Why does toluene not react with bromine water?
Bromine in water will readily react with alkenes and alkynes, but not alkanes and aromatics like toluene. The Br2 adds across the double bond in the case of alkenes, and two moles of Br2 will add across the triple bond in alkynes. Bromine will be substituted on the methyl group of toluene only under free-radical conditions with special reagents. Bromine will be substituted onto the benzene ring of toluene only with lewis acid catalysts. The reason for this is that Br2 is electrophilic enough to attack double and triple bonds, but not electrophilic enough to attack the pi bond system of the benzene ring of toluene. There is much more electron density in the alkene group (4 electrons) and alkyne group (6 electrons) than between any two carbons of the toluene ring (3 electrons on average). The “empty orbital” of lewis acids are much more reactive towards the toluene pi electron system.
