Phosphine oxides

Key takeaways

Phosphine oxides belongs to mathematics; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Phosphine oxides to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Phosphine oxides from memory before moving on to harder problems.

Reference excerpt

Phosphine oxides are phosphorus compounds with the formula OPX3. When X = alkyl or aryl, these are organophosphine oxides. Triphenylphosphine oxide is an example. An inorganic phosphine oxide is phosphoryl chloride (POCl3). The parent phosphine oxide (H3PO) remains rare and obscure.

Tertiary phosphine oxides

Tertiary phosphine oxides are the commonly encountered phosphine oxides. With the formula R3PO, they are tetrahedral compounds. They are usually prepared by oxidation of tertiary phosphines. The P-O bond is short and polar. According to molecular orbital theory, the short P–O bond is attributed to the donation of the lone pair electrons from oxygen p-orbitals to the antibonding phosphorus-carbon bonds. The nature of the P–O bond was once hotly debated. Some discussions invoked a role for phosphorus-centered d-orbitals in bonding, but this analysis is not supported by computational analyses. In terms of simple Lewis structure, the bond is more accurately represented as a dative bond, as is currently used to depict an amine oxide.

Preparation and occurrence Phosphine oxide are typically produced by oxidation of organophosphines. The oxygen in air is often sufficiently oxidizing to fully convert trialkylphosphines to their oxides at room temperature:

R3P + 1/2 O2 → R3PO Oxidation of less basic phosphines, such as methyldiphenylphosphine can be achieved using hydrogen peroxide:

PMePh2 + H2O2 → OPMePh2 + H2O (Me = CH3, Ph = C6H5) Phosphine oxides are by-product of the Wittig reaction:

R3PCR'2 + R"2CO → R3PO + R'2C=CR"2 Another route to phosphine oxides is the thermolysis of phosphonium hydroxides:

[PPh4]Cl + NaOH → Ph3PO + NaCl + PhH The hydrolysis of phosphorus(V) dihalides also affords the oxide:

R3PCl2 + H2O → R3PO + 2 HCl

Secondary phosphine oxides Secondary phosphine oxides (SPOs), formally derived from secondary phosphines (R2PH), are again tetrahedral at phosphorus. One commercially available example of a secondary phosphine oxide is diphenylphosphine oxide. SPOs are used in the formulation of catalysts for cross coupling reactions. Unlike tertiary phosphine oxides, SPOs often undergo further oxidation:

R2P(O)H + H2O2 → R2P(O)OH + H2O These reactions are preceded by tautomerization to the phosphinous acid (R2POH):

R2P(O)H → R2POH R2POH + H2O2 → R2PO2H + H2O

Syntheses A nonoxidative route is applicable secondary phosphine oxides, which arise by the hydrolysis of the chlorophosphine. An example is the hydrolysis of chlorodiphenylphosphine to give diphenylphosphine oxide:

Ph2PCl + H2O → Ph2P(O)H + HCl P-chiral phosphine oxides are valuable intermediates in the synthesis of P-chiral phosphines and phosphates, important as ligands in catalysis and in the synthesis of oligonucleotide drugs.

Primary phosphine oxides Primary phosphine oxides, formally oxidized derivatives of primary phosphines, are again tetrahedral at phosphorus. With four different substituents (O, OH, H, R), they are chiral. The primary phosphine oxides subject to tautomerization, which leads to racemization. Like SPO's they are susceptible to further oxidation. Primary phosphine oxides disproportionate to the phosphinic acid and the primary phosphine:

2 RP(O)H2 → RP(O)(H)OH + RPH2 2 RP(O)H2 → RP(O)(H)OH + 2 RPH2

Reactions Transition metal complexes of phosphine oxides are numerous. Some phosphine oxides are well-known photoinitiators in photopolymer chemistry. UV/LED exposure induces a type I Norrish fission to free radicals, which then polymerize in a radical chain. An example is 2,4,6‑trimethylbenzoyldiphenylphosphine oxide, which absorbs around 380-410nm (near UV).

Deoxygenation Phosphine oxide deoxygenation has been extensively developed because some useful reactions convert stoichiometric tertiary phosphines to the corresponding oxides. Regenerating the tertiary phosphines requires strongly oxophilic reagents, and can retain or invert chirality at P, depending on the reductant. Industrial deoxygenation usually begins with treatment with phosgene or equivalents. The resulting chlorotriphenylphosphonium chloride is then reduced. In the laboratory, phosphine oxides are usually reduced with silicon derivatives, typically inexpensive trichlorosilane. Trichlorosilane and triethylamine reduce phosphine oxides with inversion, whereas the reaction proceeds with retention absent the base:

HSiCl3 + Et3N ⇋ SiCl3− + Et3NH+ R3PO + Et3NH+ ⇋ R3POH+ + Et3N SiCl3− + R3POH+ → PR3 + HOSiCl3 Other perchloropolysilanes, e.g. hexachlorodisilane (Si2Cl6) or Si3Cl8, can reduce phosphine oxides and generally give higher yields:

R3PO + Si2Cl6 → R3P + Si2OCl6 2 R3PO + Si3Cl8 → 2 R3P + Si3O2Cl8 Boranes and alanes also deoxygenate phosphine oxides. Phosphoric acid diesters ((RO)2PO2H) catalyze deoxygenation with hydrosilanes.

Use Phosphine oxides are ligands in various types of homogeneous catalysis. In coordination chemistry, they are known to have labilizing effects to CO ligands cis to it in organometallic reactions. The cis effect describes this process. Phosphine oxides are excellent hydrogen-bond acceptors. The 31P NMR shift of triethylphosphine oxide when bound to a Lewis acid, is commonly used to determine the effective Lewis acidity.

References

Illustrations

Worked examples

Example 1 — a first encounter with Phosphine oxides

Start with the simplest possible case. Write down what Phosphine oxides claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In mathematics, 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 Phosphine oxides 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 Phosphine oxides 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 Phosphine oxides

In research: Phosphine oxides appears in mathematics 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 Phosphine oxides 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: Phosphine oxides is common in secondary-school and first-year university syllabi. It links to neighbouring topics Functional groups, Phosphine oxides, so understanding it makes those chapters shorter.
In everyday life: Look for Phosphine oxides 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.

How to study Phosphine oxides in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Phosphine oxides means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Phosphine oxides out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Phosphine oxides in simple terms?

Phosphine oxides are phosphorus compounds with the formula OPX3. When X = alkyl or aryl, these are organophosphine oxides.

Why does Phosphine oxides matter?

Because it connects several mathematics 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 Phosphine oxides?

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 Phosphine oxides.