Organic Chemistry Tutorials

The Wittig Reaction

Core Concepts

In this organic chemistry tutorial, you will learn what the Wittig reaction is, and how it compares to other organic chemistry reactions you may have seen. You will also learn about the Wittig reagent and its uses.

Background: Who is Georg Wittig?

The Wittig reaction has been a longstanding important reaction in organic chemistry, its unique nucleophile and intermediate allow for predictable and specific reactivity, which is always highly sought after in the field of organic chemistry. Georg Wittig, a German chemist, developed this reaction. He won the Nobel prize in 1981, along side of Herbert C. Brown, for his work with phosphorus compounds. Other notable work of Georg Wittig includes synthesis of phenyllithium, and 1,2-Wittg and 2,3-Wittig rearrangements.

What is the Wittig Reaction?

The Wittig reaction creates alkenes, C=C bonds, from ketones or aldehydes, C=O bonds. The general reaction scheme is below, where either a ketone or aldehyde reacts with a Wittig reagent (explained later in the article) to produce a carbon carbon double bond.

This general scheme can be applied to more specific reactions. A few specific examples are given below. Try to identify the ketone/aldehyde and alkene in each instance.

What is a Wittig reagent?

The Wittig reaction, as described above is made possible by a unique nucleophile, the phosphonium ylide. In the reaction below, triphenylphosphene (P(Ph)3) and methyl iodide (Me-I) react to create a phosphonium ylide. Notably, other alkyl halides can be used to make differently structured ylides.

So what’s so special about a phosphonium ylide? For starters, it has a positive charge positioned next to a negative charge. This is the definition of a ylide, and because the positive charge is on a phosphorus atom, we call it a phosphonium ylide. This property of our reagent is important to the mechanism of the Wittg reaction.

Secondly, the carbon on the ylide is negatively charged. It is rare to see a lone pair on a carbon, as it is not typically a very stable configuration. This allows carbon to act as a nucleophile, as it is seen to do in the Witting reaction. Forming carbon-carbon bonds is a characteristic of many noteworthy organic reactions, because it is not an easy feat.

Mechanism

Reaction Intermediate

The Witting reaction can be represented in two or more steps, but the most important aspect of the mechanism is the oxaphosphetane intermediate, shown in yellow.

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Oxaphosphetane

Three examples of an oxaphosphetane are given below, the general structure is the four membered ring in orange made of an oxygen, two carbon atoms, and P(Ph)3. You will see this as an intermediate in every Wittg reaction you encounter.

Mechanism

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The reaction is initiated by the nucleophilic carbon (the carbon with the lone pair) attaching the electrophilic carbon on the carbonyl (attached to the oxygen). The double bond between the carbonyl oxygen then breaks, and the extra electrons from the carbonyl oxygen are then transferred to the central phosphorus of the phosphonium ylide. This forms the intermediate.

Bonds Broken: C=O

Bonds Formed: C-C, O-P

Next, is a decyclization step. The C-O and P-C bonds break in the intermediate, and two new double bonds are created (P=O and C=C). The four-membered ring in the intermediate is broken, and the two products shown on the right are formed. The driving force for this reaction is contained in this last step: Phosphorus and Oxygen form a strong covalent bond.

Bonds Broken: C-O, P-C

Bonds Formed: P=O, C=C

Answer Key:

Ketone/Aldehydes are in orange. Alkenes are in blue.

Further Reading

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