The Wittig Reaction

The Wittig reaction is kind of a neat, unique reaction. You won’t see other reactions like it in the course.

Here’s what it does: the Wittig reaction converts aldehydes and ketones into alkenes. So we’re breaking a C-O double bond and forming a C-C double bond. 

The reaction works well because the phosphorus oxygen double bond is really strong: this reaction drives the reaction to completion.

Why is this useful? Carbon-carbon bond forming reactions are useful, because they let us stitch smaller molecules into larger ones. So definitely have this in your “synthetic toolbox” – because now you can tie in all the reactions you learned from Org 1 that involve alkenes.

Tomorrow: we’re going to cover a mechanism you’ve seen before, but give it a new name. It’ll be an important new addition to our toolbox of mechanisms.

What happens in the first step of the Wittig is that the ylide carbon attacks the carbonyl carbon, while the carbonyl oxygen attacks the phosphorus. This makes a 4 membered ring.

Next, the 4 membered ring breaks apart and we form a P-O double bond and the C-C double bond.

The reaction works well because the phosphorus oxygen double bond is really strong: this reaction drives the reaction to completion.

Why is this useful? Carbon-carbon bond forming reactions are useful, because they let us stitch smaller molecules into larger ones. So definitely have this in your “synthetic toolbox” – because now you can tie in all the reactions you learned from Org 1 that involve alkenes.

Tomorrow: we’re going to cover a mechanism you’ve seen before, but give it a new name. It’ll be an important new addition to our toolbox of mechanisms.

Thanks for reading! James

P.S. Further reading on the Reaction Guide – the Wittig Reaction