In arenium ions

On the described basis -li3c i were used to indicate the valent state of the carbon atom in arenium ions and their analogues. Thus, a comparison of Ji3 i for anthracene, 9-H-anthracenium ion and diphenylmethane has supported the assumption that on protonation of anthnu e the carbon atom capturing the proton passes from the sp state into the sp type. 

The highly characteristic chemical shifts of the ring carbon atom in arenium ions and the spin-spin interaction of carbons and other elements with magnetic nuclei are widely used in establishing the structure of new ions. An example is the corroboration of the bipolar structure in the complexes formed from phenols and aluminium halides and proving the structure of the ions generated by the interaction of polymethylated aromatic compounds with heteroatomic electrophilic reagents (see... 

The acidic hydrogens of the ring CH2 in arenium ions interact with the nucleophile particles of the medium. Hence, changing the anion in the salts AH " -MnYs +i causes quite distinct, though small, changes in the CHj-group absorption. Thus, in the salt of the mesitylenium ion the replacement of the Al2Brf... 

The electronic effects of substituents in arenium ions deserves quantitative estimation on a wider scale because a comparison of the data with the accumulated data on the effect of substituents in electrophilic substitutions will help to judge how close the transition states of these reactions are to the structure of arenium ions. 

Table 49. Kinetic Parameters of 1,2-Methyl Shifts in Arenium Ions... 

Among the characteristics of charge on C, the most adequate one seems to be the chemical shift of C, in the NMR- C spectrum (see Sect. 1II.2). However, other characteristics should not be excluded. For example, a linear relation exists between the activation parameters of the 1,2-migrant shift (R = CH3, CgHj) in arenium ions and the C=0 vibrations frequencies in the IR spectra of ketones formally obtained by substituting the fragment >C, —R for the group >C=0 3oi.569)... 

Schematic representation of the delocalization of positive charge in arenium ions 9.

To involve allylic resonance in stabilizing the arenium ion formed during attack at C 2 the benzenoid character of the other ring is sacrificed... 

Arenium ion (Section 12 2) The carbocation intermediate formed by attack of an electrophile on an aromatic substrate in electrophilic aromatic substitution See cyclohexadienyl cation... 

The carbocation formed in this step is a cyclohexadienyl cation. Other commonly used terms include arenium ion and a-complex. It is an allylic car bocation and is stabilized by electron delocalization which can be represented by resonance. 

Koptyug, V. A. Contemporary Problems in Carbonium Ion Chemistry III Arenium Ions — Structure and Reactivity. 122, 1-245 (1984). 

The classical syntheses of phenanthrene and fluorenone fit well into the electron transfer scheme discussed in Section 8.6 and in this chapter. The aryl radical is formed by electron transfer from a Cu1 ion, iodide ion, pyridine, hypophosphorous acid, or by electrochemical transfer. The aryl radical attacks the neighboring phenyl ring, and the oxidized electron transfer reagent (e. g., Cu11) reduces the hexadienyl radical to the arenium ion, which is finally deprotonated by the solvent (Scheme 10-76). 

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

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 ... 

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... 

In Chapter 3, it was mentioned that positive ions can form addition complexes with 7T systems. Since the initial step of electrophilic substitution involves attack by a positive ion on an aromatic ring, it has been suggested that such a complex, called a % complex (represented as 10), is formed first and then is converted to the arenium ion 11. Stable solutions of arenium ions or 7t complexes (e.g., with Br2, l2> picric... 

How can we tell if 10 is present on the reaction path If it is present, there are two possibilities (1) The formation of 10 is rate determining (the conversion of 10 to 11 is much faster), or (2) the formation of 10 is rapid, and the conversion 10 to 11 is rate determining. One way to ascertain which species is formed in the rate determining step in a given reaction is to use the stability information given in Table 11.1. We measure the relative rates of reaction of a given electrophile with the series of compounds Usted in Table 11.1. If the relative rates resemble the arenium ion stabilities, we conclude that the arenium ion is formed in the slow step but if they resemble the stabilities of the Jt complexes, the latter are formed in the slow step. When such experiments are carried out, it is found in most cases that the relative rates are similar to the arenium ion and not to the n complex stabilities. For example,... 

For the reason given above and for other reasons, it is unlikely that the encounter complex is a n complex, but just what kind of attraction exists between Y+ and ArH is not known, other than the presumption that they are together within a solvent cage (see also p. 694). There is evidence (from isomerizations occurring in the alkyl group, as well as other observations) that n complexes are present on the pathway from substrate to arenium ion in the gas-phase protonation of alkylbenzenes. ... 

We have discussed orientation in the case of monosubstituted benzenes entirely in terms of attack at the ortho, meta, and para positions, but attack at the position bearing the substituent (called the ipsoposition ) can also be important. Ipso attack has mostly been studied for nitration. When NOj attacks at the ipso position there are at least five possible fates for the resulting arenium ion (13). 

Path a. The arenium ion can lose NOj and revert to the starting compounds. This results in no net reaction and is often undetectable. 

Path b. The arenium ion can lose Z" ", in which case this is simply aromatic substitution with a leaving group other than H (see 11-36-11-44). 

In fused ring systems, the positions are not equivalent and there is usually a preferred orientation even in the unsubstituted hydrocarbon. The preferred positions may often by predicted as for benzene rings. Thus it is possible to draw more canonical forms for the arenium ion when naphthalene is attacked at the a position than when it is attacked at the p position, and the a position is the preferred site of attack,though, as previously mentioned (p. 682), the isomer formed by substitution at the p position is thermodynamically more stable and is the product if the reaction is reversible and equilibrium is reached. Because of the more extensive delocalization of charges in the corresponding arenium ions, naphthalene is more reactive than benzene and substitution is faster at both positions. Similarly, anthracene, phenanthrene, and other fused polycyclic aromatic hydrocarbons are also substituted faster than benzene. 

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