IB Chemistry HLTopic 6 — Electron Pair SharingPaper 1 & 2Organic~11 min read
Electrophilic Substitution in Benzene
Benzene is packed with electrons, so electrophiles come running. But unlike an alkene, benzene refuses to let anything add on. It hands over a hydrogen instead, and puts its ring back exactly as it was.
📚 What you need to know
Benzene has a delocalised π system: six electrons spread evenly over all six carbons.
That delocalisation makes benzene very stable and gives it high electron density.
Electrophiles attack it, but the reaction is substitution, not addition.
Substitution keeps the ring intact; addition would destroy the delocalisation.
Nitration swaps a hydrogen for a nitro group, NO2.
The electrophile is the nitronium ion, NO2+, made in situ from concentrated HNO3 and concentrated H2SO4 at 25–60°C.
The mechanism has three stages: generate the electrophile, attack, then restore aromaticity.
What makes benzene different
In an alkene the π bond belongs to two carbons. In benzene the π electrons are shared right round the ring, which is why we draw a circle inside the hexagon rather than alternating double bonds.
Spreading electrons out lowers the energy of the molecule. Benzene is far more stable than you would predict from three separate C=C bonds, and that extra stability is worth protecting.
An alkene would simply add across the double bond. Benzene will not, because that would break up the delocalisation it has just spent so much energy building.
This is the single idea the whole page hangs on. Addition would leave the ring with one fewer π bond and no delocalisation. Substitution costs a hydrogen but keeps everything else. Benzene picks the cheap option.
Nitration
Nitration replaces one of benzene’s hydrogens with a nitro group. It is the reaction the syllabus uses to teach the mechanism.
Overall equation
C6H6 + HNO3 → C6H5NO2 + H2O
The conditions matter: concentrated nitric acid, concentrated sulfuric acid as catalyst, and a temperature between 25 and 60°C. Too hot and you start substituting a second and third time.
The three stages
Stage 1: make the electrophile
Benzene’s ring is stable, so a weak electrophile will not touch it. Nitric acid on its own is not strong enough. Sulfuric acid is the stronger acid, so it protonates the nitric acid, which then falls apart to give the nitronium ion, NO2+.
The nitronium ion is generated in situ — made in the flask, used immediately, never bottled.
Stage 2: electrophilic attack
A pair of π electrons from the ring reaches out and forms a bond to the nitronium ion. That pair is now tied up in a normal covalent bond, so it is no longer delocalised. The ring is left with only four delocalised electrons and a positive charge.
One carbon now holds both a hydrogen and the nitro group. This is the unstable intermediate.
Stage 3: restore aromaticity
The C–H bond on that carbon breaks heterolytically. Both of its electrons drop back into the ring system, the delocalisation is rebuilt, and the hydrogen leaves as H+.
Only four π electrons are left in the middle structure, which is why it is drawn with partial arcs and a positive charge rather than a full circle.
🧩 Drawing the nitration mechanism
Show the generation of NO2+ if the question asks for it. Concentrated acids, 25–60°C.
Draw benzene with the circle inside, and NO2+ beside it.
Arrow 1: from the circle out to the nitrogen of NO2+.
Draw the intermediate: hexagon, partial arcs not a full circle, a + inside, and both H and NO2 on the same carbon.
Arrow 2: from the C–H bond back into the ring.
Draw nitrobenzene with the circle restored, and H+ released alongside.
Addition or substitution?
Point of comparison
Alkene
Benzene
Type of π system
Localised between two carbons
Delocalised over six carbons
Stability
Ordinary
Unusually high
Reaction with an electrophile
Addition
Substitution
What happens to the ring or bond
π bond is lost for good
Delocalisation is rebuilt
Ends up leaving
Nothing
H+
Reactivity towards Br2
Decolourises it instantly
No reaction without a catalyst
A useful check. Benzene does not decolourise bromine water. If a question gives you an unknown that leaves bromine water orange but still reacts with concentrated nitric and sulfuric acids, an arene is a very good guess.
Worked examples
WORKED EXAMPLE
Explain why benzene undergoes substitution with an electrophile while ethene undergoes addition.
Describe the bonding in each
Ethene has a localised π bond between two carbons. Benzene has a delocalised π system over six.
What delocalisation buys
Spreading the electrons out lowers the energy, so benzene is unusually stable.
What addition would cost
Adding across the ring would permanently destroy that delocalisation and the stability that comes with it.
What substitution costs instead
Losing one hydrogen, after which the ring is rebuilt exactly as before.
Substitution preserves the delocalisation; addition would destroy itthe words “delocalised” and “stability” are both doing work here — use both
WORKED EXAMPLE
State the role of concentrated sulfuric acid in the nitration of benzene, and give an equation for the formation of the electrophile.
What sulfuric acid does
It is the stronger acid, so it protonates the nitric acid, which then loses water to give the nitronium ion.
Write the equationHNO3 + 2H2SO4 → NO2+ + H3O+ + 2HSO4–Why it counts as a catalyst
The HSO4– ions can take back the H+ released at the end of the mechanism, so the sulfuric acid is not used up overall.
It generates the nitronium ion electrophile in situsay “in situ” — NO2+ is far too reactive to be added from a bottle
💡 Exam tip
Name the electrophile precisely: the nitronium ion, NO2+.
Give the conditions as a set: concentrated HNO3, concentrated H2SO4, 25–60°C.
Draw the intermediate without a full circle. Partial arcs plus a positive charge.
Show both H and NO2 on the same carbon in the intermediate. That carbon is the whole point.
Use heterolytic cleavage when describing how the C–H bond breaks in stage 3.
Both curly arrows are double-headed. Arrow 2 goes from the C–H bond into the ring.
⚠️ Common mix-up
Calling it electrophilic addition. Benzene substitutes. Only the alkenes add.
Drawing a full circle in the intermediate. The delocalisation is broken at that point.
Forgetting the H+ product. Something has to leave for it to be a substitution.
Writing NO2 instead of NO2+. Without the charge it is not an electrophile.
Saying sulfuric acid is a reactant. It is regenerated, so it is a catalyst.
Expecting benzene to decolourise bromine water. It will not, and that is often the point of the question.
That completes Electron Pair Sharing. You have now seen the same idea — one species donating a pair of electrons to another — running through nucleophilic substitution, electrophilic addition, Lewis acid–base chemistry, complex ions and aromatic substitution. Different names, one piece of chemistry.
Want this explained one-to-one?
Book a free session with an experienced IB Chemistry tutor and get your trickiest topics made simple.
