1.4- Bislocalization energy

The arguments used above show that the 1,2-bislocalization energy (SEJs of atoms s and t is given by... 

FIGURE 4.7. Calculation of 1,2-bislocalization energies ( ortholocalization energies ). 

Figure 4.7 illustrates the calculation of 1,2-bislocalization energies, using equation (4.60), and Table 4.2 shows values for various bonds in several conjugated systems. Since hardly any of these reactions are reversible, no experimental values of equilibrium constants are available however, the additions take place most easily at bonds where the 1,2-bislocalization energy is low. This admittedly is more a matter of the rates of addition rather than equilibria, a problem which will be discussed in the next chapter. 

Values from M. J. S. Dewar, J. Am. Chem. Soc. 74, 3357 (1952). Bond with lowest 1,2-bislocalization energy marked with asterisk. 

TABLE 4.1. 1,4-Bislocalization Energies for a Series of Aromatic Hydrocarbons... 

Values for 1,4-bislocalization energies of a number of conjugated and aromatic systems are shown in Table 4.1. The possibility of Diels-Alder reactions rests on a favorable equilibrium, i.e., a low enough value for the corresponding 1,4-bislocalization energy. In the reactions of dienes and aromatic... 

Hardly any quantitative data are available to check the predictions of equilibrium constants that can be made from the results in Table 4.1. However, the 1,4-bislocalization energies can be used to predict the preferred points of attack in an even AH and these are invariably the ones at which reaction is observed to take place. The values in Table 4.1 refer to the positions giving the lowest 1,4-bislocalization energy. In cases where this is low enough for reaction to occur, it occurs at those positions. 

Predict the structure of the dimer, using the 1,2 and 1,4 bislocalization energies for hexahelicene. 

Dewar and Pyron analyzed the reaction in a similar way but with localization energies and 1,4-bislocalization energies given by the more accurate MO treatment mentioned earlier. Their plot is somewhat better than Fig. 5.43. 

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