Q.Arrange the following in decreasing order of their basic strength:
C6H5NH2, C2H5NH2, (C2H5)2NH, NH3
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🔒 Start your 14-day free trial to unlock the full solution →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 (NH3), 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 RNH3+) 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 (RNH3+) has 3 H's → best solvation.
Secondary (R2NH2+) has 2 H's → good solvation.
Tertiary (R3NH+) 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 (pKb) |
|---|---|---|---|---|
| Ammonia (NH3) | Weakest | Best (3 H's) | None | 4.75 |
| Primary (RNH2) | Moderate | Good (2 H's) | Minimal | ~3.4 |
| Secondary (R2NH) | Strong | Moderate (1 H) | Some | ~3.2 |
| Tertiary (R3N) | Strongest | Poor (0 H's) | Significant | ~4.2 |
pKb is the negative log of the base dissociation constant. Lower pKb = stronger base. So secondary (pKb ≈ 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? …
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:
R3N+H2O⇌R3NH++OH−
The base dissociation constant Kb measures this:
Kb=[R3N][R3NH+][OH−]
A larger Kb 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 (−CH3, −C2H5, 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∘>2∘>1∘>NH3
✗ Solvation Effect (Hydration of the Conjugate Acid)
When the amine accepts H+, it forms R3NH+ (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 R3NH+ → stronger base (because the equilibrium shifts toward protonation).
This suggests:
1∘>2∘>3∘ (since 1∘ has 3 H's, 2∘ has 2, 3∘ has 1)
4. The Actual Order (Aqueous Solution)
In water, the observed order for aliphatic amines is:
2° > 1° > 3° > NH3
Wait — that's not simply "more alkyl = stronger". Why?
The Reasoning (Step by Step)
-
From NH3 to 1∘: Adding one alkyl group increases electron density (inductive effect) → stronger base.
So 1∘>NH3.
-
From 1∘ to 2∘: Adding a second alkyl group further increases electron density → even stronger base.
So 2∘>1∘.
-
From 2∘ to 3∘: Here, the solvation effect becomes dominant.
The 3∘ 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∘ is weaker than 2∘.
Thus, the net order is:
2° > 1° > 3° > NH3
5. The Key Formula(e) to Remember
For gas phase (no solvent):
Only inductive effect matters:
3° > 2° > 1° > NH3
For aqueous solution (common in exams): …
Concept: Basicity order of amines depends on the balance between the inductive effect (electron-donating alkyl groups increase electron density on nitrogen) and solvation/hindrance effects in aqueous medium.
Reasoning:
- In water, aliphatic amines are more basic than ammonia because alkyl groups donate electron density via the +I effect, making the lone pair more available.
- Among aliphatic amines, secondary amines ((C2H5)2NH) are generally the most basic in water due to a favourable combination of inductive effect and solvation of the conjugate acid. …
Basicity of amines depends on the balance between inductive effects and solvation. The order is (C2H5)2NH>C2H5NH2>NH3>C6H5NH2 — secondary > primary > ammonia > aniline.
The question asks us to compare basic strength across four nitrogen-containing compounds: aniline (C6H5NH2), ethylamine (C2H5NH2), diethylamine ((C2H5)2NH), and ammonia (NH3).
Basicity here means the tendency of the nitrogen atom to donate its lone pair to a proton (or to a Lewis acid). In aqueous solution — which is the standard context for such comparisons in Indian exams — two factors dominate: the electron-releasing or withdrawing effect of the groups attached to nitrogen, and the stabilisation of the conjugate acid (the protonated form) through solvation.
Let’s unpack each factor.
-
Inductive effect of alkyl groups
Alkyl groups like ethyl (−C2H5) are electron-donating via the inductive effect (+I). They push electron density toward the nitrogen, making the lone pair more available for protonation. More alkyl groups generally mean greater electron density on nitrogen — so a secondary amine (two alkyl groups) should be more basic than a primary amine (one alkyl group), which in turn is more basic than ammonia (no alkyl groups).
This suggests: (C2H5)2NH>C2H5NH2>NH3.
