Aniline (C6H5-NH2) has FIVE resonance structures because the nitrogen lone pair spreads into the benzene ring. Anilinium ion (C6H5-NH3+) has only TWO (just the plain ring), because after nitrogen grabs a proton it has NO lone pair left to donate. More resonance means more stable, so aniline is more stable than expected while anilinium is not extra-stabilised. Memory hook: "Aniline shares its lone pair with the ring, so it has little left to give a proton" - that is exactly why aniline is a weak base.
Top: in neutral aniline the nitrogen lone pair delocalises into the ring, giving extra resonance structures (negative charge on ortho/para carbons) - 5 total. Bottom: in the anilinium ion the nitrogen is -NH3+ with no lone pair, so the ring stays plain benzene - only 2 structures. This is why aniline is more resonance-stabilised yet a weaker base.
Your doubts, answered
Why does aniline have 5 resonance structures but anilinium only 2?
In aniline the nitrogen still holds a lone pair. That lone pair can shift into the benzene ring, giving three extra structures where the negative charge sits on the ortho and para carbons, plus the two normal Kekule ring structures = 5 in total. In the anilinium ion the nitrogen has already used its lone pair to bond an H+ (it is now -NH3+ with no lone pair). Nothing extra can flow into the ring, so only the 2 plain Kekule structures of the benzene ring remain.
Why is aniline a weaker base than ammonia because of resonance?
Basicity means the nitrogen must donate its lone pair to a proton. In aniline the lone pair is not fully on nitrogen - it is delocalised (shared) into the ring by resonance, so it is less available. Because the lone pair is 'busy' in resonance, aniline holds a proton less easily and is a weaker base than ammonia or methylamine, where the lone pair sits freely on nitrogen.
Does aniline become MORE or LESS stable due to resonance?
Aniline itself becomes MORE stable (extra resonance energy from 5 structures). This is the trick: because the neutral aniline is already extra-stabilised and the anilinium ion is not, the system 'prefers' to stay as neutral aniline rather than pick up a proton. Extra stability of the reactant (aniline) + no extra stability of the product (anilinium) = the proton-adding reaction is not favoured = weak base.
Why doesn't the lone pair delocalise in the anilinium ion?
Once the -NH2 grabs a proton it becomes -NH3+. All three of nitrogen's outer electron pairs are now used in N-H bonds, so there is no lone pair to push into the ring. With nothing to donate, the ring behaves like plain benzene and shows only 2 resonance structures.
How is this different from phenol resonance?
The idea is the same: in phenol the oxygen lone pair delocalises into the ring (giving extra structures), which is why phenol is acidic-friendly. In aniline the nitrogen lone pair delocalises. The key point for NEET is that this delocalisation removes electron density from N, lowering aniline's basicity, just as it lowers the availability of O's lone pair in phenol.
⚠️ The NEET trap ✗ Anilinium ion has more resonance structures than aniline, so anilinium is more stable and aniline is a strong base. ✓ Aniline (neutral) has 5 resonance structures; anilinium (protonated) has only 2. Aniline is the extra-stabilised one, and because its lone pair is tied up in resonance, aniline is a WEAK base - not a strong one. 🧠 NTA flips 'which one is more delocalised'. Rule: the LONE PAIR does the delocalising, so only the species that STILL HAS a lone pair (aniline) gets extra resonance. Protonated = no lone pair = fewer structures.
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: In aniline the nitrogen lone pair is delocalised into the benzene ring (the same resonance that gives aniline 5 structures). This spreads the lone pair away from nitrogen, so it is less available to bind a proton. Hence aryl amines like aniline are weaker bases than alkyl amines - directly the resonance effect described on this page.
Solved Amines NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Five: two normal Kekule structures of the benzene ring, plus three more where the nitrogen lone pair has moved into the ring, placing a negative charge on the two ortho carbons and the para carbon (and a positive charge on nitrogen).
How many resonance structures does the anilinium ion have?
Two - only the two Kekule structures of the benzene ring. The -NH3+ group has no lone pair, so nothing extra flows into the ring.
Why is aniline less basic than ammonia?
Because aniline's lone pair is delocalised into the ring by resonance and is therefore less available to grab a proton. In ammonia the lone pair is fully on nitrogen, so ammonia binds a proton more easily and is more basic.
Is aniline or anilinium more stabilised by resonance?
Aniline (the neutral molecule) is more stabilised, because only aniline has the lone pair needed for extra delocalisation. This is exactly why aniline resists picking up a proton and acts as a weak base.
Does resonance make the -NH2 group activating in aniline?
Yes. The same lone-pair donation that lowers basicity pushes electron density onto the ortho and para carbons, making aniline strongly activated toward electrophilic substitution at those positions - useful for reactions like bromination to 2,4,6-tribromoaniline.
