18.22:
Benzene to 1,4-Cyclohexadiene: Birch Reduction Mechanism
Recall that Birch reduction transforms benzene to 1,4-cyclohexadiene using solvated electrons as reducing agents.
The electrons are generated by dissolving the alkali metal in liquid ammonia.
Birch reduction begins with a single electron transfer to the ring to generate the radical anion. Since the anion is very basic, it abstracts a proton from the alcohol to produce a neutral cyclohexadienyl radical intermediate.
Another single electron transfer produces the cyclohexadienyl anion. A proton transfer from the alcohol gives 1,4-cyclohexadiene.
The overall mechanism involves the sequential addition of two electrons and two protons to give cyclohexadiene, and a reduction occurs via radical anion intermediates.
An electron-withdrawing substituent stabilizes the ipso and para positions of the intermediate, thereby encouraging reduction at these positions.
An electron-releasing substituent stabilizes the ortho and meta positions and promotes reduction at these positions.
18.22:
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.