Reactivity Probes of Nonbonded Interactions

Relative reactivities of molecules can also shed light upon the validity of our general theoretical approach. In this section we dull examine the following reactivity probes of nonbonded interaction  

The Diels Alder reaction has been an important testing ground for qualitative theo ries for many years In this section we consider the importance of nonbonded attractive interactions on Diels Alder reactivity. Our model reaction stem is butadiene and the three isomers of dicyanoethylene  

The key stabilizing interaction in the above reactions is between the pi HOMO of butadiene and the pi LUMO of the isomeric dicyanoethylenes. The pi LUMO s of the isomeric dicyanothylenes vary in the order 1,1 1,2-tnms 1,2-cis. Accordingly, on the basis of Eq. (1), we conclude that the stabflization of the reaction complex and, consequently, the rate of the reaction will vary in the order 1,1 , 2-trans 1,2-cis. 

Experimental evidence supporting the above prediction can be found in Table 27. Iti both examples, the 1,1 isomer reacts much faster than either the 1,2-c/s or trans isomers which exhibit comparable reactivity. 

The importance of nonbonded interactions in chemical systems is illustrated by the stereochemistry of the abnormal bimolecular substitution reaction (Sf 2 ). It has been suggested that the nucleophile attacks the same side from which the leaving group departs However, the mechanism of the reaction is still unclear  

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In Part II we examined in detail various experimental tests of nonbonded interactions. Here, we again focus on specific physical and reactivity probes in order to test the importance of n—a interactions in organic problems. Specifically, we shall examine the following two areas ... 

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