[Brad]

Hi all,

Working on the synthesis of Benzopinacol (and acetone) using isopropyl alcohol, benzophenone and an acetic acid catalyst in the presence of light but I cannot wrap my head around the mechanism (haven't covered in lecture yet). I have "Advanced Organic Chemistry, 5th. Structures and Mechanisms" by Carey and Sundberg and from what I read (page 1075) electron(s?) on the carbonyl of benzophenone are promoted from the HOMO orbital to the LUMO orbital giving rise to a radical. From here I'm not sure.

I've guessed that there is a donor (isopropyl alcohol) where a proton is transferred to the oxygen (radical) leaving the isopropyl alcohol with a radical at the C2 position. This is then used to generate the second equivalent of the benzophenone species allowing two benzophenol radicals to react making benzopinacol.

My guess is most likely wrong but all other reaction mechanisms I've read through SciFinder, Reaxys etc. uses a metal catalyst to donate the electron and coordinate the reaction. Also, I don't know where the acid catalyst enters the system.

Thanks in advance


Some key words that should appear in my report (don't know if this is useful):
HOMO and LUMO orbitals, Planck’s equation, ground and excited states, ultraviolet (uv) and visible light, the pyrex cut-off
for uv radiation, n -> pi* electronic transition, S0, S1, T1 descriptors of electronic states,
intersystem crossing, fluorescence, phosphorescence, dark reaction, hydrogen atom abstraction,
free radical.

[discodermolide]

Have a look here:

http://www.luzchem.com/edu/docstore/2-benzophenone.pdf

[Brad]

Thank you for the article. It cleared up steps one and three for me. I'm still unclear about singlet and triplet states and the effect on the reaction. I worked on the second part of the reaction and came up with this (Attached image). I still not 100% but that's my next best assumption on electron movement. The only problem is I'm still ignoring the acid catalyst from acetic acid

Thanks again

[discodermolide]

See note 2 in this Org. Syn. prep.

http://www.orgsyn.org/orgsyn/prep.asp?prep=cv2p0071

And chapter 14 in this long link.

http://books.google.ch/books?id=x77djyQHX8UC&pg=PA731&lpg=PA731&dq=Benzopinacol+synthesis&source=bl&ots=WK6o3N4f4N&sig=E7Xk3RiaF4qYW09z-hu_6QIqj8s&hl=en&sa=X&ei=avNxUKPgEcfGtAaQkYCgAg&ved=0CDAQ6AEwAg#v=onepage&q=Benzopinacol%20synthesis&f=false


They can probably explain it better than me.

[Brad]

Thank you for the second link. That one was immensely helpful! It all clicked