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.
The isomer that was not produced in this synthesis is the cis,cis-1,4-Diphenyl-1,3-butadiene.
Cinnamaldehyde, which was used as a starting material, has a trans stereochemistry. This means that the double bond connecting the carbon atoms in the benzene ring and the aldehyde group is located on the opposite sides of the molecule.
When cinnamaldehyde reacts with the benzyltriphenylphosphonium chloride, a Wittig reaction occurs, leading to the formation of the cis,trans-1,4-Diphenyl-1,3-butadiene. In this product, the double bonds are located on the same side of the molecule (cis configuration).
On the other hand, the trans,trans-1,4-Diphenyl-1,3-butadiene is obtained through elimination reaction of cis,trans-1,4-Diphenyl-1,3-butadiene.
The formation of the cis,cis-1,4-Diphenyl-1,3-butadiene requires the presence of a cis double bond in the starting material, which is not present in cinnamaldehyde. Hence, this isomer is not produced in the given synthesis.
To determine the isomer that was not produced, we need to understand the stereochemistry of the starting material, cinnamaldehyde, and the reaction conditions.
Cinnamaldehyde has trans stereochemistry, meaning that the two substituents attached to the double bond are on opposite sides of the molecule. In the reaction, cinnamaldehyde is used along with K3PO4 and benzyltriphenylphosphonium chloride to synthesize 1,4-Diphenyl-1,3-butadiene.
1,4-Diphenyl-1,3-butadiene can exist in three isomeric forms:
1. cis,cis-1,4-Diphenyl-1,3-butadiene
2. cis,trans-1,4-Diphenyl-1,3-butadiene
3. trans,trans-1,4-Diphenyl-1,3-butadiene
Considering the stereochemistry of cinnamaldehyde, only the cis,trans and trans,trans isomers can be produced through this synthesis. The cis,cis-1,4-Diphenyl-1,3-butadiene isomer cannot be produced because it would require both substituents attached to the double bonds to be on the same side of the molecule, which is not possible given the trans stereochemistry of cinnamaldehyde.
Therefore, the isomer that was not produced in this reaction is the cis,cis-1,4-Diphenyl-1,3-butadiene isomer.