-bonds can form.

Primary ammonium ( $RNH_3^+$ ) has 3 H's → best solvation.

Secondary ( $R_2NH_2^+$ ) has 2 H's → good solvation.

Tertiary ( $R_3NH^+$ ) has 1 H → poor solvation.

This favours: primary > secondary > tertiary.

3. Steric Hindrance (blocks solvation)

Bulkier alkyl groups physically block water molecules from approaching the charged nitrogen. This further reduces solvation in tertiary amines.

The Net Result: The Actual Order in Water

Amine Type	Inductive Effect	Solvation	Steric Hindrance	Net Basicity (pK $_b$ )
Ammonia ( $NH_3$ )	Weakest	Best (3 H's)	None	4.75
Primary ( $RNH_2$ )	Moderate	Good (2 H's)	Minimal	~3.4
Secondary ( $R_2NH$ )	Strong	Moderate (1 H)	Some	~3.2
Tertiary ( $R_3N$ )	Strongest	Poor (0 H's)	Significant	~4.2

Note

pK $_b$ is the negative log of the base dissociation constant. Lower pK $_b$ = stronger base. So secondary (pK $_b$ ≈ 3.2) is the strongest, then primary (≈3.4), then tertiary (≈4.2), then ammonia (4.75).

The Final Answer

The basicity order of amines in aqueous solution is:

Secondary > Primary > Tertiary > Ammonia

This is the standard order for simple alkyl amines (methyl, ethyl, etc.). The inductive effect dominates from ammonia to secondary, but the loss of solvation and steric hindrance in tertiary amines drops them below primary.

A Quick Check: Why Not Tertiary?

If you only considered electron donation, tertiary would win. But the ammonium ion $R_3NH^+$ has only one hydrogen to hydrogen-bond with water, and the three bulky alkyl groups physically block water molecules. The conjugate acid is poorly stabilised, so the equilibrium shifts back toward the free amine — making it a weaker base than expected.

Watch out

This order applies to aliphatic amines in water. Aromatic amines (like aniline) are much weaker bases because the lone pair is delocalised into the benzene ring. Also, in the gas phase (no solvent), the order reverts to tertiary > secondary > primary > ammonia — confirming that solvation is the key reason for the reversal.

The Takeaway

Basicity is a tug-of-war between:

Inductive effect (wants tertiary to win)
Solvation + sterics (wants primary to win)

Secondary amines hit the sweet spot — strong inductive donation and decent solvation — making them the strongest bases in water.

The basicity order of amines in aqueous solution is one of the most frequently asked comparison-type questions in the NCERT Class 12 Chemistry chapter on amines, regularly appearing in CBSE boards, JEE Main and NEET. Anyone searching "basicity order of amines class 12 chemistry" or trying to understand why secondary amines outrank primary and tertiary amines will find the inductive-versus-solvation trade-off above is the standard NCERT-aligned explanation.

Why this formula?

Basicity Order of Amines: Why It Holds

Let's build this from first principles — understanding why amines have different basicities is essential for exams and for deeper chemistry.

1. What Does "Basicity" Mean Here?

Basicity of an amine is its ability to accept a proton ( $H^+$ ).

The stronger the base, the more readily it grabs $H^+$ .

The equilibrium is:

R_3N + H_2O \rightleftharpoons R_3NH^+ + OH^-

The base dissociation constant $K_b$ measures this:

K_b = \frac{[R_3NH^+][OH^-]}{[R_3N]}

A larger $K_b$ means a stronger base.

2. The Key Factor: Electron Density on Nitrogen

The lone pair on nitrogen is what accepts $H^+$ .

More electron density on nitrogen → stronger base.

Why? Because a proton ( $H^+$ ) is attracted to negative charge. If the nitrogen's lone pair is more "available" (less tightly held), it binds $H^+$ more easily.

3. The Two Competing Effects

✓ Inductive Effect (Electron-Donating)

Alkyl groups ( $-CH_3$ , $-C_2H_5$ , etc.) are electron-donating via the inductive effect.

They push electron density toward nitrogen.

More alkyl groups → more electron density on N → stronger base.

