Arenium ions activating groups

The orientation and reactivity effects of each group are explained on the basis of resonance and field effects on the stability of the intermediate arenium ion. To understand why we can use this approach, it is necessary to know that in these reactions the product is usually kinetically and not thermodynamically controlled (see p. 214). Some of the reactions are irreversible and the others are usually stopped well before equilibrium is reached. Therefore, which of the three possible intermediates is formed is dependent not on the thermodynamic stability of the products but on the activation energy necessary to form each of the three... 

From these results it can be seen that the methoxy group is also an ortho/para director and is a much stronger activating group than is the methyl group. Examination of the resonance structure for the para arenium ion that has the positive charge located on the carbon bonded to the methoxy group explains these results. 

The 1,2-shifts of CH3 groups were studied in greatest detail to estimate carefully how the electron deficiency of the carbocationic centre (the carbon atom the migrant shifts to) affects the activation parameters of arenium ion rearrangements. Thus, for structurally similar ions including the heptamethylbenzenium ion, the 9,10,10-trimethylphenanthrenium ion and its disubstituted derivatives, as well as the 9,9,10-trimethylpyrenium ion the following linear relations are found... 

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)... 

Predicting activation barriers and rate coefficients of isomerization reactions For the degenerate rearrangements of arenium ions realized by the 1,2-shift of the CHj group, according to ... 

Alkyl groups are better electron-releasing groups than hydrogen. Because of this, they can activate a benzene ring toward electrophilic substitution by stabilizing the transition state leading to the arenium ion ... 

Because 70 and 72 are generated much more slowly than 67, the major product is 3-bromonitrobenzene (56). Such an intermediate is very unstable, is difficult to form with a high activation energy barrier, and will react more slowly than benzene and much more slowly than derivatives bearing an electron-releasing group. Further, the arenium ion arising from attack at the ortho and para carbons is more stabilized and will form much more slowly than the less destabilized meta arenium ion, so the major product is meta. 

Use resonance theory to explain why the hydroxyl group of phenol is an activating group and an ortho-para director. Illustfate your explanation by showing the arenium ions formed when phenol reacts with a Br ion at the ortho, meta, and para positions. 

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