Polar Reactions and How They Occur

Polar reactions occur because of the attraction between positive and negative charges on different functional groups in molecules. To see how these 

The polarity patterns of some common functional groups are shown in Table 5.1. Notice that carbon is always positively polarized except in Grig-nard reagents and alkyllithiums. 

TABLE S.1 Polarity Patterns in Some Common Functional Croups 

Compound Functional group Compound Functional group 

Because of iodine s high polarizability, the carbon-iodine bond behaves as if it were polar. 

Alkyl halide Symmetrical, nonpolar w 6-—C—X / Carboxylic acid o —c 

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Some systematic studies on the different reaction schemes and how they are realized in organic reactions were performed some time ago [18]. Reactions used in organic synthesis were analyzed thoroughly in order to identify which reaction schemes occur. The analysis was restricted to reactions that shift electrons in pairs, as either a bonding or a free electron pair. Thus, only polar or heteiolytic and concerted reactions were considered. However, it must be emphasized that the reaction schemes list only the overall change in the distribution of bonds and ftee electron pairs, and make no specific statements on a reaction mechanism. Thus, reactions that proceed mechanistically through homolysis might be included in the overall reaction scheme. 

Radical reactions are not as common as polar reactions but are nevertheless important in some industrial processes and in numerous biological pathways. Let s see briefly how they occur. 

Radicals add to unsaturated bonds to form new radicals, which then undergo addition to other unsaturated bonds to generate further radicals. This reaction sequence, when it occurs iteratively, ultimately leads to the production of polymers. Yet the typical radical polymerization sequence also features the essence of radical-induced multicomponent assembling reactions, assuming, of course, that the individual steps occur in a controlled manner with respect to the sequence and the number of components. The key question then becomes how does one control radical addition reactions such that they can be useful multicomponent reactions Among the possibilities are kinetics, radical polar effects, quenching of the radicals by a one-electron transfer and an efficient radical chain system based on the judicious choice of a radical mediator. This chapter presents a variety of different answers to the question. Each example supports the view that a multicomponent coupling reaction is preferable to uncontrolled radical polymerization reactions, which can decrease the overall efficiency of the process. 

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