Why is biphenyl formed as a by-product in a grignard reaction?
Some biphenyl can form if not all of the phenyl bromide has already reacted with magnesium solid to form the Grignard reagent. In that situation, the Grignard reagent acts as a very strong nucleophile towards the regular phenyl bromide.
First of all, biphenyl looks like this:
Now, when I look at this compound, one reaction immediately comes to mind, and it's quite an odd one (Organic Chemistry, Bruice, Ch. 16.14).
What you basically have is a nucleophilic aromatic substitution (NAS) where you see a very strong nucleophile (
This can happen because
(Note that a carbon was labeled in this reaction to demonstrate that the benzyne intermediate that forms can get attacked on either carbon that contributes to the triple bond.)
SIMILARITIES WITH THE GRIGNARD SITUATION
What you should do with the above mechanism is imagine how it would occur if you used phenyl magnesium bromide instead of
The pKa of benzene is
So, if this mechanism can occur (which it does), a very similar mechanism likely will occur to form biphenyl using the analogous Grignard situation.
Essentially, the Grignard reagent takes the role of the