Ferrocene Case

Fig. 1: EAS of bromine on a benzene ring

The three important steps should be noted in this reaction. The first is the generation of the electrophilic bromine. This occurs through the FeBr3 catalyst which forms a complex with the bromine gas. Essentially, a good leaving group, -FeBr4-, is created where one did not exist before. The second step is the attack of the aromatic π-electrons ring on the electrophile, causing the –FeBr4- to leave. The third step is the deprotonation of the carbocation intermediate to reform the aromatic ring.

Some common EAS reactions include halogenation, Friedel-Crafts alkylation, nitration, and sulfonation. These all follow the same type of three step mechanism in Figure 1.

The type of EAS that occurs is affected by the type of already present substituent groups on the ring. The substituents groups can be either activating or deactivating by donating or withdrawing charge, respectively. Examples of activating groups include alcohol, ether, and amine groups. Deactivating groups include halogens, nitro groups, and carboxylates. In general, activating groups will direct substitution at the ortho- and meta- positions, whereas deactivating groups will direct substitution at the para- positions. One important exception are the halogens, which are deactivating yet direct substitution at the ortho- and para- positions.

In this particular experiment, two different EAS reactions are explored. The first is the bromination of acetanilide, which is a benzene ring with a –NHCOCH3 amide group attached. The distinguishing feature of the amide group is its ability to donate charge to the ring through delocalization of its lone pair. This reaction is shown below as Figure 2.