Wittig reaction: push the electrons yourself
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Drag from the electrons that move (a lone pair or a bond) to where they go, or tap one and then the other. Open the demo full screen
You just pushed every arrow of the Wittig reaction.
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Organic Chemistry 2 · Free interactive demo
The phosphorus ylide Ph₃P=CH₂ turns the C=O of butanal into a C=CH₂. The two pi bonds first close a four-membered oxaphosphetane ring; then that ring falls apart the other way, giving pent-1-ene and triphenylphosphine oxide.
Draw both stages above, two arrows each. Each arrow is checked as you draw it, and Hint reveals the next move a little at a time.
Turn your own notes into exercises like this one
With the Arrow Lab student plan you can snap a photo of a reaction from your lecture notes or textbook. Arrow Lab reads the starting materials and reagents, you check that reading, and it builds the mechanism as an exercise like the one above: you draw each arrow, get hints when you are stuck, and see right away whether each move is correct.
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The Wittig mechanism, step by step
The Wittig reaction is the standard way to build a C=C bond at a former carbonyl carbon with no ambiguity about where the double bond ends up. The very strong P=O bond in triphenylphosphine oxide is what drives the second stage; follow the carbonyl oxygen and you will see it end on phosphorus.
The 3 steps of the demo, in the exercise's own words. Try drawing each one above before you read it.
Make a four-membered ring
Draw both arrows together to make the four-membered ring called an oxaphosphetane. This direct route has no separate betaine intermediate; the route can depend on the reagents and conditions. Each Ph stands for a phenyl group that stays attached to phosphorus.
Split the ring
Break the P–C and C–O bonds together. Their electrons turn the C–C and O–P bonds into double bonds, giving the alkene and Ph₃P=O. Phosphorus is drawn with more than eight electrons; both carbons keep eight.
Products
The product is pent-1-ene. Its terminal CH₂ comes from the Wittig reagent; the other alkene carbon comes from butanal. The original carbonyl oxygen is now bonded only to phosphorus in Ph₃P=O, with all three phenyl groups still attached. All atoms are still present and the total charge stays zero. A terminal CH₂ alkene has no E/Z isomers.
How the mechanism is drawn
Butanal with the ready-made Wittig reagent Ph₃P=CH₂ in dry conditions. Start with the P=C form and draw the direct route through a four-membered ring; making the reagent is not part of this exercise. Counterions are not drawn; no stereochemistry is assigned without bond markers.