Amines are basic because the nitrogen atom has one lone pair of electrons. This lone pair can grab a proton (H+) from water, just like ammonia does. We measure how strong a base is using Kb (base dissociation constant) or pKb: a HIGHER Kb or a LOWER pKb means a STRONGER base. Memory hook: "Lone pair grabs the proton — high Kb, low pKb, strong base."
The nitrogen lone pair accepts a proton (H+) to form a positive ion, which is why amines act as bases. A higher Kb (or lower pKb) means the amine grabs protons more strongly and is a stronger base.
Your doubts, answered
Why exactly are amines basic?
Nitrogen in an amine has 5 valence electrons. Three are used to make bonds (to H or carbon), so 2 electrons are left over as a lone pair. This lone pair is free and can be donated to a proton (H+). When it accepts a proton, the amine becomes a positively charged ion (R-NH3+). Accepting a proton is exactly what a base does. So the free lone pair is the real reason amines are basic. For NEET, remember: no lone pair available = not basic.
What do Kb and pKb actually mean?
When an amine is put in water, a little of it reacts: R-NH2 + H2O gives R-NH3+ + OH-. Kb is the equilibrium constant for this reaction. A large Kb means the amine pulls protons strongly and makes more OH-, so it is a strong base. pKb is just -log(Kb), a smaller number that is easier to compare. NEET often gives pKb values in questions, so you must know how to read them.
Does a higher Kb mean stronger or weaker base?
Higher Kb means STRONGER base. A big Kb tells you the amine grabs protons easily and produces lots of OH- ions. This is the direct measure, so when Kb values are given, the biggest Kb is the strongest base. Students often mix this up, so learn it as a fixed rule: big Kb equals strong base.
Is a lower pKb more basic or less basic?
Lower pKb means MORE basic (stronger base). Because pKb = -log(Kb), when Kb goes up, pKb goes down. So a small pKb number means a strong base, and a large pKb number means a weak base. NEET trap: many students think smaller number equals weaker, which is wrong. For pKb, smaller number equals STRONGER base.
Are amines a Bronsted base or a Lewis base?
Amines are BOTH. As a Bronsted base, the amine accepts a proton (H+) from water or acid. As a Lewis base, the nitrogen lone pair is donated to an electron-poor centre. NCERT specifically says amines behave as Lewis bases because of the lone pair on nitrogen. Both ideas come from the same lone pair.
What makes one amine more basic than another?
It depends on how easily the lone pair on nitrogen can be given away. If electron-pushing groups (like alkyl groups, +I effect) increase the electron density on nitrogen, the amine is more basic. If the lone pair is pulled away or spread out (like in aniline, where it goes into the benzene ring), the amine is less basic. So more available lone pair equals more basic, which means higher Kb and lower pKb.
⚠️ The NEET trap ✗ Aniline (aryl amine) is more basic than methylamine because the benzene ring pushes electrons onto nitrogen. ✓ Aniline is LESS basic than alkyl amines. In aniline the nitrogen lone pair is delocalised into the aromatic ring (resonance), so it is less available to accept a proton. Its Kb is small and pKb is large. 🧠 Ring steals the lone pair = weak base. Alkyl pushes electrons in = strong base.
Real NEET questions
NEET 2016
The correct statement regarding the basicity of aryl amines is:
A · Aryl amines are generally less basic than alkyl amines because the nitrogen lone-pair electrons are delocalized by interaction with the aromatic ring pi electron system. ✓
B · Aryl amines are generally more basic than alkyl amines because the nitrogen lone-pair electrons are not delocalized by interaction with the aromatic ring pi electron system.
C · Aryl amines are generally more basic than alkyl amines because of aryl group.
D · Aryl amines are generally more basic than alkyl amines, because the nitrogen atom in aryl amines is sp-hybridized.
Solution: Basicity depends on how freely the nitrogen lone pair can grab a proton. In an aryl amine like aniline (C6H5NH2), the lone pair on nitrogen is in conjugation with the benzene ring and gets spread out (delocalised) over the ring by resonance. This lowers the electron density on nitrogen, so the lone pair is less available for a proton. Result: aryl amines have a small Kb (large pKb) and are WEAKER bases than alkyl amines, where the +I effect of alkyl groups actually raises electron density on N. The nitrogen in aniline is sp3 (slightly flattened), not sp, so option D is wrong. Correct answer: A.
NEET 2019
The correct order of the basic strength of methyl-substituted amines in aqueous solution is:
A · (CH3)2NH > CH3NH2 > (CH3)3N ✓
B · (CH3)3N > CH3NH2 > (CH3)2NH
C · (CH3)3N > (CH3)2NH > CH3NH2
D · CH3NH2 > (CH3)2NH > (CH3)3N
Solution: In water, three effects fight each other. (1) +I effect: more CH3 groups push electrons onto N, raising basicity. (2) Steric hindrance: bulky groups block the proton, so (CH3)3N is hurt. (3) Solvation: the positive ion formed after protonation is stabilised by hydrogen bonds to water; more N-H bonds means better solvation, so CH3NH3+ and (CH3)2NH2+ are more stabilised than (CH3)3NH+. Balancing all three, the secondary amine wins in water: (CH3)2NH > CH3NH2 > (CH3)3N. Higher basic strength here means higher Kb and lower pKb. Correct answer: A.
NEET 2025
The correct order of decreasing basic strength of the given amines is (N-ethylethanamine = (C2H5)2NH; ethanamine = C2H5NH2; N-methylaniline = C6H5NH(CH3); benzenamine/aniline = C6H5NH2):
A · N-ethylethanamine > ethanamine > N-methylaniline > benzenamine ✓
B · Benzenamine > ethanamine > N-methylaniline > N-ethylethanamine
C · N-methylaniline > benzenamine > ethanamine > N-ethylethanamine
D · N-ethylethanamine > ethanamine > benzenamine > N-methylaniline
Solution: Rule 1: aliphatic amines are more basic than aromatic amines, because in aromatic amines the lone pair is pulled into the ring. So the two aliphatic amines come first. Among them, (C2H5)2NH has two +I ethyl groups, so it beats C2H5NH2. Rule 2: among the two aromatic amines, the N-CH3 group of N-methylaniline pushes electrons onto N (+I), making it more basic than plain aniline. Putting it together (most basic to least): (C2H5)2NH > C2H5NH2 > C6H5NH(CH3) > C6H5NH2. More basic means higher Kb, lower pKb. Correct answer: A.
Solved Amines NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Kb is the base dissociation constant, a raw equilibrium number for the reaction R-NH2 + H2O gives R-NH3+ + OH-. pKb = -log(Kb), a smaller and easier number. A large Kb equals a small pKb equals a strong base.
Which is more basic, one with pKb 3 or pKb 5?
pKb 3 is more basic. Lower pKb means stronger base. Since pKb 3 is smaller than pKb 5, the pKb 3 amine grabs protons more strongly.
Are amines stronger bases than water and alcohols?
Yes. Amines are more basic than water and alcohols because nitrogen is less electronegative than oxygen, so the nitrogen lone pair is held less tightly and is donated to a proton more easily.
Why is ammonia used as the reference for amine basicity?
Ammonia (NH3) is the parent molecule of all amines. We compare each amine's Kb and pKb against ammonia to see whether adding alkyl or aryl groups makes it more or less basic. This is the next concept: basicity order of aliphatic amines vs ammonia.
Do amines turn litmus paper blue?
Yes, water-soluble amines like methylamine give an alkaline solution (they produce OH- ions) and turn red litmus blue. This is direct proof that amines are basic.
