18.22 : בנזן ל-1,4-ציקלוהקסדיאן: מנגנון חיזור בירץ'

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.