This suggests:

$3^\circ > 2^\circ > 1^\circ > NH_3$

✗ Solvation Effect (Hydration of the Conjugate Acid)

When the amine accepts $H^+$ , it forms $R_3NH^+$ (a positively charged ion).

This ion is stabilized by water molecules (hydration).

More $H$ atoms on the $NH^+$ group → more hydrogen bonding with water → better stabilization of the conjugate acid.
Better stabilization of $R_3NH^+$ → stronger base (because the equilibrium shifts toward protonation).

This suggests:

$1^\circ > 2^\circ > 3^\circ$ (since $1^\circ$ has 3 $H$ 's, $2^\circ$ has 2, $3^\circ$ has 1)

4. The Actual Order (Aqueous Solution)

In water, the observed order for aliphatic amines is:

\textbf{2° > 1° > 3° > NH}_3

Wait — that's not simply "more alkyl = stronger". Why?

The Reasoning (Step by Step)

From $NH_3$ to $1^\circ$ : Adding one alkyl group increases electron density (inductive effect) → stronger base.

So $1^\circ > NH_3$ .
From $1^\circ$ to $2^\circ$ : Adding a second alkyl group further increases electron density → even stronger base.

So $2^\circ > 1^\circ$ .
From $2^\circ$ to $3^\circ$ : Here, the solvation effect becomes dominant.

The $3^\circ$ amine has only one $H$ on the $NH^+$ group → poor hydration of the conjugate acid.

The inductive effect is still present, but the loss of solvation outweighs the gain in electron density.

So $3^\circ$ is weaker than $2^\circ$ .

Thus, the net order is:

\textbf{2° > 1° > 3° > NH}_3

5. The Key Formula(e) to Remember

For gas phase (no solvent):

Only inductive effect matters:

\textbf{3° > 2° > 1° > NH}_3

For aqueous solution (common in exams):

Both effects matter — solvation dominates for $3^\circ$ :

\textbf{2° > 1° > 3° > NH}_3

For aromatic amines (e.g., aniline):

The lone pair is delocalized into the benzene ring → much weaker base.

\textbf{Aliphatic amines > Aromatic amines}

6. Quick Exam Tip

If a question asks "basicity order of amines in water", always write:

2° > 1° > 3° > NH $_3$

And explain:

Inductive effect increases from $NH_3$ to $3^\circ$
But solvation of the conjugate acid decreases from $1^\circ$ to $3^\circ$
The balance gives the above order.

7. Summary Table

Amine Type	Inductive Effect	Solvation of $R_3NH^+$	Net Basicity (aq)
$NH_3$	Weakest	Best (3 $H$ 's)	Weakest
$1^\circ$	Moderate	Good (2 $H$ 's)	Moderate
$2^\circ$	Strong	Moderate (1 $H$ )	Strongest
$3^\circ$	Strongest	Poor (0 $H$ 's)	Weaker than $2^\circ$

Final takeaway:

Basicity is not just about "more alkyl = stronger". The solvation of the conjugate acid is the deciding factor in water. Always reason from both effects.

A 3° amine has three carbon groups directly attached to the nitrogen atom. Triethylamine, with three ethyl groups on N, is the only 3° amine among the options. The correct option is (ii).

The classification of amines — primary (1°), secondary (2°), and tertiary (3°) — depends entirely on how many carbon-containing groups are directly bonded to the nitrogen atom. It has nothing to do with the total number of carbons in the molecule, nor with the complexity of the carbon skeleton elsewhere. The nitrogen's immediate neighbours decide the degree.

Let's examine each option by counting the groups attached to N.

Option (i): 1-methylcyclohexylamine

The name tells you the amine is on a cyclohexane ring, with a methyl substituent at position 1. The nitrogen is attached to the ring carbon — that's one carbon group. The other two positions on N are occupied by hydrogens. So this is a primary (1°) amine.

Watch out

Don't be fooled by the "methyl" in the name — that methyl is on the ring, not on the nitrogen. Only groups directly on N count.
Option (ii): Triethylamine

"Triethyl" means three ethyl groups, and "amine" means they are all attached to nitrogen. The structure is $(CH_3CH_2)_3N$ . Nitrogen has three carbon groups and no hydrogens. That is the definition of a tertiary (3°) amine.

