Core Concepts – Wittig Reaction
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.
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What is the Wittig Reaction?
The Wittig reaction is a chemical reaction in which an organic compound, called a phosphonium salt, reacts with an aldehyde or a ketone to form an alkene. This reaction is named after the German chemist Georg Wittig, who won the Nobel Prize in Chemistry in 1979 for his work on this and related reactions.
The Wittig reaction involves the formation of a ylide. A ylide is a species with a charge delocalized over two adjacent atoms. Deprotonation of the phosphonium salt using a strong base, such as a lithium or sodium alkoxide, forms the ylide. The ylide then reacts with the aldehyde or ketone to form an intermediate compound called a Wittig intermediate. This intermediate transforms into the final alkene product through a process called alkene stabilization.
The Wittig reaction is an important tool in organic chemistry because it allows the synthesis of alkenes, which are important building blocks in many chemical compounds. It is a highly stereoselective reaction, meaning it can produce alkenes with specific stereochemical configurations. It is also a useful method for synthesizing alkenes that are difficult to prepare using other methods.
The Wittig reaction creates alkenes, C=C bonds, from ketones or aldehydes, C=O bonds. The general reaction scheme is below. 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 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. 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, just like in the Witting reaction. Forming carbon-carbon bonds is a characteristic of many noteworthy organic reactions, because it is not an easy feat.
Mechanism of the Wittig Reaction
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.
Oxaphosphetane
Below we provide three examples of an oxaphosphetane. The general structure contains an oxygen, two carbon atoms, and P(Ph)3 in a four membered ring. You will see this as an intermediate in every Wittg reaction you encounter.
Mechanism
The nucleophilic carbon (the carbon with the lone pair) initiates the reaction, attaching the electrophilic carbon on the carbonyl (attached to the oxygen). The double bond between the carbonyl oxygen breaks, and the extra electrons from the carbonyl oxygen then transfer 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 create two new double bonds (P=O and C=C). The four-membered ring in the intermediate breaks, and the two products shown on the right form. The driving force for this reaction is contained in this last step. Phosphorus and oxygen form a strong covalent bond in the last step.
Bonds Broken: C-O, P-C
Bonds Formed: P=O, C=C
Answer Key:
Ketone/Aldehydes are in orange. Alkenes are in blue.
Background: Who is Georg Wittig?
The Wittig reaction has been a longstanding important reaction in organic chemistry. Its unique nucleophile and intermediates allows 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.