Amines — Basicity order, Kb values
Nitrogen Compounds
8
JEE Qs
8%
Hard
75
min
Always analyze amine basicity by considering the combined effects of inductive, resonance, and solvation (in aqueous medium) on the stability of the protonated conjugate acid.
🧮 Key Formulas
✅ Key Points for JEE
- 1Basicity of amines depends on the availability of the lone pair of electrons on the nitrogen atom for donation; factors that increase electron density on nitrogen enhance basicity.
- 2In the gas phase, basicity order for aliphatic amines is 3° > 2° > 1° > NH3, solely governed by the electron-donating (+I) effect of alkyl groups stabilizing the conjugate acid.
- 3In aqueous solution, the basicity of aliphatic amines is determined by a complex interplay of inductive effect, steric hindrance, and solvation (hydrogen bonding) of the protonated amine (conjugate acid).
- 4Typical aqueous basicity orders: For methyl amines: (CH3)2NH > CH3NH2 > (CH3)3N > NH3. For ethyl amines: (C2H5)2NH > (C2H5)3N > C2H5NH2 > NH3. Secondary amines are generally the strongest bases in aqueous solution due to an optimal balance.
- 5Aromatic amines are significantly weaker bases than aliphatic amines because the nitrogen lone pair is delocalized into the aromatic ring via resonance, making it less available for protonation. Electron-donating groups on the aromatic ring increase basicity, while electron-withdrawing groups decrease basicity.
⚠️ Common Mistakes
- ✕Confusing the basicity order of amines in the gas phase with that in aqueous solution, especially for aliphatic amines.
- ✕Ignoring the critical role of solvation (stabilization of the conjugate acid by hydrogen bonding with water) when determining aqueous basicity trends.
- ✕Incorrectly applying or overlooking the resonance effects of the aromatic ring or substituents, which significantly impact the basicity of aromatic amines.
- ✕Assuming tertiary amines are universally stronger bases than primary or secondary amines, without considering solvent effects or steric hindrance.
📝 Practice Questions
See allQ56.Given below are two statements : Consider the following reaction (1) Both Statement I and Statement II are false (2) Statement I is true but Statement II is false (3) Statement I is false but Statement II is true (4) Both Statement I and Statement II are true
Q56.Identify correct statements : (A) Primary amines do not give diazonium salts when treated with NaNO2 in acidic condition. (B) Aliphatic and aromatic primary amines on heating with CHCl3 and ethanolic KOH form 2025 (28 Jan Shift 2) JEE Main Previous Year Paper carbylamines. (C) Secondary and tertiary amines also give carbylamine test. (D) Benzenesulfonyl chloride is known as Hinsberg's reagent. (E) Tertiary amines reacts with benzenesulfonyl chloride very easily. Choose the correct answer from the options given below : (1) (A) and (B) only (2) (D) and (E) only (3) (B) and (D) only (4) (B) and (C) only
Q68.Match the Compounds (List - I) with the appropriate Catalyst/Reagents (List - II) for their reduction into corresponding amines. Choose the correct answer from the options given below : (1) (A)-(II), (B)-(I), (C)-(III), (D)-(IV) (2) (A)-(III), (B)-(II), (C)-(IV), (D)-(I) (3) (A)-(II), (B)-(IV), (C)-(III), (D)-(I) (4) (A)-(III), (B)-(IV), (C)-(II), (D)-(I)
Q58.The correct set of ions (aqueous solution) with same colour from the following is: 2025 (23 Jan Shift 1) JEE Main Previous Year Paper (1) Sc3+, Ti3+, Cr2+ (2) V2+, Cr3+, Mn3+ (3) Ti4+, V4+, Mn2+ (4) Zn2+, V3+, Fe3+
Q66. For reaction The correct order of set of reagents for the above conversion is : (1) Br2 ∣FeBr3, H2O(Δ), NaOH (2) H2SO4, Ac2O, Br2, H2O(Δ), NaOH (3) Ac2O, Br2, H2O(Δ), NaOH (4) Ac2O, H2SO4, Br2, NaOH
Q55.Total number of nucleophiles from the following is : ⊖ NH3, PhSH, (H3C)2 S, H2C = CH2, O H, H3O⊕, (CH3)2CO, ⇌NCH3 (1) 7 (2) 4 (3) 6 (4) 5
NCERT Chapters
- Class 12 Chemistry Ch 13: Amines