Halogens reactions

The chemistry of pyrrole is similar to that of activated benzene rings. In general, however, the heterocycles are more reactive toward electrophiles than benzene rings are, and low temperatures are often necessary to control the reactions. Halogenation, nitration, sulfonation, and Friedel-Crafts acylation can all be accomplished. For example ... 

In this section we use benzene as a typical aromatic compound to study three basic reactions halogenation, sulfonation, and nitration. In the case of halogenations, the electrophile is the X ion (X = Cl or Br). In sulfonation and nitration, the electrophiles are SO3 and NO2, respectively. In each case, part of the mechanism involves the generation of the electrophile. 

The bromide, [Cr3 en4(0H)2]Cl4.2H20, is stated by Dubsky to contain 2 molecules of water in the molecule, and by Pfeiffer 2-5 molecules. This salt is prepared by treating a dilute aqueous solution of cis-dibis-aquo-diethylenediamino-chromic bromide with pyridine.3 It separates in lustrous bluish-violet crystals, which are soluble in water with neutral reaction. Halogen acids transform it into the m-diacido-salt for example, with hydrochloric acid it forms dichloro-diethylenediamino-chromic chloride, [Cr en2Cl2]Cl.H20. Other salts of the series may be obtained from the bromide. 

The usual way to achieve heterosubstitution of saturated hydrocarbons is by free-radical reactions. Halogenation, sulfochlorination, and nitration are among the most important transformations. Superacid-catalyzed electrophilic substitutions have also been developed. This clearly indicates that alkanes, once considered to be highly unreactive compounds (paraffins), can be readily functionalized not only in free-radical from but also via electrophilic activation. Electrophilic substitution, in turn, is the major transformation of aromatic hydrocarbons. 

A variety of N-, 0- and 5-heterocyclic halides have been found to undergo the palladium-catalyzed al-kene substitution reaction. Halogen derivatives of furan, thiophene, pyridine, pyrazine, uracil, indole, quinoline and isoquinoline, for example, undergo the reaction. Iodoferrocene also reacts normally. 

Know the meaning of substitution reaction, halogenation, chlorination, bromination, free-radical chain reaction, chain initiation, propagation, termination, combustion. 

Given the reactants, write the structures of the main organic products of the common electrophilic aromatic substitution reactions (halogenation, nitration, sulfonation, alkylation, and acylation). 

Hydrogen replacement with benzene and the halobenzenes has been studied. The isomer distribution has been found to be very close to the statistical one and interpreted as the result of high-energy reactions. Halogen replacement yields increase in the series F-, C1-, Br- and iodobenzene showing dependence on the C—X bond energy both in liquid and in gas phase. 

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