Mechanism, arenium ion

commonly, the attacking species in aromatic substitution reactions is an electrophile, which is usually either a positive ion or the positive end of a dipole. Similarly, the leaving group must lack an electron pair. 

In nucleophilic substitution, the best leaving groups were those that could most readily accommodate an unshared pair of electrons. By analogy, suggest what will be the best leaving groups in an electrophilic substitution. 

The best leaving groups in an electrophilic substitution reaction will be weak Lewis acids. 

In aromatic chemistry, there is one electrophilic pathway that predominates. This is called the arenium ion mechanism. It occurs in three steps. The first is the formation of the attacking electrophile the second is the attack by this electrophile on the aromatic ring and the third is the departure of an electrofuge, which is usually a proton, with the assistance of a base. Write down the general equations for this sequence of reactions. 

The production of the attacking electrophile may occur in many different ways, and aromatic chemistry is often studied by reference to the formation of this species. This initial step is not usually rate limiting. 

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Isotope Effects. If the hydrogen ion departs before the arrival of the electrophile (SeI mechanism) or if the arrival and departure are simultaneous, there should be a substantial isotope effect (i.e., deuterated substrates should undergo substitution more slowly than nondeuterated compounds) because, in each case, the C—H bond is broken in the rate-determining step. However, in the arenium ion mechanism, the C—H bond is not broken in the rate-... 

However, in many instances, isotope effects have been found. Since the values are generally much lower than expected for either the Sgl or the simultaneous mechanisms (e.g., 1-3 for instead of 6-7), we must look elsewhere for the explanation. For the case where hydrogen is the leaving group, the arenium ion mechanism can be summarized ... 

Evidence for the arenium ion mechanism has also been obtained from other kinds of isotope-effect experiments, involving substitutions of the type... 

Isolation of Arenium Ion Intermediates. Very strong evidence for the arenium ion mechanism comes from the isolation of arenium ions in a number of instances. For example, 7 was isolated as a solid with melting point — 15°C... 

SO the Sgl mechanism and not the usual arenium ion mechanism is operating. Aromatic rings can also be deuterated by treatment with D2O and a rhodium(III) chloride or platinum catalyst or with CeDs and an alkylaluminum dichloride catalyst," though rearrangements may take place during the latter procedure. Tritium ( H, abbreviated T) can be introduced by treatment with T2O and an alkylaluminum dichloride catalyst. " Tritiation at specific sites (e.g., >90% para in... 

The decarbonylation of aromatic aldehydes with sulfuric acid" is the reverse of the Gatterman-Koch reaction (11-16). It has been carried out with trialkyl- and trialkoxybenzaldehydes. The reaction takes place by the ordinary arenium ion mechanism the attacking species is H and the leaving group is HCO, which can lose a proton to give CO or combine with OH from the water solvent to give formic acid." Aromatic aldehydes have also been decarbonylated with basic catalysts." When basic catalysts are used, the mechanism is probably similar to the SeI process of 11-38. See also 14-39. 

Where the bond between the metal and the ring is covalent, the usual arenium ion mechanism operates. Where the bonding is essentially ionic, this is a simple acid-base reaction. For the aliphatic counterpart of this reaction, see Reaction 12-23. 

Mercuration of aromatic compounds can be accomplished with mercuric salts, most often Hg(OAc)2 ° to give ArHgOAc. This is ordinary electrophilic aromatic substitution and takes place by the arenium ion mechanism (p. 675). ° Aromatic compounds can also be converted to arylthallium bis(trifluoroacetates), ArTl(OOCCF3)2, by treatment with thallium(III) trifluoroacetate in trifluoroace-tic acid. ° These arylthallium compounds can be converted to phenols, aryl iodides or fluorides (12-28), aryl cyanides (12-31), aryl nitro compounds, or aryl esters (12-30). The mechanism of thallation appears to be complex, with electrophilic and electron-transfer mechanisms both taking place. 

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... 

A semisystematic study into the hydrolysis of ethyl indole-2-carboxylate in aqueous media at high temperature, indicated that decarboxylation of the resultant acid proceeded by an arenium ion mechanism and was inhibited by base. As base pro-... 

Fluorination of aromatic rings (other than those via carbanions described above) consists principally of fluorodehydrogenation and fluorodemetallation reactions, both ofwhich are thought to proceed via an arenium ion mechanism (Scheme 25) [85]. 

Scheme 25. The arenium ion mechanism of electrophilic fluorination of aromatic rings via fluorodehydrogenation or fluorodemetallation reactions.

In the arenium ion mechanism the attacking species may be produced in various ways, but what happens to the aromatic ring is basically the same in all cases. For this reason most attention in the study of this mechanism centers around the identity of the attacking entity and how it is produced. 

In this method decarboxylation takes place by the arenium ion mechanism,440 with H+ as the electrophile and C02 as the leaving group.441 Evidently, the order of electrofugal ability... 

The simplest and most general mechanism for electrophilic aromatic substitution in solution is the so-called arenium ion mechanism, depicted in Scheme 2 [54,254]. 

As an example of a solvent-dependent electrophilic substitution reaction, the azo coupling (SsAr) reaction of 4-nitrobenzenediazonium tetrafluoroborate with N,N-dimethylaniline is given in Eq. (5-27) [504]. According to the two-step arenium ion mechanism, the activation process of the rate-limiting first step is connected with the dispersion of the positive charge. This should lead to a decrease in rate with increasing solvent polarity. 

Electrophihc aromatic substitutions are unhke nucleophilic substitutions in that the large majority proceed by just one mechanism with respect to the substrate. In this mechanism, which we call the arenium ion mechanism, the electrophile (which can be viewed as a Lewis acid) is attacked by the 71-electrons of the aromatic ring (behaving as a Lewis base in most cases) in the first step. This reaction leads to formation of a new C—X bond and a new sp carbon in a positively charged intermediate called an arenium ion, where X is the electrophile. The positively charged intermediate (the arenium ion) is resonance stabilized, but not aromatic. Loss of a proton from the sp carbon that is adjacent to the positive carbon in the arenium ion, in what is effectively an El process (see p. 1487), is driven by rearomatization of the ring from the arenium ion to give the aromatic substitution product. A proton... 

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