chemistry

Wittig reagents

Wittig reagents is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Wittig reagents rather than just read about it. In short: In organic chemistry, Wittig reagents are organophosphorus compounds of the formula R3P=CHR', where R is usually phenyl. They are used to convert ketones and aldehydes to alkenes: Preparation Because they typically hydrolyze and oxidize readily, Wittig reagents are prepared using air-free techniques.

Key takeaways

Wittig reagents belongs to chemistry; place it in that map before memorising details.
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Connect Wittig reagents to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Wittig reagents from memory before moving on to harder problems.

Reference excerpt

In organic chemistry, Wittig reagents are organophosphorus compounds of the formula R3P=CHR', where R is usually phenyl. They are used to convert ketones and aldehydes to alkenes:

Preparation Because they typically hydrolyze and oxidize readily, Wittig reagents are prepared using air-free techniques. They are typically generated and used in situ. THF is a typical solvent. Some are sufficiently stable to be sold commercially.

Formation of phosphonium salt Wittig reagents are usually prepared from a phosphonium salt, which is in turn prepared by the quaternization of triphenylphosphine with an alkyl halide. Wittig reagents are usually derived from a primary alkyl halide. Quaternization of triphenylphosphine with secondary halides is typically inefficient. For this reason, Wittig reagents are rarely used to prepare tetrasubstituted alkenes.

Bases for deprotonation of phosphonium salts The alkylphosphonium salt is deprotonated with a strong base such as n-butyllithium:

[Ph3P+CH2R]X− + C4H9Li → Ph3P=CHR + LiX + C4H10 Besides n-butyllithium (nBuLi), other strong bases like sodium and potassium t-butoxide (tBuONa, tBuOK), lithium, sodium and potassium hexamethyldisilazide (LiHMDS, NaHMDS, KHDMS, where HDMS = N(SiMe3)2), or sodium hydride (NaH) are also commonly used. For stabilized Wittig reagents bearing conjugated electron-withdrawing groups, even relatively weak bases like aqueous sodium hydroxide or potassium carbonate can be employed.

The identification of a suitable base is often an important step when optimizing a Wittig reaction. Because phosphonium ylides are seldom isolated, the byproduct(s) generated upon deprotonation essentially plays the role of an additive in a Wittig reaction. As a result, the choice of base has a strong influence on the efficiency and, when applicable, the stereochemical outcome of the Wittig reaction.

Substituent effects Electron-withdrawing groups (EWGs) enhance the ease of deprotonation of phosphonium salts. This behavior is illustrated by the finding that deprotonation of triphenylcarbethoxymethylphosphonium requires only sodium hydroxide. The resulting triphenylcarbethoxymethylenephosphorane is somewhat air-stable. It is however less reactive than ylides lacking EWGs. For example they usually fail to react with ketones, necessitating the use of the Horner–Wadsworth–Emmons reaction as an alternative. Such stabilized ylides usually give rise to an E-alkene product when they react, rather than the more usual Z-alkene.

Reactions

Olefination Wittig reagents are used for olefination reactions, i.e. the Wittig reaction.

Protonation Wittig reagents are prepared by deprotonation of alkyl phosphonium salts, and this reaction can be reversed. The methodology can be useful in the preparation of unusual Wittig reagents.

Alkylation Alkylation of Ph3P=CH2 with a primary alkyl halide R−CH2−X, produces substituted phosphonium salts:

Ph3P=CH2 + RCH2X → Ph3P+ CH2CH2R X− These salts can be deprotonated in the usual way to give Ph3P=CH−CH2R.

Deprotonation Although ylides are "electron-rich", they are susceptible to deprotonation of alkyl substituents. Treatment of Me3PCH2 with butyl lithium affords Me2P(CH2)2Li. Having carbanion-like properties, lithiated ylides function as ligands. Thus Me2P(CH2)2Li is a potential bidentate ligand.

Examples (Chloromethylene)triphenylphosphorane Methoxymethylenetriphenylphosphorane Methylenetriphenylphosphorane Triphenylcarbethoxymethylenephosphorane Hexaphenylcarbodiphosphorane

Structure Wittig reagents are usually described as a combination of two resonance structures:

Ph3P+CR2− ↔ Ph3P=CR2 The former is called the ylide form and the latter is called the phosphorane form, which is the more familiar representation. Crystallographic characterization of methylenetriphenylphosphorane shows that the phosphorus atom is tetrahedral. The PCH2 centre is planar and the P=CH2 distance is 1.661 Å, which is much shorter than the other P-C distances (1.823 Å).

External links Wittig reaction in Organic Syntheses, Coll. Vol. 10, p. 703 (2004); Vol. 75, p. 153 (1998). (Article) Wittig reaction in Organic Syntheses, Coll. Vol. 5, p. 361 (1973); Vol. 45, p. 33 (1965). (Article) Visual depiction on Tumblr of a Wittig reagent synthesis

References

Illustrations

Worked examples

Example 1 — a first encounter with Wittig reagents

Start with the simplest possible case. Write down what Wittig reagents claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Wittig reagents before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Wittig reagents ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Wittig reagents

In research: Wittig reagents appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Wittig reagents in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Wittig reagents is common in secondary-school and first-year university syllabi. It links to neighbouring topics Organophosphorus compounds, so understanding it makes those chapters shorter.
In everyday life: Look for Wittig reagents outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Wittig reagents in 20 minutes

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Frequently asked questions

What is Wittig reagents in simple terms?

In organic chemistry, Wittig reagents are organophosphorus compounds of the formula R3P=CHR', where R is usually phenyl. They are used to convert ketones and aldehydes to alkenes: Preparation Because they typically hydrolyze and oxidize readily, Wittig reagents are prepared using air-free technique…

Why does Wittig reagents matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Wittig reagents?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Wittig reagents.