Aniline Reactivity and Directing-Group Effects
The amino group on aniline is an exceptionally strong activator and o/p-director through its nitrogen lone pair, but that same lone pair is exactly what gets compromised under strongly acidic conditions or by acetylation — and tracking what happens to that lone pair explains almost every surprising result in this cluster.
Spotting the False Statement About Aniline Electrophilic Substitution is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Why aniline over-reacts, and why nitration seems to break the rules
Anilines nitrogen lone pair donates powerfully into the ring by resonance, making EVERY position ortho/para to it strongly activated — so strongly that aniline reacts with bromine water alone (no Lewis acid catalyst needed) to instantly give 2,4,6-tribromoaniline, substituting all three available o/p positions at once. Direct nitration with a nitrating mixture (HNO3/H₂SO₄) looks like an exception to the o/p-directing rule, giving significant META product — but this is not really an exception at all: under the strongly acidic nitrating conditions, a large fraction of the aniline is protonated to the anilinium ion, -NH₃⁺, which has NO lone pair left to donate (it is now a full positive charge, not a donor at all) and instead acts as a deactivating META director, just like any other electron-withdrawing group. The meta product comes from the protonated fraction of the molecule, not from the free amine reacting unusually.
Acetylation makes the amino group a WEAKER donor, not a stronger one
Acetylating aniline to acetanilide (-NHCOCH₃ in place of -NH₂) delocalises the nitrogen lone pair into the adjacent carbonyl group (competing resonance with the C=O), leaving LESS electron density available to donate into the aromatic ring than the free amino group has. This makes the acetamido group a WEAKER activator than free -NH₂, not a stronger one — despite being less basic. This weaker, more controlled activation is exactly why acetylation is used synthetically: it tones aniline down from tribromination-on-contact to clean, controlled MONOsubstitution at the ortho/para positions, precisely because the ring is no longer activated enough to react three times over.
Reading which substituted benzene favours the para-nitro product
Among a set of substituted benzenes competing for nitration, only genuine o/p-directing, activating (or mildly deactivating but still donor-capable) groups give a predominant para product. A quaternary ammonium substituent (like trimethylanilinium, with no lone pair on nitrogen at all, since all four bonds are already used) is a deactivating META director, just like -NO₂ or -CN — none of these three give a predominant para product. Acetanilide, by contrast, retains just enough nitrogen lone-pair donation (even though weakened by the competing carbonyl resonance covered above) to act as an o/p-director, and sterically favours the LESS crowded para position over ortho for its major product — making it the one substrate in a mixed list that predominantly gives the para-nitro isomer.
OlympIQ exam lens
- ✓Aniline free amino group is such a strong activator that bromination proceeds instantly to 2,4,6-tribromoaniline with no catalyst
- ✓Direct nitration of aniline gives significant meta product because much of the aniline is protonated to -NH₃⁺, a deactivating meta director with no lone pair to donate
- ✓Acetylation makes the nitrogen a WEAKER ring activator than free -NH₂ (lone pair delocalised into the carbonyl instead), which is why it enables controlled monosubstitution
- ✓A quaternary ammonium substituent (no lone pair at all) is a deactivating meta director, unlike acetanilide (a weakened but genuine o/p-director)
- ✓Acetanilide nitration favours the less hindered para position, making it the substrate that predominantly gives the para-nitro product
Do not assume the acetamido group (-NHCOCH₃) activates the ring MORE strongly than free -NH₂ just because it is less basic. Less basic here means the lone pair is more tied up (delocalised into the carbonyl), which makes it a WEAKER ring activator, not a stronger one — this is exactly the inverted claim these questions test.
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