chemistry

Organophosphorus chemistry

Organophosphorus chemistry 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 Organophosphorus chemistry rather than just read about it. In short: Organophosphorus chemistry is the scientific study of the synthesis and properties of organophosphorus compounds, which are organic compounds containing phosphorus. They are used primarily in pest control as an alternative to chlorinated hydrocarbons that persist in the environment.

Key takeaways

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

Reference excerpt

Organophosphorus chemistry is the scientific study of the synthesis and properties of organophosphorus compounds, which are organic compounds containing phosphorus. They are used primarily in pest control as an alternative to chlorinated hydrocarbons that persist in the environment. Some organophosphorus compounds are highly effective insecticides, although some are extremely toxic to humans, including sarin and VX nerve agents. Phosphorus, like nitrogen, is in group 15 of the periodic table, and thus phosphorus compounds and nitrogen compounds have many similar properties. The definition of organophosphorus compounds is variable, which can lead to confusion. In industrial and environmental chemistry, an organophosphorus compound need contain only an organic substituent, but need not have a direct phosphorus-carbon (P−C) bond. Thus a large proportion of pesticides (e.g., malathion), are often included in this class of compounds. Phosphorus can adopt a variety of oxidation states, and it is general to classify organophosphorus compounds based on their being derivatives of phosphorus(V) vs phosphorus(III), which are the predominant classes of compounds. In a descriptive but only intermittently used nomenclature, phosphorus compounds are identified by their coordination number σ and their valency λ. In this system, a phosphine is a σ3λ3 compound.

Organophosphorus(V) compounds, main categories

Phosphate esters and amides

Phosphate esters have the general structure P(=O)(OR)3 feature P(V). Such species are of technological importance as flame retardant agents, and plasticizers. Lacking a P−C bond, these compounds are in the technical sense not organophosphorus compounds but esters of phosphoric acid. Many derivatives are found in nature, such as phosphatidylcholine. Phosphate ester are synthesized by alcoholysis of phosphorus oxychloride. A variety of mixed amido-alkoxo derivatives are known, one medically significant example being the anti-cancer drug cyclophosphamide. Also derivatives containing the thiophosphoryl group (P=S) include the pesticide malathion. The organophosphates prepared on the largest scale are the zinc dithiophosphates, as additives for motor oil. Several million kilograms of this coordination complex are produced annually by the reaction of phosphorus pentasulfide with alcohols.

Phosphoryl thioates are thermodynamically much stabler than thiophosphates, which can rearrange at high temperature or with a catalytic alkylant to the former:

SP(OR)3 → OP(OR)2SR In the environment, all these phosphorus(V) compounds break down via hydrolysis to eventually afford phosphate and the organic alcohol or amine from which they are derived.

Phosphonic and phosphinic acids and their esters

Phosphonates are esters of phosphonic acid and have the general formula RP(=O)(OR')2. Phosphonates have many technical applications, a well-known member being glyphosate, better known as Roundup. With the formula (HO)2P(O)CH2NHCH2CO2H, this derivative of glycine is one of the most widely used herbicides. Bisphosphonates are a class of drugs to treat osteoporosis. The nerve gas agent sarin, containing both C–P and F–P bonds, is a phosphonate. Phosphinates feature two P–C bonds, with the general formula R2P(=O)(OR'). A commercially significant member is the herbicide glufosinate. Similar to glyphosate mentioned above, it has the structure CH3P(O)(OH)CH2CH2CH(NH2)CO2H.

The Michaelis–Arbuzov reaction is the main method for the synthesis of these compounds. For example, dimethylmethylphosphonate (see figure above) arises from the rearrangement of trimethylphosphite, which is catalyzed by methyl iodide. In the Horner–Wadsworth–Emmons reaction and the Seyferth–Gilbert homologation, phosphonates are used in reactions with carbonyl compounds. The Kabachnik–Fields reaction is a method for the preparation of aminophosphonates. These compounds contain a very inert bond between phosphorus and carbon. Consequently, they hydrolyze to give phosphonic and phosphinic acid derivatives, but not phosphate.

Phosphine oxides, imides, and chalcogenides

Phosphine oxides (designation σ4λ5) have the general structure R3P=O with formal oxidation state +5. Phosphine oxides form hydrogen bonds and some are therefore soluble in water. The P=O bond is very polar with a dipole moment of 4.51 D for triphenylphosphine oxide. Compounds related to phosphine oxides include phosphine imides (R3PNR') and related chalcogenides (R3PE, where E = S, Se, Te). These compounds are some of the most thermally stable organophosphorus compounds. In general, they are less basic than the corresponding phosphine oxides, which can adduce to thiophosphoryl halides:

R3PO + X3PS → R3P+–O–P+X2–S− + X− Some phosphorus sulfides can undergo a reverse Arbuzov rearrangement to a dialkylthiophosphinate ester. Phosphorus selenides are susceptible to further alkylation on the selenium atom.

Phosphonium salts and phosphoranes

Compounds with the formula [PR4+]X− comprise the phosphonium salts. These species are tetrahedral phosphorus(V) compounds. From the commercial perspective, the most important member is tetrakis(hydroxymethyl)phosphonium chloride, [P(CH2OH)4]Cl, which is used as a fire retardant in textiles. Approximately 2 Gg are produced annually of the chloride and the related sulfate. They are generated by the reaction of phosphine with formaldehyde in the presence of the mineral acid:

PH3 + HX + 4 CH2O → [P(CH2OH)4+]X− A variety of phosphonium salts can be prepared by alkylation and arylation of organophosphines:

PR3 + R'X → [PR3R'+]X− Phosphoranes (σ5λ5) are formally derived from the unknown PH5. Those with five organic substituents are rare, although P(C6H5)5 is the product of P(C6H5)4+ and phenyllithium. Related compounds containing both halide and organic substituents on phosphorus are fairly common. Phosphorus ylides are unsaturated phosphoranes, known as Wittig reagents, e.g. CH2P(C6H5)3. These compounds feature tetrahedral phosphorus(V) and are related to phosphine oxides. They too are derived from phosphonium salts; Wittig's original preparation begins by methylating triphenylphosphine. But Wittig reagent synthesis proceeds to deprotonation, not a second alkylation.

Organophosphorus(III) compounds, main categories

Phosphites, phosphonites, and phosphinites

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Organophosphorus chemistry

Start with the simplest possible case. Write down what Organophosphorus chemistry 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 Organophosphorus chemistry 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 Organophosphorus chemistry 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 Organophosphorus chemistry

In research: Organophosphorus chemistry 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 Organophosphorus chemistry 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: Organophosphorus chemistry is common in secondary-school and first-year university syllabi. It links to neighbouring topics Functional groups, Organophosphorus compounds, so understanding it makes those chapters shorter.
In everyday life: Look for Organophosphorus chemistry 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 Organophosphorus chemistry in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Organophosphorus chemistry 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 Organophosphorus chemistry out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Organophosphorus chemistry in simple terms?

Why does Organophosphorus chemistry 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 Organophosphorus chemistry?

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 Organophosphorus chemistry.