The rest is according to the general mechanism of electrophilic aromatic substitution: The electrophile in the nitration of benzene is the + NO 2 (the nitronium ion), which is formed by protonation of HNO 3 by H 2SO 4 (yep, sulfuric acid is powerful). Instead, it is done by converting benzene into an arenediazonium salt which is then replaced by fluorine by reacting it with fluoroboric acid (HBF 4). Plus, handling F 2 is not really what you want to do unless you absolutely have to, and are trained to do so. They oxidize the I 2 to I + and after this, it follows the standard mechanism of the electrophilic aromatic substitution.įluorination of benzene, on the other hand, is a violent reaction and cannot be achieved directly. Iodine is unreactive under identical conditions and the iodination of benzene is achieved in the presence of an oxidizing agent such as nitric acid or a mixture of hydrogen peroxide and sulfuric acid. Sometimes, Fe may be shown instead of FeBr 3, but don’t worry, it is the same thing as Fe as it reacts with Br 2 to form the catalyst FeBr 3 in situ (in the reaction mixture). The rest of the mechanism is identical to what we saw for the chlorination of benzene.
#Chemdoodle ion bonds Activator#
In the same way, FeBr 3 is used as the Lewis acid activator for generating the source of Br +. First, the addition of the electrophile, forming the sigma complex which is then deprotonated by –AlCl 4. Once the electrophile is formed, it follows the same general mechanism as we have discussed earlier.
The Lewis acids are usually aluminum chloride (AlCl 3) or iron chloride (FeCl 3) used for the chlorination, and iron bromide (FeBr 3) for the bromination of the aromatic ring: The activation energy of this step is a lot smaller and the reaction occurs very fast:īenzene only reacts with bromine and chlorine in the presence of Lewis acids as they coordinate to the halogens and generate strong electrophilic species. The deprotonation is the driving force of the reaction making it energetically possible to proceed. In the second step, the hydrogen on the sp 3-hybridized carbon is removed by a counterion/conjugate base restoring the aromaticity to the ring: It has three resonance forms, where the positive charge appears on three carbons and the resonance hybrid can be shown with these carbons having a partial positive charge: It is secondary, there two conjugated double bonds, which in turn are conjugated with the empty p orbital of the positively charged carbon. On the other hand, the arenium ion is not the worst carbocation you will ever see. This, energetically unfavorable process of interrupting aromaticity, is the slow- rate determining step of the reaction. Notice that this breaks the highly stable aromatic system since in the intermediate arenium ion, the carbon connected to the electrophile becomes sp 3 hybridized and, therefore, cannot be part of the conjugated π bond system. Because of the new sigma bond formed, this intermediate is called a sigma complex. This forms a σ bond between one carbon atom of the benzene ring and the electrophile. In step 1 the π electrons of benzene attack the electrophile which takes two electrons of the six-electron aromatic system. Regardless of what electrophile is used, the electrophilic aromatic substitution mechanism can be divided into two main steps. The Mechanism of Electrophilic Aromatic Substitution Even though the reaction goes through an intermediate where the aromaticity is broken, it still ends up restored because that brings a lot of stability and energetically is very favorable. And in fact, this is still related to the stability of the aromatic ring. The second difference is that the Br in the electrophilic aromatic substitution reaction replaces the hydrogen while both hydrogens are still there when they are on the alkene. The first difference of benzene being less reactive brings the need for using a Lewis acid FeBr 3 which turns the Br 2 into a stronger electrophile and makes the reaction possible. However, there are two key differences between their reactions with electrophiles.įirst, benzene is very stable and thus less reactive. Second, unlike the alkenes, it undergoes an electrophilic substitution and not an electrophilic addition reaction: Benzene has 3 π bonds and as expected shows some similarities to alkenes in being reactive towards electrophilic species.
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