Why does Birch reduction on benzene derivatives with an electron donating group happen at ortho and meta positions? When the substitute is electron withdrawing, ipso and para reduction take place because the electron withdrawing group stabilizes the anion in these positions. What's the explanation for electron donating group directivity?
The explanation lies in the distribution of electron density in the intermediate radical anions that appear during the Birch reduction. I use the following reactions as guiding examples:
Electron-withdrawing groups stabilize electron density at the ipso and para positions through conjugation and so the negative charge will mainly be found in these positions; subsequent protonation occurs para.
Electron-donating groups destabilize a negative charge (when compared to simple benzene) through conjugation and this effect would be strongest at the ipso and para positions.
Thus the negative charge will mainly be found at the ortho and meta position.