I've come across an interesting protocol for photochemical reduction of benzophenone to benzhydrol (diphenyl methanol) with sodium alcoholate in alcohol, the 2nd protocol in https://prepchem.com/synthesis-of-diphenylmethanol/. The reaction proceeds via ketyl radical initially to benzopinacol, which is immediately cleaved by a catalytic amount of sodium alcoholate to the benzhydrol and benzophenone. The latter repeats the reaction until it's used up. The alcohol acts as a reducing agent and solvent.
I'm particularly interested in the following part:
the flask is filled to the neck with more of isopropyl alcohol ... The flask is tightly stoppered and inverted upside down in a beaker.
I want to know why is the flask inverted?
My best guess is to avoid exposure to air, since that would interfere with the reduction. But then why isn't it enough to just stopper the full flask?
Might this have more to do with the quality of exposure to the light source (neck and stopper producing a shadow in the upright position)?
Also, is there some official term for this inverted flask technique?
Notes (extra info for the interested):
- The ketyl radical (which is actually a radical anion) is brightly colored, which provides a convenient way to monitor the reaction. Upon exposure to air, the color disappears, presumably because the radical anion gets oxidized back to the ketone.
- Any strong enough base should work for cleavage of the pinacol. In a related synthesis where benzopinacol is the desired product, a small amount of acid is added, because the glass itself is alkaline enough to cleave the product. The sodium alcoholate is convenient in that it's the conjugate base of the alcohol. It's formed by reaction of the alcohol with sodium metal.
- Cleavage of the pinacol proceeds by deprotonation to the pinacolate anion. This rearranges to the ketone and alkoxide, which protonates to the alcohol, regenerating the base.