Benzenium ion intermediate

Electron-donor substituents are known to accelerate the rate of electrophilic substitution on benzene, while electron-withdrawing groups are known to retard the reaction. One explanation is that electron donors stabilize the positive charge in the benzenium ion intermediate while electron-withdrawing substituents destabilize the positive charge. 

In aromatic electrophilic substitution, " the initial interaction between an electrophile and the aromatic n system is a multicenter interaction (of n-complex nature). The lack of substrate selectivity observed in some reactions of aromatic compounds with strong electrophiles (e.g., N()2 ) indicates that the initial multicenter complex is a separate well-defined intermediate,- " " Its nature was much discussed. Schofield et al. suggested it to be a solvent cage, whereas Perrin preferred a radical ion pair. There was general agreement of an initial intermediate involving the aromatic as an entity. The subsequent step affords a trivalent benzenium ion intermediate or a complex (Scheme 6.42). 

The benzenium ion intermediate 29 is formed by participation of the j8-phenyl group of 37. For stereoelectronic reasons the participation should occur from the trans position with respect to the nucleofuge. This can be confirmed by comparing the product distributions of the E and Z isomers of the substrate. 

In this exercise, we will try to explain the observed outcome of fhe nitration of mefhyl benzoafe. The major product of this reaction is methyl ra-nitrobenzoate, where the nitro group has been added to the meta position of fhe ring. The rate-defermining sfep of fhis reaction is the attack of the nitronium ion on the benzene ring. Three benzenium ion intermediates ortho, meta, and para) are possible ... 

Make models of each of fhe three benzenium ion intermediates (separately), and calculate their heats of formation using an AMl-level calculation with geometry optimization. Don t forget to specify a positive charge when you submit the calculation. What do you conclude ... 

Under normal conditions of electrophilic substitution, these benzenium ions are short-lived intermediates. The fact that the structures are stable in nonnucleophilic media clearly demonstrates the feasibility of such intermediates. 

Electrophilic nitration of a substituted benzene may lead to ortho, meta or para products, depending on the substituent. According to the Hammond Postulate, the kinetic product will be that which follows from the most stable intermediate benzenium ion, i.e. 

Display the electrostatic potential map for 2-methyl anisole. For which ring site, para to methyl or para to methoxy, is the electrostatic potential more negative Where do you expect electrophilic attack to occur Is your result consistent with the relative stabihties of intermediate benzenium ions formed upon addition of the electrophile Compare energies for 3-methyl-4-methoxybenzenium ion and 4-methyl-3-methoxybenzenium ion. 

Benzene and substituted benzenes reaet with electrophiles, leading to new functionality. The two-step mechanism involves initial attack by an electrophile to form an intermediate (benzenium ion), followed by elimination of a proton to generate the substituted benzene. 

Silylium ions, which are not protected sterically or are not stabilized either electronically or by intramolecular interaction with a remote substituent do interact strongly with the solvent and/or the counteranion. The reaction of the transient silylium ion with solvents like ethers, nitriles and even aromatic hydrocarbons lead to oxonium, nitrilium and arenium ions with a tetrahedral environment for the silicon atom. These new cationic species can be clearly identified by their characteristic Si NMR chemical shifts. That is, the oxonium salt [Me3SiOEt2] TFPB is characterized by S Si = 66.9 in CD2CI2 solution at —70°C. " Similar chemical shifts are found for related silylated oxonium ions. Nitrilium ions formed by the reaction of intermediate trialkyl silylium ions with nitriles are identified by Si NMR chemical shifts S Si = 30—40 (see also Table VI for some examples). Trialkyl-substituted silylium ions generated in benzene solution yield silylated benzenium ions, which can be easily detected by a silicon NMR resonance at 8 Si = 90—100 (see Table VI). ... 

Fig. 21. A reaction mechanism proposed by Xiong et al. to rationalize the toluene disproportion reaction on zeolites. The benzyl cations and benzenium ions were proposed as reactive intermediates for this zeolite-catalyzed, high-temperature reaction. (Reprinted with permission from Xiong et al. (122). Copyright 1995 American Chemical Society.)...

