Wittig Reaction
Click to watch video
Wittig reaction – making alkene
Surprisingly, so far in organic chemistry class, we have learned very few ways to form carbon-carbon bonds. Georg Wittig won the 1979 Nobel Prize in Chemistry for his discovery of a reaction that forms alkene carbon-carbon double bonds from a ketone or aldehyde and a phosphine reagent.
The phosphine for the Wittig reaction is formed by the reaction of triphenylphosphine with the appropriate alkyl halide followed by deprotonation with butyl lithium.
The phosphine reagent can be written with a phosphorus-carbon double bond, or it can be written with a positive and negative charge. A compound with a positive and negative charge separation is called an ylide.
Many people prefer the ylide form, but I prefer the phosphorus-carbon double bond form, because then the mechanism is two arrows to form a square intermediate followed by two more arrows for form the final alkene and the very stable triphenylphosphine oxide. The driving force for this reaction is that phosphorus atoms love to bond to oxygen atoms. Overall, you can think of the reaction as removing the oxygen atom and the phosphine in the shaded box. You can simply cover the shaded box up with your hand and push together the two carbon atoms to make a new alkene.
The mechanism is below.
1. Fill in the blank boxes.
a)
b)
Answers
1. Fill in the appropriate reagent or product.
a)
b)
