Aryl halides from aromatics + halogens

There are three major classes of organohalogen compounds the alkyl halides, the vinyl halides, and the aryl halides. An alkyl halide simply has a halogen atom bonded to one of the sp hybrid carbon atoms of an alkyl group. A vinyl halide has a halogen atom bonded to one of the sp hybrid carbon atoms of an alkene. An aryl halide has a halogen atom bonded to one of the sp hybrid carbon atoms of an aromatic ring. The chemistry of vinyl halides and aryl halides is different from that of alkyl halides because their bonding and hybridization are different. We consider the reactions of vinyl halides and aryl halides in later chapters. The structures of some rqjresentative alkyl halides, vinyl halides, and aryl halides are shown here, with their most common names and uses. 

An interesting case is provided by the nitration of aryl halides where the effect of the halogen is to deactivate the aromatic nucleus (by the -I effect) but to direct the incoming nitronium ion to the ortho and para positions as a result of the mesomeric interaction of the halogen lone electron pair with the charge developed in the corresponding intermediates [e.g. (5), in the formation of p-bromonitrobenzene from bromobenzene, Expt 6.20],... 

The synthesis of unsymmetrical biaryls 8 from two monoaryl species involves the coupling of a metallated aromatic molecule 6 with an aryl halide or triflate 4 under the action of palladium(O) catalysis. The reaction involves a catalytic cycle in which palladium(O) inserts into the C-halogen bond via an oxidative addition to generate an arylpalladium(II) species 5 (Scheme 10.18). This undergoes a trans-metallation with the metallated component, producing a biarylpalladi-um(II) complex 7. The biaryl product is formed by reductive elimination. In the process, Pd(0) is regenerated and this can then react with a second molecule of aryl halide. Pd(0) is therefore a catalyst for the reaction. 

From Aryl Halides. Kekule Synthesis.—Aromatic halides in which the halogen is in the ring yield ring carboxy acids by the action of carbon dioxide and sodium. 

Compounds in which the halogen atom is attached directly to an aromatic ring (aryl halides, e.g., bromobenzene) differ so much from the alkyl halides in their preparations and properties that they will be taken up in a separate chapter (Chap. 25). For the present we need to know that—in the kinds of reaction typical of alkyl halides—most aryl halides are extremely mreactive. 

Aryl halides are distinguished from aromatic hydrocarbons by the presence of halogen, as shown by elemental analysis. Aryl halides are distinguished from most alkyl halides by their inertness toward silver nitrate in this respect they resemble vinyl halides (Sec. 25.5). 

Halogenated aromatic compounds can be condensed with alkyl or aryl halides by using copper to remove the halogen Zincke350 prepared ethyl phenylacetate in this way from bromobenzene and ethyl chloroacetate and Ullmann and his co-workers351 prepared biaryls from 2 moles of aryl halide and copper (for a review see Fanta352). 

When relatively unreactive (unactivated) aryl halides are treated with powerful bases such as potassium amide (KNH2) or rerubutyllithium (sec. 8.5.B), a hydrogen atom is removed from the aromatic ring in an acid-base reaction. The presence of the halogen makes the ortho hydrogens more acidic. Removal of the... 

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