[chynagrl]

Hi everyone,

I'm stumped on a question--which is more polar, benzene or biphenyl? I'm trying to sort out a slew of products to obtained from TLC data analysis of a grignard reaction with an anhydride. There's like 6 spots on the TLC product column, all corresponding to starting material, the product intended to be obtained, and lots of side products.

Thank you very much! 
chynagrl

[nj_bartel]

It seems to me that they'd both have an effective dipole moment of 0, but bisphenyl would be more polarizable.

[sanderol]

I would say biphenyl/diphenyl would be more polar, although negligible. All the dipoles in benzene cancel each other, since the ring is fully symmetrical. But in the case of biphenyl, instead of all sp2 carbons in the ring being connected to hydrogens, there is one carbon per ring which is connected to another sp2 carbon, which would give an equal charge distribution. Which will not cancel the charge distribution on the opposite site of the ring...

Hope that makes any sence.

Anyone knows what the main difference is between bi- or diphenyl. In other words, when do you use di or bi???

[James Newby]

Regards to all the spots on the TLC plate, the guy next to me in lab has a similar situation when aluminium salts are used.  If this is a pre-workupthe spots should disappear after workup.  If post-workup, try washing 6 times instead of the usual 3.

Another reason may be the remaining grignard is decomposing on the silica TLC plate.  Try running a TLC, turn it 90 degrees then run the same plate in the same eluent.  If more than one spot appears on the second run you know one product is unstable on silica.

[sanderol]

wrong topic? nice tip tho

[kanan]

i think the biphenyl is more polar than benzene, and this feature is attributed to the mesomeric effect( resonance) that goes from one ring- creating a positive charge-to another  creating a negatively charge ring.
consequently two opposite charges which are the main requirment to talk about polarity