![]() ![]() Instead almost all of the product consists of the ortho and para isomers. The actual distribution of these products shows that very little of the meta product is obtained. The result is that only three products are obtained. Similar considerations apply to the meta position. Attack at either of these positions gives the same product. We'll use nitration as our example of electrophilic aromatic substitution here.Īlthough there are five hydrogens available to be replaced on the benzene ring of toluene, two of those are directly adjacent ( ortho) to the methyl group. As an example let's look at toluene - which is methyl benzene. There are three products which can arise in such a system. The reagent - catalyst - electrophile - product pattern works well when the aromatic compound is benzene itself, but things become more complicated when a substituted benzene (a molecule in which one of the hydrogens of benzene has been replaced by another atom or group) is used. The reaction is completed by the loss of an H +. To summarize, the function of the catalyst is to convert the reagent into a strong electrophile, which then attacks the pi electrons on the aromatic ring to make a new covalent bond. Its departure makes the active electrophile: When the OH group of nitric acid is involved, the OH 2 +group which is formed is a good leaving group. Just as was the case in the S N1 and S N2 reactions of alcohols, the function of the acid is to protonate an unshared electron pair on oxygen. Similar mechanisms are used in the halogenation and Friedel-Crafts acylation reactions.įor sulfonation and nitration, the catalyst is sulfuric acid. This makes the formation of the electrophile (carbocation in this case) much easier so that there is more electrophile around to attack the benzene ring. Notice that the role of the catalyst is to bond with the leaving group and make it into a better leaving group. The formation of a carbocation from an alkyl halide and aluminum trichloride is a typical example of the first process: One applies when the electrophile is made by removing a halide ion from the reagent (halogenation and the two Friedel-Crafts reactions). What is not clear is how the catalyst transforms the reagent into the electrophile. The connection between the electrophile and the product is clear from these examples. The following table outlines these relationships in more detail for several reactions which follow the electrophilic aromatic substitution pathway. Conversely, if we know the electrophile, we can predict the structure of the product. Let's begin by recalling the key steps in an electrophilic aromatic substitution mechanism.Īn important feature of this mechanism is that we can identify the electrophile if we know the product because it is the atom or group which replaces the H +. ![]()
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