Organic Chemistry · Chemistry of Benzene: Electrophilic Aromatic Substitution

Other Aromatic Substitutions

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In 30 seconds
Why this matters
The college version
Eli explains
Worked example
Key takeaway
Check yourself
Study tools
Sources & references

In 30 seconds

Bromination (Topic 1) is one member of a large EAS family. This topic covers the other classic substitutions — nitration, , and halogenation with chlorine, fluorine, and iodine — plus a preview of Friedel–Crafts. Every member follows the same two-step arenium-ion mechanism; only the electrophile changes:

Nitration: C₆H₆ + HNO₃ ^H₂SO₄⟶ C₆H₅NO₂ + H₂O. The electrophile is the nitronium ion, NO₂⁺, generated by the action of sulfuric acid on nitric acid.
Sulfonation: C₆H₆ + SO₃ → C₆H₅SO₃H (fuming sulfuric acid/oleum). Uniquely among EAS reactions, sulfonation is reversible — hot water or steam removes the SO₃H group — and that reversibility enables blocking strategies in synthesis.
Halogenation: Cl₂/FeCl₃ mirrors bromination; F₂ is too reactive; I₂ too unreactive (needs an oxidant).

The nitro and sulfonic-acid groups are strongly deactivating and meta-directing; the halogens are weakly deactivating but ortho/para-directing. Those effects (a later topic) explain why nitration of chlorobenzene works while nitrobenzene needs very harsh conditions.

Why this matters

These reactions build aromatic chemistry's molecular variety. Nitration is the entry point to anilines: reduction of the nitro group (Fe/HCl or catalytic hydrogenation) converts a strongly deactivating meta-director into a strongly activating ortho/para-director — a transformation used in dye chemistry and pharmaceutical synthesis. Sulfonation delivers the sulfonic-acid group of sulfa drugs, detergents, and ion-exchange resins; its reversibility makes it the textbook blocking tool in synthesis. Chlorination produces chlorobenzene and chlorotoluenes, bulk intermediates for agrochemicals, polymers, and solvents. On exams these appear as "complete the reaction," "predict the product," and "propose a synthesis" questions.

The college version

Core Concepts

Nitration: making the nitronium ion

Nitric acid alone is a weak electrophile toward benzene. Concentrated sulfuric acid protonates and dehydrates it, generating the powerful electrophile NO₂⁺ (nitronium ion):

$HNO 3 + 2 H 2 SO 4 ⟶ NO 2 + + H 3 O + + 2 HSO 4 -$

In words: sulfuric acid protonates the OH of nitric acid; loss of water gives NO₂⁺. The nitronium ion then attacks the benzene π cloud to form the arenium ion, and deprotonation regenerates the aromatic ring, giving nitrobenzene and water. Nitrobenzene is strongly deactivated toward further EAS — the nitro group withdraws electrons by induction and resonance — so a second nitration needs harsher conditions and delivers predominantly 1,3-dinitrobenzene.

From nitro to amino

Reduction of the nitro group (iron and aqueous acid, or H₂ over a metal catalyst) gives an aniline:

$C 6 H 5 NO 2 Fe/HCl or H 2 / Pd ⟶ C 6 H 5 NH 2$

The amino group is strongly activating and ortho/para-directing — the opposite personality of the nitro group it came from. This nitro → amino sequence is one of the most-used "substituent makeover" tools in aromatic synthesis.

Sulfonation: reversible, and therefore useful

Sulfonation uses SO₃ as the electrophile, generated in fuming sulfuric acid (oleum); in concentrated H₂SO₄ the active species is written as SO₃ or HSO₃⁺. The product, benzenesulfonic acid, is a strong organic acid; the reaction is reversible:

$C 6 H 6 + SO 3 ⇌ C 6 H 5 SO 3 H$

Hot water or steam drives the reverse reaction (), removing SO₃H and returning benzene. Because the SO₃H group can be removed, it serves as a : sulfonate the position that would otherwise direct the next substituent, force the new substituent elsewhere, then remove the sulfonate with steam. Sulfonic acids are also precursors of sulfonamides, the sulfa-drug class.

