CD 12.4 Reactions of Arenes: Chlorination of Benzene

The chlorination of benzene is very much similar to the bromination of benzene. The video above does a very good job serving as a visual aid, thanks to dnseducation. The only difference is that Chemical Ideas gives the catalyst to be aluminium chloride rather than iron (III) chloride (however they both have the same mechanism.) So in the exam I'd use aluminium chloride as the catalyst.

Anyway, the reaction must take place in anhydrous conditions as the aluminium chloride reacts with water, which we do not want do we? Noooo, we do not my precious. (Sorry about the movie quotes, I need to cheer myself up since I just wrote this out 10 minutes ago then accidentally logged out of google and lost it all, rant over, let the learning begin.)

The overall reaction equation is shown below:

The first step in the reaction is the polarisation of the chlorine molecule, caused by approaching the benzene. Benzene is electron rich, so causes the electrons in the bond between the chlorines to repel away. This makes it have a slightly positive and negative end.

Aluminium chloride also has areas of slightly positive and negative charge. The chlorine atoms are highly electronegative and so the aluminium atom becomes positive. To fix this, the aluminium wants to gain electrons, and so accepts a lone pair of electrons from the chlorine. This in turn, makes the chlorine even more polarised, to the point where it splits apart. This forms the electrophile, a chloride ion, and the other chloride ion binds to the catalyst to form AlCl₄^-.

An electrophile loves electrons, it's like a gold digger... for electrons. So the electron rich benzene is a prime 'gold digging' target. The chloride ion therefore 'attacks' the benzene ring, and binds the to carbon.

This upsets the delocalised system of electrons momentarily, because it loses one to the bond with the chloride ion. That bitch of a chloride ion stole your electrons and your delocalised system is in trouble, so what are you gonna do?! You're gonna get them back by kicking off a hydrogen ion.

So the benzene takes the electrons from the bond with hydrogen, releasing it into the wild. It doesn't have any electrons of it's own so is positively charged. So now chlorobenzene has been formed - it lost two electrons to form a bond with the chloride ion, and gained two from kicking off the hydrogen, it is now stable.

The hydrogen ion goes on to react with the AlCl₄^- to form HCl and regenerate the catalyst AlCl₃.

And that is the story of the gold digging, chloride electrophile and benzene. It is a classic tale of electrophilic substitution.

I've not managed to find any past exam questions on it, so be prepared for it to come up. But by the looks of things, the exams focus on bromination, nitration and the Friedel-Crafts reactions in this chapter.