-
Solvation of the conjugate acid
When an amine accepts a proton, it forms an ammonium ion (R3NH+). This cation is stabilised in water by hydrogen bonding between the N–H hydrogens and water molecules. The more N–H bonds the conjugate acid has, the better it is solvated, and the more stable it is — which shifts the equilibrium toward the protonated form, increasing basicity.
Ammonia’s conjugate acid (NH4+) has four N–H bonds. Primary amine conjugate acids have three, secondary have two, and tertiary have only one. So solvation favours: NH3>C2H5NH2>(C2H5)2NH.
These two factors — inductive effect and solvation — pull in opposite directions. The observed order in aqueous solution for aliphatic amines is a compromise: secondary > primary > tertiary > ammonia is the usual pattern for simple alkyl amines, but here we have only up to secondary. For ethylamines, the order is indeed (C2H5)2NH>C2H5NH2>NH3.
-
The special case of aniline
Aniline is dramatically less basic than aliphatic amines. Why? The lone pair on nitrogen is delocalised into the benzene ring through resonance. This conjugation makes the lone pair less available for protonation — it’s partly “tied up” in the π system. …
Method: Inductive Effect + Solvation Effect Analysis (for Aliphatic vs Aromatic Amines)
This method compares electron availability on nitrogen by considering:
- +I effect of alkyl groups (pushes electrons toward N)
- Resonance delocalisation (in aniline, lone pair is shared with the ring)
- Solvation of the conjugate acid (more H atoms on N → better H-bonding with water)
Step 1 — Identify the type of each amine
| Amine | Type | Key feature |
|---|---|---|
| C6H5NH2 | Aromatic primary | Lone pair delocalised into benzene ring |
| C2H5NH2 | Aliphatic primary | One ethyl group (+I) |
| (C2H5)2NH | Aliphatic secondary | Two ethyl groups (+I) |
| NH3 | Ammonia | No alkyl group |
Step 2 — Compare aliphatic amines with ammonia
- Alkyl groups are electron-donating (+I effect) → increase electron density on N.
- More alkyl groups → stronger +I effect → higher basicity in gas phase.
- In aqueous solution, solvation matters: The conjugate acid RNH3+ is stabilised by H-bonding with water. More H atoms on N → better solvation.
Result in water:
Secondary > Primary > Ammonia
So:
(C2H5)2NH>C2H5NH2>NH3
Step 3 — Compare aromatic amine with ammonia
- In C6H5NH2, the lone pair on N is delocalised into the benzene ring (resonance).
- This makes the lone pair less available for protonation.
- Hence, aniline is weaker base than ammonia.
So:
NH3>C6H5NH2
Step 4 — Combine all comparisons
From strongest to weakest base: …
Common Mistakes: Basicity Order of Amines
Students often lose marks on this exact question. Here are the most frequent errors and how to avoid them.
Mistake 1: Importing compounds that are not in the question
The error: While reciting the memorised ethyl-series order (2∘>1∘>3∘>NH3 etc.), students write triethylamine, (C2H5)3N, into their final answer — even though this question's list contains only four species: C6H5NH2, C2H5NH2, (C2H5)2NH and NH3.
Why it's wrong: An ordering answer must contain exactly the species asked about — nothing added, nothing dropped. There is no tertiary amine in this list, so the whole 2∘-vs-3∘ complication never arises here.
How to avoid: Before writing the final order, tick off each formula against the question's own list. For these four, both the inductive effect and simple solvation reasoning agree:
(C2H5)2NH>C2H5NH2>NH3>C6H5NH2
Mistake 2: Forgetting that aniline is much weaker than ammonia
The error: Students place C6H5NH2 somewhere in the middle, thinking the phenyl ring is just a mild electron-withdrawing group.
Why it's wrong: The lone pair on N in aniline is delocalised into the benzene ring via resonance. This drastically reduces electron density on N, making it a very weak base — weaker than ammonia.
Correct placement: C6H5NH2 is the weakest among these.
How to avoid: Draw the resonance structures of aniline. See how the lone pair is shared with the ring. Compare pKb values:
- NH3: pKb≈4.75
- C6H5NH2: pKb≈9.38
That difference of about 4.6 pKb units corresponds to roughly a 40,000-fold (≈4×104) difference in Kb.
