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 CH₃substituent 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.