Chemical Significance of the f Factor

The /factor defines the intrinsic selectivity of the reaction series to a change in the vertical gap (2,5,33), that is, 

As we just argued, for reactions of electrophiles and nucleophiles/increases as the nucleophile becomes more delocalized. Thus, the series of delocalized nucleophiles, in Fig. 6.10, is more selective to changes (of any kind) that affect the gap, G, compared with the series of localized nucleophiles. This would be general for other processes as well delocalization of the single electrons in the R states of the diagram results in higher/values, and vice versa. Such trends abound in electrophile—nucleophile combinations they were analyzed also for radical addition to olefins (40), and are likely to be a general feature of reactivity. 

6 Making Stereochemical Predictions with the VBSCD Model 

The R state correlates to product, X—A/ Y, since it contains a cpx — a AY bond pair that becomes the X—A bond, and at the same time the occupancy of the ct y orbital causes the cleavage of the A—Y bond to release the Y anion. Furthermore, the R state contains information about the stereochemical pathway. Since the bond pair involves a px—i°ay overlap, due to the nodal properties of the aAY orbital the bond pair will be optimized when the X is coupled to the substrate in a collinear X—A—Y fashion. Thus, the steepest 

Therefore, it follows that in a backside trajectory, we obtain both the lowest crossing point as well as the largest TS resonance energy. Computationally, the backside barrier is smaller by 10—20 kcal/mol compared with a front side attack (42). Equation 6.18 defines an orbital selection rule for an Sn2 reaction. Working out this rather trivial prediction is nevertheless necessary since it constitutes a prototypical example for deriving orbital selection rules in other reactions, using FO—VB configurations. Thus, a simple rule may be stated as follows  

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