Tip

The prefix "tri-" in the name is a dead giveaway: trialkylamines are always 3°.
Option (iii): tert-butylamine

The name "tert-butylamine" means a tert-butyl group $(CH_3)_3C-$ attached to $NH_2$ . Nitrogen is bonded to exactly one carbon (the quaternary carbon of the tert-butyl group) and two hydrogens. This is a primary (1°) amine, despite the bulky, highly branched alkyl group.

Note

tert-Butylamine is a classic example of a primary amine that looks "tertiary" in its carbon skeleton — but the nitrogen itself is only singly substituted.
Option (iv): N-methylaniline

Aniline is $C_6H_5NH_2$ . "N-methyl" means one hydrogen on nitrogen is replaced by a methyl group. So the structure is $C_6H_5NH(CH_3)$ . Nitrogen is attached to two carbon groups (the phenyl ring and the methyl) and one hydrogen. That makes it a secondary (2°) amine.

✓Final answer

The correct option is (ii) Triethylamine .

Common Mistakes Students Make with 3° Amine Identification

The Concept: What is a 3° (Tertiary) Amine?

A tertiary amine has the nitrogen atom bonded to three carbon atoms (alkyl or aryl groups). The general structure is:

R_3N

where each $R$ is a carbon-containing group (not hydrogen).

Mistake #1: Confusing "tertiary" on carbon vs. nitrogen

The error: Students see "tert-butylamine" (option C) and think "tert" means tertiary amine — but tert-butyl refers to the carbon's substitution, not the nitrogen's.

tert-butylamine = $(CH_3)_3C-NH_2$ → nitrogen is bonded to only one carbon → 1° amine
A tertiary amine requires the nitrogen to be bonded to three carbons.

How to avoid: Always check the nitrogen's bonds, not the carbon's. Draw the structure:

Compound	Structure	Amine class
tert-butylamine	$(CH_3)_3C-NH_2$	1°
Triethylamine	$(CH_3CH_2)_3N$	3° ✓

Mistake #2: Forgetting that N-substituted rings count as carbon bonds

The error: In 1-methylcyclohexylamine (option A), students count only the cyclohexyl ring as one carbon bond and miss the methyl group.

Structure: cyclohexyl ring with $NH_2$ and a $CH_3$ on the same carbon
Nitrogen is bonded to one carbon (the ring carbon) → 1° amine

How to avoid: Count every atom directly bonded to nitrogen. If it's a carbon, it counts. If it's hydrogen, it doesn't.

Mistake #3: Misidentifying aromatic amines

The error: In N-methylaniline (option D), students see "methyl" and "aniline" and assume it's tertiary.

Structure: $C_6H_5-NH-CH_3$
Nitrogen is bonded to two carbons (one phenyl, one methyl) and one hydrogen → 2° amine

How to avoid: Draw the full structure. Aniline ( $C_6H_5NH_2$ ) is 1°. Adding one methyl to nitrogen makes it 2°. Only adding two alkyl groups to nitrogen makes it 3°.

Mistake #4: Rushing and not checking all options

The error: Students see "triethylamine" and think "tri = three ethyl groups" but don't verify that all three are bonded to nitrogen.

How to avoid: Triethylamine is $(CH_3CH_2)_3N$ — three ethyl groups on nitrogen. That's correctly 3°. But always check the other options too — sometimes a trick option looks similar.

Quick Summary Table

Option	Name	Nitrogen bonds	Amine class
A	1-methylcyclohexylamine	1 C, 2 H	1°
B	Triethylamine	3 C, 0 H	3° ✓
C	tert-butylamine	1 C, 2 H	1°
D	N-methylaniline	2 C, 1 H	2°

Final Tip for Exams

Always draw the nitrogen's immediate neighbours. Count only the atoms directly bonded to N. If three are carbon → 3° amine. If two are carbon → 2°. If one is carbon → 1°. If zero → ammonia (not an amine).

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Q.Which of the following is a 3° amine?