posted by Caleb .
1,4-Diphenyl-1,3-butadiene was synthesied using cinnamaldehyde, K3PO4, and benzyltriphenylphosphonium chloride.
In addition to cis,trans and trans,trans-1,4-Diphenyl-1,3-butadiene, there is another isomer of this compound that has not been shown. What is it and why was it not produced? (Hint: Cinnamaldehye has trans stereochemistry)
It is most likely cis,cis-1,4-dephenyl-1,3-butadiene. If you draw it you would see a U-shaped molecule with the two phenyl groups getting close together at the the ends of the "U". Formation of this isomer is probably prevented by crowding (steric hindrance) at the ends of the U-shaped structure.
Another argument I just overlooked is that one of the reactants, cinnamaldehyde, contains a trans structure around the c=c double bond that is preserved in the final compound, 1,4-Diphenyl-1,3-butadiene. Thus, we cannot have two cis structures in the C=C-C=C chain which is the backbone of the the final compound. This may be a better argument than the one in my previous answer.