Benzene: Delocalisation and Electrophilic Substitution
Benzene's six electrons spread around the ring instead of sitting in three double bonds, which makes it much more stable than the alternative structure. That stability is why it substitutes rather than adds.
What a learner can do afterwards
- Compares the enthalpy of hydrogenation of benzene with three times that of cyclohexene and says what the gap shows
- Gives two further pieces of evidence against alternating double bonds, such as equal bond lengths and the failure to decolourise bromine water
- Draws the mechanism for nitration, including how the electrophile is generated
- Explains why the ring is restored at the end of the mechanism
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Benzene spreads its six pi electrons around the whole ring instead of fixing them in three double bonds. Burning it with hydrogen releases clearly less energy than three cyclohexenes would, so the ring starts from a calmer, more stable place.
Two more clues agree. Every carbon to carbon bond in benzene shares one length, and benzene will not decolourise bromine water the way alkenes do.
In nitration, concentrated nitric and sulfuric acids make the NO2+ electrophile, the ring offers its electrons to it, and then a proton drops off so the shared loop closes again as nitrobenzene.
Adding would smash the shared ring and its bonus stability, so benzene substitutes instead and keeps the ring intact. Whenever you write the mechanism, end it by restoring the loop.
Shared electrons bring bonus stability, tests deny fixed double bonds, and substitution protects the ring.
More to read
20.1 Hydrocarbons
Most hydrocarbons are chains and rings that react much as you would expect, but benzene breaks the pattern. Its six carbon ring shares its pi electrons across the whole loop instead of fixing them into three double bonds. That sharing is called delocalisation and it makes benzene far more stable than three separate alkenes. This section covers the hydrocarbon families, including the aromatic ring at the heart of benzene chemistry. Learn how structural formulas reveal the ring, then compare the hydrogenation enthalpy of benzene with three times that of cyclohexene. The gap between them shows delocalisation.
21.1 Hydrocarbons
Normal alkenes decolourise bromine water in an addition reaction, but benzene leaves it orange, which already argues against three fixed double bonds. X-ray data adds a second fact: every carbon to carbon bond in the ring shares one length, halfway between a single and a double bond, instead of alternating long and short. Taken with the hydrogenation enthalpy gap, each fact points the same way, toward one shared ring of six electrons rather than three separate double bonds.
More EAS & Benzylic Reactions: Crash Course Organic Chemistry #39
Electrophilic aromatic substitution is the reaction family that lets benzene swap a ring hydrogen for a new group while keeping its stable ring. Nitration, sulfonation and halogenation all follow the same script: make a strong electrophile, let the ring attack it, then restore the ring by losing a proton. This resource walks through those reactions with worked examples. Watch how the nitrating mixture of concentrated nitric and sulfuric acids generates the nitronium ion, and notice that the ring is rebuilt at the end. That rebuilt ring is why substitution wins and addition loses.
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.