Pasrules - 20-9-2015 at 18:36
I performed a textbook Grignard reaction of phenylmagnesium bromide with benzophenone to produce triphenylmethanol. The grignard reagent synthesis
went swimmingly however the second stage with the addition of benzophenone did not reach reflux temperature. Manual stated 50degC I increased to
70degC but still no reflux. I stopped the reaction with ice and did an acid work up. After the drying steps a black oily substance was produced.
I ran this exact reaction in tandem with another that did reflux at 90degC and received an 80% yield.
I would appreciate any explanation to the first pot. As I don't think temperature should have had that much of an effect.
Note: glass ware dried in a oven an atmospheric filters in place as standard.
Steam - 20-9-2015 at 19:24
hmmm, Grignards are very touchy, as you just found out! If both rxns were carried out in identical conditions the product should have been the same.
Perhaps your solvent was the problem? Grignards require a good dry solvent which can interact with that electropositive Mg (Ether, THF, generic polar
aprotic solvent).
Another possibility CO2 contamination. R-Mg-Br class organo-metalics will react with CO2 to produce a carboxylic acid group. You must make sure that
there is a good inert atmosphere (which you said there was). If you used ether ( which I am guessing you didn't based on your reflux temperatures) the
vapors from the ether will purge the apparatus for you.
[Edited on 21-9-2015 by Steam]
Pasrules - 20-9-2015 at 21:00
Both pots were identical with an anhydrous THF solvent which behaved strangely for the temperatures I had to use.
In the first stage of synthesizing the Grignard reagent the pot was bubbling away with drop wise addition. I am sure that I hadn't boiled off all the
THF so the only variable here is temperature.
Thank you for you reply.
Edit spelling
[Edited on 21-9-2015 by Pasrules]