Chemguide: Support for CIE A level Chemistry
Learning outcome 15.4(c)
This statements deals with the difference in reactivity between benzene and chlorobenzene.
This statement is new to the 2016 syllabus, and replaces a different comparison between chlorobenzene and halogenoalkanes. At the moment, it isn't clear what detail CIE want for this. What you will find below answers the syllabus statement, but we won't find out whether that is actually enough until 2016 at the earliest.
Before you go on, you should find and read the statement in your copy of the syllabus.
The relative lack of reactivity in chlorobenzene towards electrophilic substitution
Chlorobenzene reacts with electrophiles more slowly than benzene does. The chlorine is said to be a deactivating group.
Electrophilic substitution relies on the electrophile being attracted to the delocalised electrons in the benzene ring. But chlorine is more electronegative than carbon, and so draws electrons in the ring towards itself.
That makes the electron density around the ring rather less in chlorobenzene. It is less attractive for electrophiles, and so the reaction is slower.
© Jim Clark 2014