Halobenzenes, halogenation

For halobenzenes, halogen-Li exchanges and benzyne formation become easier in the sequence I > Br > Cl > F indeed, for iodides and bromides, the most probable reactions are halogen-lithium exchanges (see 5.5.2.3.3.). Halogenoanisoles,... 

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... 

Approximate Relative Rates of Halogen Displacement FROM 4-Substituted Halobenzenes and 4-Substituted 2-Nitrohalobenzenes... 

Many variations of the reaction can be carried out, including halogenation, nitration, and sulfonation. Friedel-Crafts alkylation and acylation reactions, which involve reaction of an aromatic ling with carbocation electrophiles, are particularly useful. They are limited, however, by the fact that the aromatic ring must be at least as reactive as a halobenzene. In addition, polyalkylation and carbocation rearrangements often occur in Friedel-Crafts alkylation. 

When arenesulfonyl radicals are generated in benzene the only reported products are those of disproportionation50,95 (vide supra). However, Camaggi and coworkers95 have found that arenesulfonyl radicals in halobenzene replace the halogen atom at 150-190 °C, the relative reactivities being for Cl Br I, 1 5.9 18.6. These authors95 proposed that the reaction proceeds via a reversible ip.vo-substitution,... 

Halogen substituents withdraw electron density from the aromatic nucleus but direct olp-through resonance effects. The result is that halobenzenes undergo nitration with more difficulty relative to benzene. The nitration of chlorobenzene with strong mixed acid gives a mixture of 2,4- and 2,6-isomeric dinitrochlorobenzenes in which the former predominates." The nitration of 2,4-dinitrochlorobenzene to 2,4,6-trinitrochlorobenzene (picryl chloride) requires an excess of fuming nitric acid in oleum at elevated temperature. Both are useful for the synthesis of other polynitroarylene explosives but only 2,4-dinitrochlorobenzene finds industrial importance (Sections 4.8.1.2 and 4.8.1.3). 

The a-, f3- and y-halogeno substituents in pyridines and their benzo analogues are each more susceptible to nucleophilic substitution than is the case for halobenzenes because of the overall electron deficiency of the heteroaromatic ring. Furthermore, halogen substituents a and y to nitrogen are usually more reactive than /3-halogens and this is particularly so in pyridinium type systems. 

In the presence of anhydrous Lewis acid (e.g. FeCls or FeBrs), benzene reacts readily with halogens (bromine or chlorine) to produce halobenzenes (bromobenzene or chlorobenzene). Fluorine (Fy reacts so rapidly with benzene that it requires special conditions and apparatus to carry out fluorination. On the other hand, iodine (I2) is so unreactive that an oxidizing agent (e.g. HNO ) has to be used to carry out iodination. 

Arynes are formed when haloarenes are treated with a strong base. Deprotonation at the ortho position of the halogen atom is favored because of polarity alternation, and the facility ofbenzyhe formation from halobenzene (PhF > PhCl > PhBr p Phi) also corresponds with the relative strength of the donor. 

In l,2,3,4-tetrafluoro-5-halobenzenes 1 and l,2.3,5-tetrafluoro-4-halobenzenes 4 the fluorine atoms in the 4-position relative to hydrogen are replaced preferentially to those in the 4-position relative to the halogen atom.83... 

Nirmalakhandan and Speece [36] introduced the polarizability factor,, as an additional molecular descriptor. They derived the following model for halogenated alkanes and alkenes, alkylbenzenes, halobenzenes, and alkanols ... 

---