The Wittig Reaction: Making Alkenes Where You Want Them

Why it matters

Elimination reactions make alkenes, but the position of the double bond is decided by which proton is removed, and mixtures are common. The Wittig reaction fixes the position absolutely: the alkene forms between the former carbonyl carbon and the ylide carbon, and nowhere else.

That regiochemical certainty is why it is used in synthesis even though it produces a stoichiometric phosphorus byproduct.

Making the ylide

A phosphine displaces halide from an alkyl halide to give a phosphonium salt. A base then removes a proton from the carbon adjacent to phosphorus, giving the ylide — a species with adjacent positive and negative formal charges.

The proton is acidic because the resulting negative charge is stabilised by the adjacent positively charged phosphorus. The strength of base required depends on how much additional stabilisation is present.

The mechanism

The ylide carbon, being nucleophilic, attacks the carbonyl carbon. The resulting species closes to a four-membered ring containing phosphorus and oxygen — the oxaphosphetane. That ring then fragments to give the alkene and a phosphine oxide.

Stereochemistry — the part that is examined

Which geometric isomer forms depends on the ylide.

The reasoning is kinetic versus thermodynamic control. A reactive non-stabilised ylide adds irreversibly and fast, and the transition state that minimises steric interaction as the ring forms leads to the Z alkene. A stabilised ylide adds reversibly, so the system equilibrates and settles on the more stable E product.

Being asked to predict geometry from the ylide, or to choose an ylide to obtain a required geometry, is the standard question — and the answer needs the reversibility argument, not just the table.

Scope and limitations

• Works well with aldehydes and ketones; esters and amides are generally unreactive toward ylides.
• Tolerates many functional groups, particularly with stabilised ylides which need only a mild base.
• The phosphine oxide byproduct must be separated, which can be inconvenient on scale.
• Hindered ketones react sluggishly with bulky ylides.

Related reactions worth knowing

The Horner–Wadsworth–Emmons variant uses a phosphonate rather than a phosphonium salt. Its advantages are that the byproduct is water-soluble and therefore easy to remove, and that it gives E alkenes with high selectivity. Where a question emphasises ease of purification or strong E selectivity, this is the intended answer.

Frequently asked questions

Why is the Wittig preferred over elimination?

Because the double bond position is unambiguous. Elimination can give several alkenes depending on which proton is removed, and the major product must then be predicted rather than chosen.

What exactly is an ylide?

A neutral molecule with adjacent atoms carrying opposite formal charges, here a carbanion adjacent to positively charged phosphorus. The arrangement is stabilised by that adjacency.

Why do stabilised ylides give the E alkene?

Because their addition is reversible, so the intermediate can revert and re-form, allowing the system to reach the thermodynamically preferred arrangement leading to the more stable E product.

Can the reaction be used on an ester carbonyl?

Not usefully with an ordinary ylide, since esters are much less electrophilic than aldehydes and ketones. More reactive reagents are needed for that transformation.