Substitution, aromatic, electrophilic halogenation

Halogenation Electrophilic Aromatic Substitution to Yield 4-Bromoacetanilide... 

SUBSTITUENT EFFECTS IN ELECTROPHILIC AROMATIC SUBSTITUTION HALOGENS... 

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... 

Substituent Effects in Electrophilic Aromatic Substitution Halogens... 

The two mam methods for the preparation of aryl halides halogenation of arenes by electrophilic aromatic substitution and preparation by way of aryl diazomum salts were described earlier and are reviewed m Table 23 2 A number of aryl halides occur natu rally some of which are shown m Figure 23 1... 

Halogenation (Sections 4 14 and 12 5) Replacement of a hy drogen by a halogen The most frequently encountered ex amples are the free radical halogenation of alkanes and the halogenation of arenes by electrophilic aromatic substitution... 

Many of the common electrophilic aromatic substitution reactions can be conducted on indole. CompHcations normally arise either because of excessive reactivity or the relative instabiUty of the substitution product. This is the case with halogenation. 

The reaction exhibits other characteristics typical of an electrophilic aromatic substitution. Examples of electrophiles that can effect substitution for silicon include protons and the halogens, as well as acyl, nitro, and sulfonyl groups. The feet that these reactions occur very rapidly has made them attractive for situations where substitution must be done under very mild conditions. ... 

Electrophilic aromatic substitution (Section 12.14) Halogen substituents are slightly deactivating and ortho, para-directing. Br... 

The hydroxyl group is a strongly activating, ortho- and para-directing substituent in electrophilic aromatic substitution reactions (Section 16.4). As a result, phenols are highly reactive substrates for electrophilic halogenation, nitration, sulfonation, and lTiedel-Crafts reactions. 

Unlike benzene, pyridine undergoes electrophilic aromatic substitution reactions with great difficulty. Halogenation can be carried out under drastic conditions, but nitration occurs in very low yield, and Friedel-Crafts reactions are not successful. Reactions usually give the 3-substituted product. 

Hydro-de-diazoniation seems to be an unnecessary reaction from the synthetic standpoint, as arenediazonium salts are obtained from the respective amines, reagents that are normally synthesized from the hydrocarbon. Some aromatic compounds, however, cannot be synthesized by straightforward electrophilic aromatic substitution examples of these are the 1,3,5-trichloro- and -tribromobenzenes (see below). These simple benzene derivatives are synthesized from aniline via halogenation, diazotization and hydro-de-diazoniation. Furthermore hydro-de-diazoniation is useful for the introduction of a hydrogen isotope in specific positions. 

We may ask How does Y know which side will give the more stable carbocation As in the similar case of electrophilic aromatic substitution (p. 681), we invoke the Hanunond postulate and say that the lower energy carbocation is preceded by the lower energy transition state. Markovnikov s rule also applies for halogen substituents because the halogen stabilizes the carbocation by resonance ... 

Successful thermal decarboxylation of metal arenoates other than poly-halogenoarenoates are restricted to mercury compounds and fall into three categories, namely (i) those where electron-withdrawing substituents other than halogens are present in the organic groups, (ii) those where substituents and/or conditions are used which favor a different mechanism, e.g., classic electrophilic aromatic substitution, or (iii) those where the conditions are sufficiently forcing for both mercuration and decarboxylation to occur. 

Equipped with these reference trends for steric and electronic effects, one is prepared to survey more general classes of electrophilic aromatic substitution on benzocycloalkenes. Such reactions include nitration, halogenation, sulfonation, and alkylation. Each has its own mechanistic peculiarities, but their product distributions can be rationalized by consideration of the appropriate reference. 

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