Halogenation of benzene beyond bromine

Chlorination: Cl₂ with FeCl₃ (or Fe, which generates FeCl₃ in situ) is the direct analogue of bromination: C₆H₆ + Cl₂ ^FeCl₃⟶ C₆H₅Cl + HCl; chlorine is more reactive than bromine.
Iodination: I₂ alone fails because the reaction is thermodynamically unfavorable — HI would reduce the product back to benzene. An oxidant (e.g., HNO₃) converts I₂ to an I⁺-like electrophile and consumes HI.

All halogenations pass through the same arenium-ion intermediate.

Preview: Friedel–Crafts reactions (next topic)

Alkylation (RCl/AlCl₃) and acylation (RCOCl/AlCl₃) form C–C bonds. Alkylation proceeds through a carbocation that can rearrange and can polyalkylate; acylation proceeds through a resonance-stabilized acylium ion (R–C≡O⁺), does not rearrange, and stops at the monoacyl product because the ketone deactivates the ring. Both fail on strongly deactivated rings (nitrobenzene) and on basic N–H rings (aniline complexes with AlCl₃).

Common Confusions

Do Not Confuse	With	The Difference
"Nitric acid is the electrophile"	The actual electrophile NO₂⁺	HNO₃ alone is weak; H₂SO₄ generates the nitronium ion.
Sulfonation (reversible)	Other EAS reactions (irreversible)	Only sulfonation is reversed by hot water/steam — the basis of blocking.
F₂, Cl₂, Br₂, I₂ reactivity	One-size-fits-all halogenation	F₂ is violent/uncontrollable; Cl₂/Br₂ need FeX₃; I₂ needs an oxidant.
"Friedel–Crafts works on any arene"	Reality	Fails on strongly deactivated rings (nitrobenzene) and on basic N–H rings (aniline).
Deactivating = meta-directing	Halogens	Halogens deactivate but direct ortho/para; only strong π-withdrawing groups (NO₂, SO₃H, C=O) direct meta.
"A deactivating group stops all reaction"	Reality	It slows EAS and redirects it; harsher conditions can still force substitution (e.g., chlorobenzene nitration).

Eli explains

The same idea, in plain words

Explain it like I’m 10

Bromination swaps a hydrogen for a bromine — these reactions are the same swap with different visitors. Nitration uses a special "nitronium" messenger made by mixing nitric and sulfuric acids to plant a NO₂ tag on the ring. Sulfonation plants an SO₃H tag that steam can peel off later — a temporary sticky note that blocks one spot so a new tag is forced elsewhere. Chlorination works like bromination with a different helper; iodine is too shy to join alone and needs a push from an oxidant.

Worked example

Example 1: Degree of unsaturation of nitrobenzene

Nitrobenzene is C₆H₅NO₂. The DBE formula including nitrogen is DBE = C - H2 + N2 + 1:

$DBE = 6 - 52 + 12 + 1 = 6 - 2.5 + 0.5 + 1 = 5$

Interpretation: the benzene ring contributes 4 units (1 ring + 3 π bonds) and the N=O double bond contributes 1 more — total 5. DBE counts all rings and π bonds, including a substituent's.

Example 2: Stoichiometry of nitration

How much HNO₃ is needed to nitrate 5.00 g of benzene? Balanced equation:

$C 6 H 6 + HNO 3 H 2 SO 4 ⟶ C 6 H 5 NO 2 + H 2 O$

Moles of benzene:

$n = 5.00 g 78.11 g\cdotmol -1 = 0.0640 mol$

1:1 stoichiometry: nitric acid required is 0.0640 mol × 63.01 g·mol^-1 = 4.03 g (excess is used in practice). Theoretical yield of nitrobenzene:

$m = 0.0640 mol \times 123.11 g\cdotmol -1 = 7.88 g$

If 6.50 g is isolated, the percent yield is (6.50/7.88) × 100% = 82.5%. Units chain: g → mol → g.

