Benzene is similar to
alkenes in that it
reacts with
electrophiles.
As shown below the outcome of the reactions is different, with
benzene forming a
substitution product and
alkenes forming an
addition product.
Furthermore the reaction with
benzene is slower and
requires a catalyst and/or heating.
Both of these differences can be rationalised on the basis of the
mechanism for the reaction.
As shown below, the
first step is the same, namely
formation of a bond to the electrophile using the electrons of the π bond in either case.
For benzene:
 + Br2 | Step 1
 |  + Br– | Step 2
|  + HBr |
The
second step for
benzene is
abstraction of a proton (H
+) to restore the aromatic system.
The
second step for
alkenes is combination of the
carbocation with Br– to give the addition product.
For but-2-ene:
 + Br2 | Step 1
|
The
difference in the rates of these reactions is due to the
high activation energy required to convert the aromatic system to a non-aromatic carbocation intermediate.
It is, however, noteworthy that the carbocation intermediate formed from benzene is more stable than the one formed from but-2-ene due to delocalisation of the charge to positions ortho- and para- to the position new bond to the substituent as shown below.
+ Br–
