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The Wittig Reaction & Alkene Synthesis: Definition & Examples

Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

This lesson will explore the process and mechanism of the Wittig reaction. We'll learn how this reaction works and its application as a nucleophilic addition reaction.

What Are Alkenes?

Alkenes might sound foreign, but chances are you've already come in contact with them today. Have you used any plastic? All items made with a type of plastic called PVC, or polyvinyl chloride, are made from alkenes. The antifreeze that keeps our engines running in the winter is also made from an alkene, ethylene glycol. Alkenes are also used in the manufacturing of solvents, paints, lacquers and other materials.

Alkenes are hydrocarbon molecules with a carbon to carbon double bond. Hydrocarbons are molecules that have long chains of carbon with hydrogen atoms attached. Alkenes may have other functional groups attached too, but the main feature is a carbon to carbon double bond.

The Wittig Reaction

Clearly, there is abundant uses for alkenes. One way that scientists can manufacture alkenes is through the Wittig reaction. The Wittig reaction combines a ylide with an aldehyde or ketone to form an alkene. The Wittig reaction is useful because it forms the alkene bond at the specific carbon that is the location of the aldehyde or ketone functional group. Let's take a look at the requirements.

Wittig reactions are used to produce alkenes
Wittig

First, scientists must synthesize a ylide. A ylide is a molecule that has a positive charge on one atom and a negative charge on the atom next to it. The overall charge of the molecule is neutral. Ylides are prepared by combining a triphenylphosphine with an alkyl halide and concludes with treatment with a strong base to remove the halide.

An example of a ylide that could be used in the Wittig reaction
Triphenyl phosphonium ylide

Once a ylide is formed it can be reacted with the aldehyde or ketone of choice to form an alkene. An aldehyde is an organic molecule that have a terminal carbon attached to an oxygen and a hydrogen atom. Ketones have a carbon attached to two other carbons as well as to an oxygen atom. Both aldehydes and ketones form alkenes at the oxygen bond when combined with a ylide.

Mechanism

So, how does this reaction happen?

Step 1: Nucleophilic Attack

First, the negatively charged atom on the ylide attacks the carbonyl carbon, the carbon attached to the oxygen atom, on the aldehyde or ketone. This nucleophilic attack allows the ylide to donate an electron pair to the carbonyl carbon.

Step 2: Formation of a Four Membered Ring

Next, the oxygen atom in the aldehyde or ketone attacks one of the phenyl groups on the ylide to form a four membered ring.

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