Question

The para product is not formed to a significant extent. Draw the carbocationic intermediate that would be involved in the formation of the para product (had it actually formed), and its resonance structures. Explain why para product formation is much slower than meta-product formation. (Hint: is the CO2Me substituent an electron donor or withdrawer? A cation stabilizer or destabilizer?) Circle the most problematic resonance of the three resonance structures.

Answer 1

The selectivity of disubstitution on the ring depends upon the group which is already repsent on the ring. If the group is electron donating it would increase elevtron density at ortho and para and thus ortho and para products areformed in major amount. On the other hand If the group is electron withdrawing like in -CO2Me, the ortho and para becomes electron deficient and thus meta product is formed as major product.

Drawn below are the resonance structure for the same.

In ortho and para, we have a resonance structure wherein, the positive charge lies on the carbon next to -CO2Me, which is least stable form and thus, ortho and para products are not preferred in this case.