Carbonium ions electrophilic aromatic substitution

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). 

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 observed effects of structure on rate and on orientation, confirmed by the Brown selectivity relationship, show that there is no basic difference between heterogeneous catalytic alkylation of aromatic compounds and homogeneous electrophilic aromatic substitution, cf. nitration, sul-phonation etc. This agreement allows the formulation of the alkylation mechanism as an electrophilic attack by carbonium ion-like species formed on the surface from the alkene on Br0nsted acidic sites. The state of the aromatic compound attacked is not clear it may react directly from the gas phase (Rideal mechanism ) [348] or be adsorbed weakly on the surface [359]. 

Now, we know that, whatever the specific reagent involved, the rate of electrophilic aromatic substitution is determined by the same slow step -attack of the electrophile on the ring to form a carbonium ion ... 

In electrophilic aromatic substitution the intermediate carbonium ion is a hybrid of structures I, 1, and III, in which the positive charge is distributed about the ring, being strongest as the positions ortho and para to the carbon atom being attacked. 

In summary, we can say that both reactivity and orientation in electrophilic aromatic substitution are determined by the rates of formation of the intermediate carbonium ions concerned. These rates parallel the stabilities of the carbonium ions, which are determined by the electron-releasing or electron-withdrawing tendencies of the substituent groups. 

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. 

We have already seen that the —NH2, —NHR, and —NR2 groups act as powerful activators and ortho para directors in electrophilic aromatic substitution. These effects were accounted for by assuming that the intermediate carbonium ion is stabilized by structures like I and 11 in which nitrogen bears a positive charge... 

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. 

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. 

A tertiary carbonium ion is more stable than a secondary carbonium ion, which is in turn more stable than a primary carbonium ion. Therefore, the alkylation of ben2ene with isobutylene is much easier than is alkylation with ethylene. The reactivity of substituted aromatics for electrophilic substitution is affected by the inductive and resonance effects of a substituent. An electron-donating group, such as the hydroxyl and methyl groups, activates the alkylation and an electron-withdrawing group, such as chloride, deactivates it. 

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. 

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. 

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. 

Like the amino group, the phenolic group powerfully activates aromatic rings toward electrophilic substitution, and in essentially the same way. The intermediates are hardly carbonium ions at all, but rather oxonium Ions (like I and II), in which every atom (except hydrogen) has a complete octet of electrons ... 

Like benzene, naphthalene typically undergoes electrophilic substitution this is one of the properties that entitle it to the designation of aromatic. An electrophilic reagent finds the ir cloud a source of available electrons, and attaches itself to the ring to form an intermediate carbonium ion to restore the stable aromatic system, the carbonium ion then gives up a proton. 

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