Example 3: Synthesis planning with the sulfonation blocking group (words-only walkthrough)

Target: o-nitrotoluene from toluene. Toluene's methyl group is ortho/para-directing, so direct nitration gives o- and p-nitrotoluene. To force nitration ortho only: (1) sulfonate toluene — the bulky, reversible SO₃H installation favors para, blocking it; (2) nitrate the p-toluenesulfonic acid — para is occupied, and both the methyl (ortho-directing) and the sulfonate (meta-directing) groups agree on C-2, so nitration delivers the 2-nitro product selectively; (3) desulfonate with steam to leave o-nitrotoluene. The sequence works precisely because sulfonation is reversible and everything else is not.

Example 4: Predicting the product of nitration of chlorobenzene (words-only)

Chlorine is a weak deactivator and an ortho/para-director, so nitration of chlorobenzene occurs at the ortho and para positions — a mixture of o- and p-chloronitrobenzene — needing somewhat more forcing conditions than nitration of benzene. The same logic explains why nitrobenzene is hard to nitrate twice: NO₂ strongly deactivates, and further nitration goes meta.

Key takeaways

Nitration: HNO₃/H₂SO₄ → NO₂⁺ (nitronium ion); product nitrobenzene; NO₂ is strongly deactivating, meta-directing.
Sulfonation: electrophile SO₃ (oleum); reversible — steam removes SO₃H; used as a blocking group; product is a strong organic acid.
Chlorination: Cl₂/FeCl₃, analogous to bromination; F₂ is dangerously vigorous; I₂ needs an oxidant (I⁺ equivalent).
All EAS reactions share the two-step arenium-ion mechanism; halogenation/nitration/sulfonation show no rearrangements.
Friedel–Crafts (next topic): alkylation rearranges and polyalkylates; acylation is clean; both fail on rings like nitrobenzene.

Check yourself

6 review questions from the chapter. Try each one, then open the answer.

What is the electrophile in nitration, and how is it generated?
Show answer
The nitronium ion, NO₂⁺, generated when concentrated H₂SO₄ protonates and dehydrates HNO₃.
Why is sulfonation reversible, and how is that exploited in synthesis?
Show answer
Hot water or steam removes the SO₃H group (desulfonation). In synthesis, SO₃H blocks a position (e.g., para of toluene), forces the next substituent elsewhere, then is removed by steam.
Which halogen is too reactive for direct use, and which needs an oxidant?
Show answer
F₂ is too vigorous (explosive, polyfluorination); I₂ is too unreactive because the I₂/HI equilibrium lies toward starting materials — an oxidant makes an I⁺-like electrophile and consumes HI.
Calculate the DBE of nitrobenzene and account for each unit.
Show answer
DBE = 6 - 5/2 + 1/2 + 1 = 5: ring (1) + ring π bonds (3) + N=O double bond (1).
What happens when a nitro group is reduced, and why is that transformation valuable?
Show answer
Reduction (Fe/HCl or H₂/Pd) converts −NO₂ to −NH₂: a strongly deactivating meta-director becomes a strongly activating ortho/para-director — a key move in aromatic synthesis.
Why does Friedel–Crafts chemistry fail on nitrobenzene?
Show answer
The nitro group withdraws electrons so strongly that the ring cannot supply enough density to attack even the powerful Friedel–Crafts electrophiles.

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Study tools & related lessonsKey vocabulary · Related

Key vocabulary

Nitronium ion (NO₂⁺): The electrophile of nitration, from HNO₃ + H₂SO₄
Nitro group (−NO₂): Strongly electron-withdrawing, meta-directing substituent
Sulfonation: EAS installing −SO₃H using SO₃
Desulfonation: Removal of −SO₃H by hot water/steam
Blocking group: Removable substituent occupying a reactive position
Deactivating group: Substituent that slows further EAS (NO₂, SO₃H, halogens)

Sources & references

openstax.org — Organic Chemistry

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