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
1 · Read
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.
2 · Watch
Take it off screen
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.