Halogens ortho

To eliminate arylating activity, replace halogen ortho/para to the nitro group(s) to the meta position in nitroaromatic compounds. 

Ring-cleavage dioxygenases (Halogenated) 1,2-dihydroxy-aromatics Ar(OH)2 -h O2 — (halogenated) ortho-or meta-nng fission products... 

Carbon nucleophiles can also be used deprotonated nitriles will displace a halogen electron-rich aromatic compounds will displace a-halogen ortho or para to nitro or cyano, using aluminium chloride... 

The enhancing effect of the chlorine ortho to the electron-withdrawing acyl substituent is reminiscent of the enhancing activity of halogen ortho to the sulfamoyl group in the thiazide series. 

The resonance effect of halogens. A halogen ortho or para to the site of electrophilic attack stabilizes the cation intermediate by delocalization of the positive charge ... 

We noted earlier that with the exception of the halogens ortho, para directors activate the ring toward electrophilic substitution by supplying eleetton density to the ring. But why are the ortho and para positions especially susceptible to attack To answer this question, consider the stability of the cyclohexa-dienyl carbocation that forms in the first step of the electrophilic aromatic substitution mechanism. The r ioselectivity of the reaction is controlled by the stability of the carbocation. To determine the stability of a cydohexadienyl carbocation, we must compare all the possible resonance forms. Thus, we compare the stabihty of the intermediate carbocations resulting from attack at the ortho and para positions with those resulting when an electrophile attacks at the meta position. 

Halogen substituents are slightly deactivating but are ortho para directing... 

Activating Standard of comparison Deactivating —R —Ar —CH=CR —H —X (X = F Cl —CH2X (alkyl) (aryl) 2 (alkenyl) (hydrogen) (halogen) Br 1) (halomethyl) Ortho para directing Ortho para directing... 

Returning to Table 12 2 notice that halogen substituents direct an incoming electrophile to the ortho and para positions but deactivate the ring toward substitution Nitration of chlorobenzene is a typical example of electrophilic aromatic substitution m a halobenzene... 

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... 

Like hydroxyl groups and ammo groups however halogen substituents possess unshared electron pairs that can be donated to a positively charged carbon This electron donation into the TT system stabilizes the intermediates derived from ortho and from para attack... 

The mix of inductive and resonance effects varies from one halogen to another but the net result is that fluorine chlorine bromine and iodine are weakly deactivating ortho para directing substituents... 

Unprotected arylamines are so reactive toward halogenation that it is difficult to limit the reaction to monosubstitution Generally halogenation proceeds rapidly to replace all the available hydrogens that are ortho or para to the ammo group... 

One group of aryl halides that do undergo nucleophilic substitution readily consists of those that bear a nitro group ortho or para to the halogen... 

Halogenation Bromination and chlorination of phenols occur readily even in the absence of a cata lyst Substitution occurs primarily at the position para to the hydroxyl group When the para position IS blocked ortho substitution is observed... 

Nucleophilic Displacement Reactions. The strong electron-withdrawing effect of a trifluoromethyl group activates ortho and para halogen toward nucleophilic attack. Such chlorine labiUty is utili2ed in the manufacture of crop control chemicals containing trifluoromethyl and nitro groups. 

Halogenation. The presence of the amino group activates the ortho and para positions of the aromatic ring and, as a result, aniline reacts readily with bromine or chlorine. Under mild conditions, bromination yields 2,4,6-trihromoaniline [147-82-0]. 

Aromatic compounds may be chlorinated with chlorine in the presence of a catalyst such as iron, ferric chloride, or other Lewis acids. The halogenation reaction involves electrophilic displacement of the aromatic hydrogen by halogen. Introduction of a second chlorine atom into the monochloro aromatic stmcture leads to ortho and para substitution. The presence of a Lewis acid favors polarization of the chlorine molecule, thereby increasing its electrophilic character. Because the polarization does not lead to complete ionization, the reaction should be represented as shown in equation 26. 

Substituted Anthraquinones. Commercially important blue disperse dyes are derived from 1,4,5,8-substituted anthraquiaones. Among them, diaminodihydroxyanthraquiaone derivatives are most important in view of their shades and affinity. Representative examples are Cl Disperse Blue 56 [31810-89-6] (11) Cl 63285) (126), and Cl Disperse Blue 73 (113) (115). Introduction of a halogen atom ortho to the amino group improves affinity and lightfastness. 

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