Question

chem lab: Solvent Free Wittig Reaction Lab

Wittig reaction= reaction of a phosphonium ylide with an adehyde or ketone to make a new alkene

A.) Another isomer of the product is formed to a small extent. Draw the structure. Why might we expect this product to be formed in only small amounts?

B.) Give a sequence of reactions showing how the ylide used in the lab could be synthesized from triphenylphosphine

Answer 1

Hi, since you have not mentioned the details of you reactants, I will try to explain it with a general reaction that is usually done in the labs.

I have drawn both the isomers. Remember, the E-isomer is produced is larger amounts while the Z-isomer is only produced in smaller amounts. This is due to the steric crowding in the Z-alkene as you can see that there are two large phenyl substituents across the double bond, the steric hinderance reduced the stability of the molecule. One the other hand, E-alkene is more stable since the two phenyl substituents are as far away as possible from each other and there is minimum steric hinderance.

If you were to synthesize the ylide, remember triphenyl phosphine is nucleophilic, so the phosphonium salt can be prepared by reacting it with benzyl chloride.

the proton next to the phosphonium group can be removed by reacting it with a base and the ylide will form.

If you need more information, please do not hesitate to ask.