3.2: Electrophilic Aromatic Substitution Reactions - Bromination
After completing this section, you should be able to
- write the detailed mechanism for the reaction of bromine with benzene in the presence of a suitable catalyst.
- draw the resonance contributors for the carbocation which is formed during the reaction of bromine with benzene.
- compare the reaction which takes place between bromine and benzene and the reaction which takes place between bromine and an alkene.
- draw an energy diagram for the reaction of bromine with benzene.
- identify the reagents required to bring about aromatic bromination.
- write an equation to represent aromatic bromination.
Before seeing how electrophilic aromatic substitutions occur, let’s briefly recall what we learned about alkenes: Structure and Reactivity about electrophilic alkene additions. When a reagent such as Br 2 adds to an alkene, the \(\pi\) electrons of the double bond approach the electrophilic Br and form in a second step, the bromonium reacts with Br− ion to yield the addition product. (First Reaction in Figure 1).
An electrophilic aromatic substitution reaction begins in a similar way, but there are a number of differences. One difference is that aromatic rings are less reactive toward electrophiles than alkenes. For example, \(\ce{Br2}\) in \(\ce{CH2Cl2}\) solution reacts instantly with most alkenes but does not react with benzene at room temperature (Second Reaction in Figure 1). For the bromination of benzene to take place, a catalyst such as \(\ce{FeBr3}\) is needed, which results in the substitution of hydrogen. (Third Reaction in Figure 1)
A Mechanism for Electrophilic Substitution Reactions of Benzene
The catalyst makes the \(\ce{Br2}\) molecule more electrophilic by polarizing it to give a \(\ce{FeBr4^{–}Br^{+}}\) species that reacts as if it were \(\ce{Br^{+}}\).
A two-step mechanism has been proposed for these electrophilic substitution reactions. In the first, slow or rate-determining step, the polarized \(\ce{Br2}\) molecule reacts with the nucleophilic benzene ring forming a sigma-bond to yield a nonaromatic carbocation intermediate that is doubly allylic and has three resonance forms. This intermediate is called arenium intermediate .
Although more stable than a typical alkyl carbocation because of resonance, the intermediate in electrophilic aromatic substitution is nevertheless much less stable than the starting benzene ring itself, with its 150 kJ/mol (36 kcal/mol) of aromatic stability. Thus, the reaction of an electrophile with a benzene ring is endergonic, has a substantial activation energy, and is rather slow. Figure \(\PageIndex{3}\) shows an energy diagram comparing the reaction of an electrophile with an alkene and with benzene. The benzene reaction is slower (higher ∆ G ‡ ) because the starting material is more stable.
Another difference between alkene addition and aromatic substitution occurs after the carbocation intermediate has formed. Instead of adding Br – to give the product of an addition reaction, the carbocation intermediate loses H + from the bromine-bearing carbon to give a substitution product. Note that this loss of H + is similar to what occurs in the second step of an E1 reaction. The net effect of reaction of Br 2 with benzene is the substitution of H + by Br + by the overall mechanism shown in Figure \(\PageIndex{5}\).
Why does the reaction of Br 2 with benzene take a different course than its reaction with an alkene? The answer is straightforward. If addition occurred, the 150 kJ/mol stabilization energy of the aromatic ring would be lost and the overall reaction would be endergonic. When substitution occurs, though, the stability of the aromatic ring is retained and the reaction is exergonic. An energy diagram for the overall process is shown in Figure \(\PageIndex{6}\).
Exercises
What reagents would you need to get the given product?
- Answer
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Cl 2 and AlCl 3 or Cl 2 and FeCl 3
What product would result from the given reagents?
- Answer
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No Reaction
What is the major product given the reagents below?
- Answer
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Draw the formation of Cl + from AlCl 3 and Cl 2 .
- Answer
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Draw the mechanism of the reaction between Cl + and a benzene.
- Answer
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The mechanism presented in Figure 5 for electrophilic aromatic substitution should be considered in context with other mechanisms involving carbocation intermediates. These include S N 1 and E1 reactions of alkyl halides, and Brønsted acid addition reactions of alkenes.
To summarize, when carbocation intermediates are formed one can expect them to react further by one or more of the following modes:
1.
The cation may bond to a nucleophile to give a substitution or addition product.
2.
The cation may transfer a proton to a base, giving a double bond product.
3.
The cation may rearrange to a more stable carbocation, and then react by mode #1 or #2.
S N 1 and E1 reactions are respective examples of the first two modes of reaction. The second step of alkene addition reactions proceeds by the first mode, and any of these three reactions may exhibit molecular rearrangement if an initial unstable carbocation is formed. The carbocation intermediate in electrophilic aromatic substitution (the arenium ion) is stabilized by charge delocalization (resonance) so it is not subject to rearrangement. In principle it could react by either mode 1 or 2, but the energetic advantage of reforming an aromatic ring leads to exclusive reaction by mode 2 ( ie. proton loss).