Mistake 3: Confusing the order of alkyl amines in water vs. gas phase
The error: Students memorise one order and apply it everywhere.
The correct orders (for the species in this question):
| Phase | Order (decreasing basicity) |
|---|---|
| Gas phase | (C2H5)2NH>C2H5NH2>C6H5NH2>NH3 (only intrinsic electronic effects operate — with no solvent, even aniline edges above ammonia, because its large polarisable ring spreads the conjugate acid's charge) |
| Aqueous | (C2H5)2NH>C2H5NH2>NH3>C6H5NH2 (solvation of the small NH4+ lifts ammonia above aniline) |
Showing the 12 most recent of 13 on this concept.
- KEAM 2026Set eng-2026-04174 marksMCQQ.Choose the correct decreasing order of basic strength of amines in aqueous solution: (A) NH3 > CH3NH2 > (CH3)2NH > (CH3)3N (B) (CH3)2NH > (CH3)3N > NH3 > CH3NH2 (C) (CH3)3N > NH3 > CH3NH2 > (CH3)2NH (D) (CH3)2NH > CH3NH2 > (CH3)3N > NH3 (E) CH3NH2 > (CH3)2NH > (CH3)3N > NH3
›Reveal solutionSolution
Because solvation of the ammonium ion opposes the pure inductive trend, the aqueous basicity order of methylamines is (CH3)2NH>CH3NH2>(CH3)3N>NH3.
In water, base strength of amines depends on three competing effects: the electron-donating +I of methyl groups (raises basicity), steric hindrance to protonation, and stabilisation of the protonated cation by hydrogen-bonded solvation (fewer N–H bonds in more-substituted amines lowers solvation). …
- KEAM 2026Set eng-2026-04184 marksMCQQ.Which of the following amine has the highest pKb value in aqueous phase? (A) Methanamine (B) N-methylmethanamine (C) Ethanamine (D) N-Methylbenzenamine (E) Benzenamine
›Reveal solutionSolution
Weakest base = highest pKb; aniline, with lone-pair delocalisation into the ring, is the weakest here.
Basicity depends on availability of the N lone pair. In aniline (benzenamine) the lone pair is delocalised into the benzene ring, sharply lowering basicity, so pKb≈9.4. …
- KEAM 2026Set eng-2026-04204 marksMCQQ.Which of the following amine has lowest pKb value in aqueous phase? (A) Ethanamine (B) Methanamine (C) N-Methylmethanamine (D) N-Ethylethanamine (E) N, N-Diehtylmethanamine
›Reveal solutionSolution
Diethylamine (secondary amine) is the strongest base ⇒ lowest pKb.
In aqueous solution basic strength reflects a balance of +I effect and solvation of the cation, giving the order secondary > primary ≈ tertiary for aliphatic amines. Among the options, N-Ethylethanamine (C2H5)2NH is a secondary amine with two electr …
- KEAM 2026Set eng-2026-04214 marksMCQQ.The descending order of basic strength of the following amines is(i) N-Methylbenzenamine(ii) N.N'-Dimethylbenzenamine(iii) Benzenamine(iv) Phenylmethanamine (A)(i) >(ii) >(iv) >(iii) (B)(iv) >(i) >(ii) >(iii) (C)(iv) >(ii) >(i) >(iii) (D)(iv) >(iii) >(ii) >(i) (E)(i) >(iv) >(ii) > (iii)
›Reveal solutionSolution
Benzylamine (nitrogen not on the ring) is the strongest base; for the ring-N anilines, basicity rises with the number of electron-donating alkyl groups: (iv) benzylamine > (ii) N,N-dimethylaniline > (i) N-methylaniline > (iii) aniline.
Reasoning
- (iv) Phenylmethanamine (benzylamine, C6H5CH2NH2): the –NH₂ is on an sp³ carbon, not conjugated with the ring, so the lone pair is fully available → most basic.
