18.28: Benzene to 1,4-Cyclohexadiene: Birch Reduction Mechanism
Birch reduction uses solvated electrons as reducing agents. The reaction converts benzene to 1,4-cyclohexadiene. The reaction proceeds by the transfer of a single electron to the ring to form a benzene radical anion. This anion is highly basic—it abstracts a proton from the alcohol to form a cyclohexadienyl radical. Another single electron transfer gives the cyclohexadienyl anion. A proton transfer from the alcohol forms 1,4-cyclohexadiene. Since this reduction occurs via radical anion intermediates, the presence of an electron-withdrawing group stabilizes the ipso and para positions, favoring reduction at these positions. On the other hand, the presence of an electron-donating group stabilizes the ortho and meta positions, favoring reduction at these positions.