Organic reaction mechanisms (A2)
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I’m revising benzene at the moment and I don't understand how iron catalyses the reaction between methylbenzene and bromine. Hope you can help. Thanks.
Igloo writes
Diagrams don’t travel from the React website so here is a verbal description.
The iron first reacts with the bromine to form iron(III) bromide, FeBr3.
The iron(III) ion in this compound considerably polarises the bromide ions, giving a structure with a highly covalent character. The iron within the molecule is also capable of polarising bromine molecules themselves and this results in the formation of a “loose” complex:
Br-Br -> FeBr3
As shown above, the electrons in the bromine molecule are displaced over to the right towards the iron atom, and so the left-hand side becomes electron deficient, i.e. delta positive. In this condition the structure can act as an electrophile and is attracted to methylbenzene molecules. Once the delta positive bromine atom forms an intermediate bond with the benzene ring, the FeBr3 molecules become detached and are available to form new complexes with fresh bromine molecules.
If not for the creation of the complex, the bromine molecule has no permanent dipole. The interaction with FeBr3 gives a bromine molecule proper electrophilic credentials.
Strictly speaking, then, FeBr3 is the catalyst, since it takes part in the mechanism for this process but is reformed when the complex is destroyed.
If you have studied the function of AlCl3 as a catalyst in the alkylation of methylbenzene (the Friedel-Crafts reaction), you should note that the mechanism I have described for bromination is a very similar one.
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updated: 16 December 2006