- Anilines have the N lone pair delocalised into the ring, lowering basicity. Adding alkyl groups (+I effect) partially restores basicity: …
- KEAM 2026Set pha-2026-0420F4 marksMCQQ.Which one of the following compounds is strongly basic in aqueous medium? (A) Benzenamine (B) N-ethylethanamine (C) Phenylmethanamine (D) N,N-Dimethylbenzenamine (E) Ammonia
›Reveal solutionSolution
Diethylamine (secondary aliphatic amine) is most basic in water thanks to +I of two ethyl groups plus favourable solvation.
Basicity of amines in aqueous solution depends on the electron density on N (inductive effect) and on solvation of the ammonium ion. Aromatic amines (benzenamine/aniline, N,N-dimethylbenzenamine) are weak bases because the lone pair is delocalised into the ring. Among the aliphatic amines, N-ethylethanamine = diethylamine (C2H5)2NH is a secondary amine whose two electron-donating ethyl groups increase electron density o …
- KEAM 2025Set eng-2025-04234 marksMCQQ.The order of basic strength of following amines is(i) CH3NH2(ii) (C2H5)2NH(iii) C6H5NH2(iv) C6H5NHCH3 (A)(ii) <(i) <(iv) <(iii) (B)(iii) <(iv) <(ii) <(i) (C)(ii) <(iii) <(iv) <(i) (D)(i) <(ii) <(iii) <(iv) (E)(iii) <(iv) <(i) < (ii)
›Reveal solutionSolution
The increasing order of basicity is aniline < N-methylaniline < methylamine < diethylamine: (iii) < (iv) < (i) < (ii).
Concept and Intuition
Aromatic amines are much weaker bases than aliphatic amines because the nitrogen lone pair is delocalised into the benzene ring. An added alkyl group increases basicity by electron donation. Among aliphatic amines, a secondary amine (diethylamine) is more basic than a primary one (methylamine).
Step-by-Step Solution
- Aniline (iii): lone pair delocalised → weakest base.
- N-Methylaniline (iv): +I of CH3 makes it slightly more basic than aniline, but still aromatic and weak. …
- KEAM 2025Set eng-2025-04274 marksMCQQ.The amine with the highest pKb value is (A) Methanamine (B) N-methylmethanamine (C) Benzeneamine (D) N-Methylaniline (E) Ethanamine
›Reveal solutionSolution
Highest pKb means weakest base. Aromatic amines are far weaker than aliphatic ones; of the two aromatic amines, unsubstituted aniline is weaker than N-methylaniline, so aniline has the highest pKb.
Reasoning
Basicity depends on availability of the nitrogen lone pair. In aromatic amines the lone pair is delocalised into the benzene ring, sharply lowering basicity (raising pKb).
Approximate pKb values:
- (A) Methanamine (CH3NH2): 3.36
- (B) N-methylmethanamine ((CH3)2NH): 3.27
- (E) Ethanamine (C2H5NH2): 3.29
- (D) N-Methylaniline: ~9.2
- (C) Benzeneamine (aniline): ~9.4 …
- KEAM 2024Set eng-2024-06074 marksMCQQ.The decreasing order of basic strength in aqueous solution of amines is (A) Dimethylamine > Methylamine > Trimethylamine > Ammonia (B) Methylamine > Dimethylamine > Trimethylamine > Ammonia (C) Trimethylamine > Dimethylamine > Methylamine > Ammonia (D) Ammonia > Trimethylamine > Dimethylamine > Methylamine (E) Ammonia > Dimethylamine > Trimethylamine > Methylamine
›Reveal solutionSolution
In aqueous solution the basicity order for methyl amines is dimethylamine > methylamine > trimethylamine > ammonia, reflecting the balance of the +I effect, solvation (H-bonding of the conjugate acid) and steric hindrance.
Basicity in water is governed by three factors: the electron-donating (+I) effect of alkyl groups (increases basicity), stabilisation of the protonated ammonium ion by hydrogen bonding/solvation (favours more N–H bonds), and steric hindrance (cro …
- KEAM 2024Set eng-2024-06084 marksMCQQ.The decreasing order of basic strength of amines in aqueous medium is (A) CH3NH2>(CH3)2NH>(CH3)3N>NH3 (B) (CH3)2NH>CH3NH2>(CH3)3N>NH3 (C) (CH3)2NH>(CH3)3N>CH3NH2>NH3 (D) (CH3)2NH>NH3>(CH3)3N>CH3NH2 (E) NH3>CH3NH2>(CH3)3N>(CH3)2NH
›Reveal solutionSolution
For methylamines in water the basic-strength order is (CH3)2NH>CH3NH2>(CH3)3N>NH3, because solvation of the ammonium ion and steric hindrance offset the +I effect.
