Chemguide: Support for CIE A level Chemistry
Learning outcome 15.4(c)
The first part of this page is for exams up to 2018.
This statement 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.
For exams from 2019, this statement has been replaced by the one from previous syllabuses
The relative lack of reactivity of compounds like chlorobenzene compared with halogenoalkanes
The lack of reactivity of compounds like chlorobenzene is covered on the page about reactions of aryl halides. It is important that you follow the link in the section about the strength of the carbon-halogen bond to the introductory page about aryl halides. You can ignore the stuff about physical properties on that second page.
For exam purposes, CIE will ask you about the lack of reactivity of chlorobenzene in terms of the extra strength of the carbon-chlorine bonds. Don't use the additional explanation about the repulsion between the incoming nucleophile and the ring electrons.
If you are asked why chlorobenzene is unreactive towards nucleophiles, say "Delocalisation of a lone pair on a chlorine atom over the benzene ring strengthens the carbon-chlorine bond."
© Jim Clark 2014 (modified February 2017)