Carbonium ions electrophilic aromatic substitution reactions

Hammond postulate has been used to explain the effect of substituents on the rate of benzilic acid rearrangements, mechanism of electrophillic aromatic substitution reactions and reactions involving highly reactive intermediates such as carbonium ions and carbon ions. 

The selectivity relationship merely expresses the proportionality between intermolecular and intramolecular selectivities in electrophilic substitution, and it is not surprising that these quantities should be related. There are examples of related reactions in which connections between selectivity and reactivity have been demonstrated. For example, the ratio of the rates of reaction with the azide anion and water of the triphenylmethyl, diphenylmethyl and tert-butyl carbonium ions were 2-8x10 , 2-4x10 and 3-9 respectively the selectivities of the ions decrease as the reactivities increase. The existence, under very restricted and closely related conditions, of a relationship between reactivity and selectivity in the reactions mentioned above, does not permit the assumption that a similar relationship holds over the wide range of different electrophilic aromatic substitutions. In these substitution reactions a difficulty arises in defining the concept of reactivity it is not sufficient to assume that the reactivity of an electrophile is related... 

A specific case of the carbonium ion mechanism [Eq. (5)] with reasonable plausibility is decarboxylation of metal arenoates by classic electrophilic aromatic substitution [Eq. (12)]. This mechanism would be favored by electron-donating substituents and has been invoked to explain the relative ease of decarboxylation of p-methoxybenzoic acid in molten mercuric trifluoroacetate (77) as well as the very facile decarboxylation on reaction of polymethoxybenzoic acids with mercuric acetate (18) (see below). 

Electrophilic substitution, then, like electrophilic addition, is a stepwise process involving an intermediate carbonium ion. The two reactions differ, however, in the fate of the carbonium ion. While the mechanism of nitration is, perhaps, better established than the mechanisms for other aromatic substitution reactions, it seems clear that all these reactions follow the same course. 

We have accounted for the facts of electrophilic aromatic substitution in exactly the way that we accounted for the relative ease of dehydration of alcohols, and for reactivity and orientation in electrophilic addition to alkenes the more stable the carbonium ion, the faster it is formed the faster the carbonium ion is formed, the faster the reaction goes. 

In al this we have estimated the stability of a carbonium ion on the same basis the dispersal or concentration of the charge due to electron release or electron withdrawal by the substituent groups. As wc shall see, the approach that has worked so well for elimination, for addition, and for electrophilic aromatic substitution works for still another important class of organic reactions in which a positive charge develops nucleophilic aliphatic substitution by the S l mechanism (Sec. 14.14). It works equally well for nucleophilic aromatic substitution (Sec. 25.9), in which a negative charge develops. Finally, we shall find that this approach will help us to understand acidity or basicity of such compounds as carboxylic acids, sulfonic acids, amines, and phenols. 

In our study of electrophilic aromatic substitution (Sec. 11.19 and Sec. 30.9), we found that we could account for orientation on the following basis the controlling step is the attachment of the electrophilic reagent to the aromatic ring, which takes place in .uch a way as to yield the most stable intermediate carbonium ion. Let us apply this approach to the reactions of pyrrole. 

Since A is electron-seeking, this process is said to be an electrophilic substitution reaction and is given the symbol aSe- Many carbonium ion reactions (p. 45) and aromatic substitution reactions (p. 235) are initiated by electrophilic attack. 

It is not surprising that electrophilic aromatic substitutions were the first organic reactions investigated using acidic room-temperature chloro-almninate(III) ionic liquids. Indeed, chloroaluminate(ni) species combine their properties of good solvents for simple arenes to their role as Lewis acid catalysts. In Friedel-Craft alkylations, polyalkylation is common as well as the isomerisation of primary halides to secondary carbonium ions. 

The mechanism of the aromatic substitution may involve the attack of the electrophilic NOj ion upon the nucleophilic aromatic nucleus to produce the carbonium ion (I) the latter transfers a proton to the bisulphate ion, the most basic substance in the reaction mixture... 

