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Carbocations and factors affecting instability



Let’s consider the following carbocations and rank them in terms of their stability:

 

Fig. 1: Carbocation 1 is stabilized by the –OCH3  substituent  since it is electron donating. The + charge is delocalized and the carbocation is stabilized. Carbocation 2 is destabilized by the electron withdrawing substituent NO2. By drawing the resonance structure we see that there are two adjacent positive charges. This resonance structure is highly unfavorable!



The substituent (-OCH3) stabilizes carbocation 1 by resonance and the resulting charge delocalization. The + charge is delocalized to the O atom.
The substituent (-NO2+) in carbocation 2 is electron-withdrawing and the carbocation is destabilized. Notice the two adjacent + charges.

In general, carbocations are destabilized by neighboring electron-withdrawing groups if:

  • Contain an atom more electronegative than C

  • There is no atom with an electron pair that is directly attached to the carbocation (-CF3, -CCl3, -C(O)R, -+NR3, -CN, -NO2, -C(O)-OH, -S(O)2OH)


For example let us consider the following carbocations and rank them in terms of stability:

 

Fig. 2: The carbocation with the CH3 substituent is more stable than the carbocation with the electron-withdrawing CF3 substituent.


The carbocation with the CH3 group in Figure 1 is more stable than the carbocation with the -CF3 group. Notice the partial + charge (d+) on the carbon atom due to the electron withdrawing effect of the fluorine atoms. The carbocation is destabilized due to the adjacent positive charge and partial positive charge.




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