Activating and deactivating groups

E. Activation and Deactivation by Other Groups i. General Effects and Summary... 

For the aromatic carboxylic acids, substituents on the aromatic ring may also influence the acidity of the acid. Benzoic acid, for example, has = 4.3 x 10 . The placements of various activating groups on the ring decrease the value of the equilibrium constant, and deactivating groups increase the value of the equilibrium constant. Table 12-2 illustrates the influence of a number of para-substituents upon the acidity of benzoic acid. 

Nitration is a very general reaction, and satisfactory conditions can normally be developed for both activated and deactivated aromatic compounds. Because each successive nitro group reduces the reactivity of the ring, it is easy to control conditions to obtain a mononitration product. If polynitration is desired, more vigorous conditions are used. Scheme 11.1 gives some examples of nitration reactions. 

In 2000, Dubac s group reported the microwave-assisted Friedel-Crafts acylation of slightly activated and deactivated arenes under solvent-free conditions with FeCl3 as catalyst. Here, for the acylation of toluene a 90% product yield is obtained after 5 min of irradiation and an overall reaction time of 30 min in the presence of only 5 mol% of FeCl3. A sequential MW irradiation at 300 W afforded the acylation of fluorobenzene with 2 -chlorobenzoyl chloride, with a surprisingly high yield of 92% of 2-chloro-4 -fluorobenzophenone (Scheme 6.11). 

In summary, the data for a wide range of activating and deactivating substituents in aromatic substitution reactions involving the replacement of hydrogen or other metalloid groups are correlated with truly remarkable accuracy by the o-+-parameters. 

Few reliable procedures are available for estimating O even among a closely related group of compounds, because its value depends on competition among several activation and deactivation steps for excited state species, only a few of which are known with any certainty for a few well-studied compounds (Turro, 1967). Mill and Mabey (1985) and Mill (1995) list O values for several types of photoreactions in aerated water. Calvert and Pitts (1967) present extensive compilations of O values for many photoreactions, but only a few of these measurements were conducted in the presence of air with light >300 nm. Absence... 

Sterically hindered alkylmonophosphines provided improved catalyst systems (Table 5) [163]. In this case, reactions occurred within 8 h at 100 °C for both activated and deactivated aryl bromides and with electron-poor or electron-neutral aryl chlorides. Reactions of ortho-substituted aryl halides were unusual, providing a mixture of 1- and 3-substituted indoles, but these aryl halides were suitable substrates when the 3-position of the indole was also substituted. The origin of this C- vs. N-arylation is unknown. The Tosoh group has also used this catalyst system for the arylation of the parent pyrrole, indole, and carbazole. They observed that Rb2C03 was a particularly effective base [187]. 

Alkyl groups and halogen atoms behave normally as weakly activating and deactivating substituents, respectively. 

Use Figure 16.11 to find the activating and deactivating effects of groups. 

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