CH3Y/CH2 = Y- systems

The CH3Y/CH2=Y- Systems 261 The NCCH2Y/NCCH=Y- Systems 280 Aromatic and anti-aromatic systems 282... 

The barriers for all CH3Y/CH2=Y- systems are lower than for the CH4/ CHj" system. This means that the stabilization of the transition states by the Y-group is greater than that of the respective anions. The situation is illustrated in Fig. 4 for the case of CH3N02 it shows that the transition state for the CH3NO2 reaction is more stable than the transition state for the methane reaction by 73.3 kcal mol-1 while CH2=N02 is more stable than Cl I3 by only 59.1 kcal mol-1. 

Of particular interest is a comparison of the relative contributions of field, resonance, and polarizability effects to the barriers for the NCCH2Y/ NCCH=Y systems relative to those for the CH3Y/CH2=Y systems. These relative contributions were obtained from the correlation according to Equation (32) which yields pR =-7.01, pR = 36.6, and p = 11.9 (Table 17). These... 

The barrier enhancement by the resonance effect is much greater than in the CH3Y/CH2=Y systems. As stated before, a positive pR value does not mean that the transition state is destabilized by the resonance effect it only means that the resonance stabilization of the transition state is weaker than that of the anion. In quantitative terms, transition state resonance stabilization is given by PrOr + p or which yields (PrOr +plop)/p oR = (p + p )/ pR = (-135 + 36.6)/(—135) = 0.73 as the fraction of transition state stabilization relative to resonance stabilization of the anion. This compares with (pR + pR)/ pR = 0.95 for the CH3Y/CH2=Y systems. The smaller resonance stabilization of the transition state in the NCCH2Y/NCCH=Y systems is a direct consequence of the larger imbalance. 

Table 13 Relative contraction of C-Y bond in the anion and negative charge on Y-group of aniona in the CH3Y/CH2=Y and NCCH2Y/NCCH=Y systems...

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