Cycloalkadienyl cations, particularly cyclohexadienyl cations (benzenium ions), the intermediate of electrophilic aromatic substitution, frequently show remarkable stability. Protonated arenes can be readily obtained from aromatic hydrocarbons244 251 in superacids and studied by 1H and 13C NMR spectroscopy.252,253 Olah et al.252 have even prepared and studied the parent benzenium ion (C6H7+) 88. Representative 1H NMR spectra of benzenium253 and naphthalenium ions25488 and 89 are shown in Figures 3.11 and 3.12, respectively. 

The work of Swain, de la Mare, and Roberts indicated that caution was necessary because of the differences in the intermediates. However, the resonance-stabilized carbonium ion (12) formed in the rate-determining step of the solvolysis offered an attractive model for the benzenium ions (13) formed in the slow step of the substitution reaction. The preliminary correlation of rate data for solvolysis of substituted phenyl-dimethylcarbinyl chlorides and aromatic substitution reactions revealed... 

There are, however, examples indicating that in ion molecule reactions between a protonated species (AH+) and benzene (B), two isomeric forms of the intermediate complex may exist (AH+)(B) and (A)(BH+) [74,286]. In the cases of water [287] and propene [74], quantum chemical calculations clearly indicate that the former corresponds to a n complex where A-H acts as a hydrogen bond donor towards the centre of the benzene ring, while the latter is a hydrogen bonded complex between the benzenium ion and A. In neither case has a barrier been located, but is probably rather low in both cases. The role of the n complex has still not been clarified, since direct downhill routes from the reactants to the a complex exist. It has been pointed out that n complex formation between a pro electrophile and the substrate may be important in solution and in biological systems for molecular recognition purposes. In such cases the proelectrophile is activated to form the actual electrophile subsequent to n complexation, thereupon giving rise to the a complex. This has been shown by quantum chemistry to provide a reasonable scenario for the reaction between HF and benzene, in which BF3 is ultimately required to promote ion formation of the HF/benzene tt complex [288]. 

The formation of norcaradiene derivatives with naphthalene [reaction (22)] lends some support to this scheme. This mechanism resembles a bimolecular two-step process suggested for the reaction of chloromethyl-aluminum compounds with olefins (199-201). On the other hand, a bimolecular one-step methylene transfer mechanism is generally accepted for the formation of cyclopropane derivatives by the reaction of halo-methylzinc compounds with olefins. This difference between the mechanism proposed for the cyclopropane formation from olefin and that for the ring expansion of aromatic compound may be ascribable to the difference in the stability of intermediates the benzenium ion (XXII) may be more stable than an alkylcarbonium ion (369). 

Rates for the key step of Friedel-Crafts alkylations, i.e., the attack of carbocations on arenes have only recently been reported [214,215]. Problems arising from the reversibility of the CC-bond-forming step have been overcome by performing experiments in presence of R4N + MC1,7+i salts, where MCl,r+1 acts as a base for the rapid deprotonation of the intermediate benzenium ions (Scheme 52). 

The hexamethylbenzene skeleton was also used by Olah et al. (1972b) in the study of 1-nitro- and 1-chloro-benzenium ions under conditions of long life. Less substituted ions decompose by proton elimination and make difficult the study of the intermediates in electrophilic aromatic substitution. Treatment of hexamethylbenzene with NO+BF in FSO H-SOa gave a solution of l-nitro-l,2,3,4,5,6-hexamethylbenzenium ions [3(X)] undergoing degenerate rearrangement via 1,2-nitro shifts with an activation energy of... 

An interesting rationalization of the mechanistic pathways involved in the duorination of aromatic compounds by high-valency metal fluorides has been provided. The main features of the mechanism [outlined for the conversion of fluorobenzene into /)-difluorobenzene by cobalt(in) fluoride in Scheme 2] are oxidation of the aromatic system to a radical-cation (1) and its subsequent reaction with a fluorine atom from the metal fluoride to give the type of Wheland intermediate [or benzenium ion (c/. p. 362)] (2) which would be formed in a conventional electrophilic process. Such a scheme implies that the relative stabilities of Wheland intermediates might be used to rationalize orientation in this type of reaction in much the same way as for more traditional processes, and some examples of such applications have been provided. Alternative routes from the radical-cation (1) are discussed and possible... 

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