COMMON ELECTROPHILIC AROMATIC SUBSTITUTION REACTIONS

Many of the common electrophilic aromatic substitution reactions can be conducted on indole. CompHcations normally arise either because of excessive reactivity or the relative instabiUty of the substitution product. This is the case with halogenation. 

Common electrophilic aromatic substitution reactions taking piace in phenol are as follows ... 

Given the reactants, write the structures of the main organic products of the common electrophilic aromatic substitution reactions (halogenation, nitration, sulfonation, alkylation, and acylation). 

The following are the five most common electrophilic aromatic substitution reactions ... 

Benzene s aromaticity causes it to undergo electrophilic aromatic substitution reactions. The electrophilic addition reactions characteristic of alkenes and dienes would lead to much less stable nonaromatic addition products. The most common electrophilic aromatic substitution reactions are halogenation, nitration, sulfonation, and Friedel-Crafts acylation and alkylation. Once the electrophile is generated, all electrophilic aromatic substitution reactions take place by the same two-step mechanism (1) The aromatic compound reacts with an electrophile, forming a carbocation intermediate and (2) a base pulls off a proton from the carbon that... 

All electrophilic aromatic substitution reactions share a common mechanism. This mechanism consists of a series of steps. 

Similar intramolecular electrophilic aromatic substitution reactions are common, especially when five- or six-membered rings are formed. 

Also like benzene, naphthalene undergoes electrophilic aromatic substitution reactions. Substitution occurs preferentially at the 1-position. In common nomenclature, the 1-position is called the a-position and the 2-position is called the j8-position. 

Now we can remove the "blocking group" - SO3H and next form the final product 2,4-D. To do this we treat 2-hydroxy 3,5-dichloropbenzenesulfonic acid with steam at 120°C with a trace of acid catalyst. This is essentially the reverse of sulfonation, or "desulfonation." We note that sulfonation is the only one of the common electrophilic aromatic substitutions that is reversible and the question arises why this is so. This calls for an analysis of the general mechanism of electrophilic aromatic substitution. The idealized energy profile for such a reaction (chlorination) is ... 

AICI3 is a commonly used Lewis acid in electrophilic aromatic substitution reactions. Here it activates the bromine to form the electrophile Br+ , which attacks the aromatic ring. Methyl groups are ortho,para directors, so any of the four unsubstituted positions could be attacked, but steric hindrance directs the first bromine to go to one of the positions that does not lead to a 1,2,3-trisubstituted ring. 

Electrophilic aromatic substitution reactions are a very important class of chemical reactions that allow the introduction of substituents on to arenes by replacing a hydrogen atom covalently bonded to the aromatic ring structure by an electrophile. The most common reactions of this type are aromatic nitrations, halogenations, Friedel-Crafts alkylations and acylations, formylations, sulfonations, azo couplings and carboxylations - to name just a few. 

In this section, we study several types of electrophilic aromatic substitution reactions—that is, reactions in which a hydrogen of an aromatic ring is replaced by an electrophile, E. The mechanisms of these reactions are actually very similar. In fact, they can be broken down into three common steps ... 

Issues of regioselectivity in the Blanc chloromethylation and related Friedel-Crafts reactions have been studied extensively. As is common with a majority of electrophilic aromatic substitution reactions, substitution typically occurs ortho or para to electron-donating substituents, with issues of steric strain playing a role in the relative ratio of ortho and para products. The Blanc reaction is t3q)ically somewhat regioselective, favoring the para-isomer but accompanied by lesser amounts of the ortho product. ... 

Electrophilic aromatic substitution reactions are common with phenols and in these processes, the hydroxyl group is retained. However, as pointed out above, preparation of suitable derivatives can induce loss of that hydroxyl group too. 

In this chapter, we will learn about the most common reactions of aromatic rings, with the main focus on electrophilic aromatic substitution reactions. During the course of our discussion, we will see that many common food colorings are aromatic compounds that are synthesized using this reaction type, and we will also see how extensive research of aromatic compounds in the early twentieth century made significant contributions to the field of medicine. 

Resonance effects are the primary influence on orientation and reactivity in electrophilic substitution. The common activating groups in electrophilic aromatic substitution, in approximate order of decreasing effectiveness, are —NR2, —NHR, —NH2, —OH, —OR, —NO, —NHCOR, —OCOR, alkyls, —F, —Cl, —Br, —1, aryls, —CH2COOH, and —CH=CH—COOH. Activating groups are ortho- and para-directing. Mixtures of ortho- and para-isomers are frequently produced the exact proportions are usually a function of steric effects and reaction conditions. 

The rate-determining step is the electrophilic aromatic substitution as in the closely related Friedel-Crafts reaction. Both reactions have in common that a Lewis acid catalyst is used. For the Blanc reaction zinc chloride is generally employed, and the formation of the electrophilic species can be formulated as follows ... 

The most common reaction of aromatic compounds is electrophilic aromatic substitution. That is, an electrophile reacts with an aromatic ring and substitutes for one of the hydrogens. The reaction is characteristic of all aromatic rings, not just benzene and substituted benzenes. In fact, the ability of a compound to undergo electrophilic substitution is a good test of aromaticity- . 

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