Aromatic substitution electrophilic species

A nitro group behaves the same way m both reactions it attracts electrons Reaction is retarded when electrons flow from the aromatic ring to the attacking species (electrophilic aromatic substitution) Reaction is facilitated when electrons flow from the attacking species to the aromatic ring (nucleophilic aromatic substitution) By being aware of the connection between reactivity and substituent effects you will sharpen your appreciation of how chemical reactions occur... 

Electrophilic aromatic substitution (Section 12 1) Fundamen tal reaction type exhibited by aromatic compounds An electrophilic species (E" ) attacks an aromatic ring and re places one of the hydrogens... 

A wide variety of electrophilic species can effect aromatic substitution. Usually, it is a substitution of some other group for hydrogen that is of interest, but this is not always the case. Scheme 10.1 lists some of the specific electrophilic species that are capable of carrying out substitution for hydrogen. Some indication of the relative reactivity of the electrophiles is given as well. Most of these electrophiles will not be treated in detail until Part B. Nevertheless, it is important to recognize the very broad scope of electrophiUc aromatic substitution. 

Scheme 10.1. Electrophilic Species Active in Aromatic Substitution...

The active electrophile is formed by a subsequent reaction, often involving a Lewis acid. As discussed above with regard to nitration, the formation of the active electrophile may or may not be the rate-determining step. Scheme 10.1 indicates the structure of some of the electrophihc species that are involved in typical electrophilic aromatic substitution processes and the reactions involved in their formation. 

In a first reaction step the formaldehyde 2 is protonated, which increases its reactivity for the subsequent electrophilic aromatic substitution at the benzene ring. The cationic species 4 thus formed loses a proton to give the aromatic hydroxymethyl derivative 5, which further reacts with hydrogen chloride to yield the chloromethylated product 3 ... 

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 synthesis of an alkylated aromatic compound 3 by reaction of an aromatic substrate 1 with an alkyl halide 2, catalyzed by a Lewis acid, is called the Friedel-Crafts alkylation This method is closely related to the Friedel-Crafts acylation. Instead of the alkyl halide, an alcohol or alkene can be used as reactant for the aromatic substrate under Friedel-Crafts conditions. The general principle is the intermediate formation of a carbenium ion species, which is capable of reacting as the electrophile in an electrophilic aromatic substitution reaction. 

Mechanistically it is an electrophilic aromatic substitution reaction. The electrophilic species (4—its exact structure is not known) is generated in a reaction of hydrogen cyanide and hydrogen chloride (gas) and a Lewis acid ... 

Resole syntheses entail substitution of formaldehyde (or formaldehyde derivatives) on phenolic ortho and para positions followed by methylol condensation reactions which form dimers and oligomers. Under basic conditions, pheno-late rings are the reactive species for electrophilic aromatic substitution reactions. A simplified mechanism is generally used to depict the formaldehyde substitution on the phenol rings (Fig. 7.21). It should be noted that this mechanism does not account for pH effects, the type of catalyst, or the formation of hemiformals. Mixtures of mono-, di-, and trihydroxymethyl-substituted phenols are produced. 

The first step is usually, but not always, rate determining. It can be seen that this mechanism greatly resembles the tetrahedral mechanism discussed in Chapter 10 and, in another way, the arenium ion mechanism of electrophilic aromatic substitution. In all three cases, the attacking species forms a bond with the... 

The behaviour of the frontier electrons was also attributed to a certain type of electron delocalization between the reactant and the reagent 40). A concept of pseudo-n-orbital was introduced by setting up a simplified model, and the electron delocalization between the 71-electron system of aromatic nuclei and the pseudo-orbital was considered to be essential to aromatic substitutions. The pseudo-orbital was assumed to be built up out of the hydrogen atom AO attached to the carbon atom at the reaction center and the AO of the reagent species, and to be occupied by zero, one, and two electrons in electrophilic, radical, and nucleophilic reactions. A theoretical quantity called "superdelocalizability was derived from this model. This quantity will be discussed in detail later in Chap. 6. 

If we are correct in our assumption that the electrophilic substitution of aromatic species involves such a complexes as intermediates—and it has proved possible actually to isolate them in the course of some such substitutions (p. 136)—then what we commonly refer to as aromatic substitution really involves initial addition followed by subsequent elimination. How this basic theory is borne out in the common electrophilic substitution reactions of benzene will now be considered. 

Many other aromatic electrophilic substitution reactions are found to follow the general pathway [2] discussed above, usually corresponding to [2a] though a number of [2b] examples are known. The major point still requiring elucidation is very often the exact nature of the electrophilic species that is involved in attack on the aromatic nucleus. 

Attack on aromatic species can occur with radicals, as well as with the electrophiles (p. 131) and nucleophiles (p. 167) that we have already considered as with these polar species, homolytic aromatic substitution proceeds by an addition/elimination pathway ... 

There is, however, no very satisfactory explanation of why such m-attack as does take place on QH5Y should also be faster than attack on QHg or of why attack on the o-position in C6H5Y is commonly faster than attack on the p-position. The relatively small spread of the partial rate factors for a particular QH5Y means that homolytic aromatic substitution normally leads to a more complex mixture of products than does electrophilic attack on the same species. 

On the other hand, the involvement of vinyl cationic species in the reaction cannot be ruled out in some cases, as shown in Scheme 4. In this context, it was found that the reaction of 3-butyn-2-one with mesitylene can occur without Pd(OAc)2, clearly indicating the involvement of vinyl cations generated from alkynes and H+ in this reaction.47 1 The yield difference in the presence and in the absence of Pd(OAc)2 may be explained by the competition between [Pd(n)02CF3]+ and vinyl cationic species in the electrophilic substitution of aromatic G-H bonds. Recent kinetic isotope experiments suggest a mechanism involving alkyne coordination to Pd(n) followed by electrophilic aromatic substitution.476... 

No investigations have as yet been undertaken to elucidate the mechanism of these reactions, although ordinary electrophilic aromatic substitutions are the most likely. The attacking species is apparently RfS" " or an activated complex formed through coordination of the per-fluorohalogenosulfenyl chloride with a Lewis or a Bronsted acid (4). 

Aniline is an important derivative of benzene that can be made in two steps by nitration to nitrobenzene and either catalytic hydrogenation or acidic metal reduction to aniline. Both steps occur in excellent yield. Almost all nitrobenzene manufactured (97%) is directly converted into aniline. The nitration of benzene with mixed acids is an example of an electrophilic aromatic substitution involving the nitronium ion as the attacking species. The hydrogenation of nitrobenzene has replaced the iron-... 

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