The Wittig reaction is one of the most significant reactions in organic chemistry, invented by German chemist George Wittig. He was awarded the Nobel Prize for this work in 1979. The Wittig reaction converts a carbonyl group into a double bond using phosphonium ylides.
There are three ways to achieve the Wittig reaction:
- Making the ellipse from scratch or from the alkyl halide.
- From the neutral ellipse form.
- From the cationic ellipse form.
To illustrate this, let’s take a closer look at George Wittig’s face. He is wearing glasses and smiling, which we can use to represent the double bond in C=C groups as a smiley face. The reaction will produce a P=O group and a C=C group, as demonstrated by Wittig’s face.
Mechanism of the Wittig Reaction
Starting with an alkyl halide, triphenylphosphine attacks the carbon holding the leaving group (usually bromide). This results in the formation of a phosphonium salt and the leaving group is kicked off. The intermediate can then be treated with embutyl lithium to form the cationic ellipse.
The cationic ellipse has a resonance form where the negative charge attacks the phosphorus atom, forming the neutral ellipse. The Wittig reaction can also start from the alkyl halide or the neutral ellipse forms. In this case, embutyl lithium is added to convert the neutral ellipse into a cationic ellipse.
Now let’s analyze the viric reaction mechanism starting from the alkyl halide. The reagents and reaction scheme include triphenylphosphine, an alkyl halide (e.g., 1-bromopropane), embutyl lithium as the base, and a ketone (e.g., cyclohexanone).
The first step is the attack of triphenylphosphine on the carbon holding the bromide, followed by the leaving group being kicked out. Embutyl lithium then protonates one of the hydrogens next to the phosphine plus group, transferring electrons to form the cationic ellipse.
The cationic ellipse has a resonance form where the negative charge attacks the phosphorus atom, forming a double bond between carbons 1 and 2. The ketone (cyclohexanone) is then introduced, attacking on the carbonyl group and transferring electrons to form the desired alkene product and triphenylphosphine oxide as a byproduct.
The driving force for this reaction is the formation of a strong phosphorus-oxygen double bond in the byproduct, which has one of the strongest bonds in nature. The Wittig reaction remains an essential tool for creating double bonds in organic chemistry and continues to be widely used today.
