Substituents deactivating

Table 12 2 summarizes orientation and rate effects m electrophilic aromatic sub stitution reactions for a variety of frequently encountered substituents It is arranged m order of decreasing activating power the most strongly activating substituents are at the top the most strongly deactivating substituents are at the bottom The mam features of the table can be summarized as follows... 

Substituent Effects in Electrophilic Aromatic Substitution Strongly Deactivating Substituents... 

This makes the nitro group a powerful electron withdrawing deactivating substituent and a meta director... 

Because we have come to associate activating substituents with ortho para directing effects and deactivating substituents with meta the properties of the halogen substituents appear on initial inspection to be unusual... 

Aluminum chloride is a stronger Lewis acid than iron(lll) bromide and has been used as a catalyst in electrophilic bromination when as in the example shown the aromatic ring bears a strongly deactivating substituent... 

Neither Friedel-Crafts acylation nor alkylation reactions can be earned out on mtroben zene The presence of a strongly deactivating substituent such as a nitro group on an aromatic ring so depresses its reactivity that Friedel-Crafts reactions do not take place Nitrobenzene is so unreactive that it is sometimes used as a solvent m Friedel-Crafts reactions The practical limit for Friedel-Crafts alkylation and acylation reactions is effectively a monohalobenzene An aromatic ring more deactivated than a mono halobenzene cannot be alkylated or acylated under Friedel-Crafts conditions... 

Deactivating substituent (Sections 12 11 and 12 13) A group that when present in place of hydrogen causes a particular reaction to occur more slowly The term is most often ap plied to the effect of substituents on the rate of electrophilic aromatic substitution... 

Mercury(II) acetate tends to mercurate all the free nuclear positions in pyrrole, furan and thiophene to give derivatives of type (74). The acetoxymercuration of thiophene has been estimated to proceed ca. 10 times faster than that of benzene. Mercuration of rings with deactivating substituents such as ethoxycarbonyl and nitro is still possible with this reagent, as shown by the formation of compounds (75) and (76). Mercury(II) chloride is a milder mercurating agent, as illustrated by the chloromercuration of thiophene to give either the 2- or 2,5-disubstituted product (Scheme 25). 

The caibonyl group in aldehydes, ketones, acids, esters, and amides is deactivating and wcto-directing. There are distinct limitations on the types of substitution reactions that are satisfiictory for these deactivating substituents. In general, only those electrophiles in category A in Scheme 10.1 react readily. 

Product complex 7 as well as the free product 3 are much less reactive towards further electrophilic substitution as is the starting material thus the formation of polyacylated products is not observed. If the starting material bears one or more non-deactivating substituents, the direction of acylation can be predicted by the general rules for aromatic substitution. 

Scheme 78) [89]. Aryl chlorides with activating as well as deactivating substituents could also be coupled under the same conditions in high yields, ranging from 60% to 95%, within 30-60 min of microwave irradiation. The process does not require an inert atmosphere. The increased conversion observed with the addition of the ionic liquid reveals that it might have an additional function besides simply acting as a molecular irradiator . It cannot be excluded for instance that carbene palladium complexes are formed in situ and implicated in the catalytic cycle. 

A wide variety of aromatic compounds can be brominated. Highly reactive ones, such as anilines and phenols, may undergo bromination at all activated positions. More selective reagents such as pyridinium bromide perbromide or tetraalkylammonium tribromides can be used in such cases.18 Moderately reactive compounds such as anilides, haloaromatics, and hydrocarbons can be readily brominated and the usual directing effects control the regiochemistry. Use of Lewis acid catalysts permits bromination of rings with deactivating substituents, such as nitro and cyano. 

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