Three factors govern basicity in water:
- +I (electron-donating) effect of methyl groups increases electron density on N (raises basicity).
- Solvation/stabilisation of the protonated cation by water — more N–H bonds allow more H-bonding (raises effective basicity).
- Steric hindrance — bulky groups around N hinder protonation and solvation (lowers basicity). …
- KEAM 2024Set pha-2024-06104 marksMCQQ.The correct increasing order of basic strength is (A) $NH_3 < C_2H_5NH_2 < C_6H_5NH_2 < C_6H_5CH_2NH_2$ (B) $C_6H_5NH_2 < NH_3 < C_6H_5CH_2NH_2 < C_2H_5NH_2$ (C) $C_6H_5NH_2 < C_6H_5CH_2NH_2 < NH_3 < C_2H_5NH_2$ (D) $C_6H_5CH_2NH_2 < NH_3 < C_2H_5NH_2 < C_6H_5NH_2$ (E) $C_6H_5NH_2 < NH_3 < C_2H_5NH_2 < C_6H_5CH_2NH_2$
›Reveal solutionSolution
Basic strength order (pKb): aniline (9.4) < NH3 (4.75) < benzylamine (4.66) < ethylamine (3.25).
Basic strength depends on availability of the N lone pair.
- Aniline (C6H5NH2): lone pair delocalised into the ring ⇒ weakest (pKb ≈ 9.4).
- Ammonia (NH3): pKb ≈ 4.75.
- Benzylamine (C6H5CH2NH2): the CH2 insulates N from the ring; slightly more basic than ammonia (pKb ≈ 4.66). …
- KEAM 2022Set eng-2022-P1-A14 marksMCQQ.Among methanamine, ethanamine, benzenamine, N-methylaniline and N, N-dimethylaniline, the weakest and the strongest base in aqueous phase, respectively are (A) benzenamine and methanamine (B) N-methylaniline and ethanamine (C) N, N-dimethylaniline and ethanamine (D) benzenamine and ethanamine (E) N-methylaniline and methanamine
›Reveal solutionSolution
The weakest base is benzenamine (aniline) and the strongest is ethanamine.
Concept and Intuition
Aromatic amines are weaker bases than aliphatic amines because the lone pair on nitrogen is delocalised into the ring. Among aromatic amines, N-methyl and N,N-dimethyl substitution increases electron density on nitrogen, so plain aniline is the weakest. Among aliphatic amines in water, ethanamine is more basic than methanamine due to a better balance of inductive and solvation effects.
Step-by-Step Solution
- Aromatic set: aniline < N-methylaniline < N,N-dimethylaniline in basicity; aniline is weakest.
- Aliphatic set: in aqueous phase ethanamine > methanamine. …
- KEAM 2021Set eng-2021-P1-A14 marksMCQQ.Which one of the following is not correct with respect to properties of amines? (A) pKb of aniline is more than that of methylamine. (B) Ethylamine is soluble in water whereas aniline is not. (C) Ethanamide on reaction with Br2 and NaOH gives ethylamine. (D) Ethylamine reacts with nitrous acid to give ethanol. (E) Aniline does not undergo Friedel-Crafts reaction.
›Reveal solutionSolution
Statement (C) is wrong: Hofmann degradation of ethanamide gives methylamine, not ethylamine.
Concept and Intuition
The Hofmann bromamide reaction converts an amide RCONH2 to an amine RNH2 with the loss of one carbon (the carbonyl C leaves as carbonate). So a two-carbon amide yields a one-carbon amine.
Step-by-Step Solution
- CH3CONH2Br2, NaOHCH3NH2 (methylamine), one carbon fewer.
- Statement (C) claims ethylamine — incorrect. …
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