A reaction in which an electrophile participates in het-erolytic substitution of another molecular entity that supplies both of the bonding electrons. In the case of aromatic electrophilic substitution (AES), one electrophile (typically a proton) is substituted by another electron-deficient species. AES reactions include halogenation (which is often catalyzed by the presence of a Lewis acid salt such as ferric chloride or aluminum chloride), nitration, and so-called Friedel-Crafts acylation and alkylation reactions. On the basis of the extensive literature on AES reactions, one can readily rationalize how this process leads to the synthesis of many substituted aromatic compounds. This is accomplished by considering how the transition states structurally resemble the carbonium ion intermediates in an AES reaction. 

Electrophiles for aromatic substitution include the halonium ion, the nitronium ion and the carbonium ion. The latter may be generated from alkyl and acyl halides using Lewis acid catalysis in the Friedel-Crafts reactions. 

Thus far the reaction is like addition to alkenes an electrophilic particle, attracted by the rr electrons, attaches itself to the molecule to form a carbonium ion. But the fate of this carbonium ion is different from the fate of the ion formed from an alkene. Attachment of a basic group to the benzenonium ion to yield the addition product would destroy the aromatic character of the ring. Instead, the basic ion, HS04", abstracts a hydrogen ion (step 3) to yield the substitution product, which retains the resonance-stabilized ring. Loss of a hydrogen ion, as we have seen, is one of the reactions typical of a carbonium ion (Sec. 5.20) it is the preferred reaction in this case. 

This interpretation is consistent with our mechanism. The rate of the overall substitution is determined by the slow attachment of the electrophilic reagent to the aromatic ring to form the carbonium ion. Once formed, the carbonium ion rapidly loses hydrogen ion to form the products. Step (1) is thus the rate-determining step. Since it docs not involve the breaking of a carbon- hydrogen bond, its rate- and hence the rate of the overall reaction- is independent of the particular hydrogen isotope that is present. 

Substitution of a trimethylsilyl group directly onto an aromatic ring leads to an increase in the rate of electrophilic attack relative to that for hydrogen usually via ipso substitution. Cleavage of the C-Si bond is in the direction C SiR3 in the same sense as aryl-H bonds are broken C"H +. This reaction proceeds by the familiar aromatic substitution mechanism (equation 21). Activation to electrophilic attack arises from / stabilization of the carbonium ion in one of the resonance forms, 18, of the intermediate79,80. 

Friedel-Crafts reactions are important methods for introducing carbon substituents on aromatic rings. The reactive intermediates are electrophilic carbon species. In some reactions, discrete carbonium ions or acylium ions are involved in other cases, however, the electrophile no doubt consists of the alkyl or acyl group still bonded to a potential leaving group, which is displaced in the substitution step. Whether discrete carbonium ions are involved depends primarily on the stability of the potential carbonium ion. 

Alkylation is the paramount electrophilic substitution reaction in industrial aromatic chemistry, for example, in the production of ethylbenzene, cumene, diisopropylbenzenes and diisopropylnaphthalenes. A carbonium ion generally acts as the electrophilic agent and is produced by reaction of a Lewis add with an olefin. The most stable of the possible carbonium ions normally predominates in the reaction nevertheless, attention must also be paid to the formation of isomers. 

A typical benzene substitution reaction involves preliminary addition of an electrophilic agent to the ring forming a carbonium ion which is followed by loss of a proton to regenerate the conjugated aromatic system. 

In somewhat related work. Siskin et ah reported that 4-hydroxyphenyl benzyl ether undergoes complete conversion in decalin and water at 343°C (Fig. 9.47) during 2 days to give toluene (63 and 44%), hydroquinone (32 and 35%), and 2-benzylhydroquinone (4 and 12%). It was postulated that the intermediate benzyl alcohol was formed, which subsequently reacted with hydroquinone to produce the 2-benzylhydroquinone. The benzyl carbonium ion was proposed as the reactive species in the aromatic electrophilic substitution reaction. 

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