chemistry

Phosphorus trichloride

Phosphorus trichloride 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 Phosphorus trichloride rather than just read about it. In short: Phosphorus trichloride is an inorganic compound with the chemical formula PCl3. A colorless liquid when pure, it is an important industrial chemical, being used for the manufacture of phosphites and other organophosphorus compounds.

Key takeaways

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

Reference excerpt

Phosphorus trichloride is an inorganic compound with the chemical formula PCl3. A colorless liquid when pure, it is an important industrial chemical, being used for the manufacture of phosphites and other organophosphorus compounds. It is toxic and reacts readily with water or air to release hydrogen chloride fumes.

History Phosphorus trichloride was first prepared in 1808 by the French chemists Joseph Louis Gay-Lussac and Louis Jacques Thénard by heating calomel (Hg2Cl2) with white phosphorus. Later during the same year, the English chemist Humphry Davy produced phosphorus trichloride by burning white phosphorus in chlorine gas.

Preparation World production exceeds one-third of a million tonnes. Phosphorus trichloride is prepared industrially by the reaction of chlorine with white phosphorus, using phosphorus trichloride as the solvent. In this continuous process PCl3 is removed as it is formed in order to avoid the formation of PCl5.

P4 + 6 Cl2 → 4 PCl3

Structure and spectroscopy It has a trigonal pyramidal shape. Its 31P NMR spectrum exhibits a singlet around +220 ppm with reference to a phosphoric acid standard.

Reactions The phosphorus in PCl3 is often considered to have the +3 oxidation state and the chlorine atoms are considered to be in the −1 oxidation state. Most of its reactivity is consistent with this description.

Oxidation PCl3 is a precursor to other phosphorus compounds, undergoing oxidation to phosphorus pentachloride (PCl5), thiophosphoryl chloride (PSCl3), or phosphorus oxychloride (POCl3).

PCl3 as an electrophile PCl3 reacts vigorously with water to form phosphorous acid (H3PO3) and hydrochloric acid:

PCl3 + 3 H2O → H3PO3 + 3 HCl Phosphorus trichloride is the precursor to organophosphorus compounds. It reacts with phenol to give triphenyl phosphite:

3 PhOH + PCl3 → P(OPh)3 + 3 HCl (Ph = C6H5) Alcohols such as ethanol react similarly in the presence of a base such as a tertiary amine:

PCl3 + 3 EtOH + 3 R3N → P(OEt)3 + 3 R3NH+Cl− With one equivalent of alcohol and in the absence of base, the first product is alkoxyphosphorodichloridite:

PCl3 + EtOH → PCl2(OEt) + HCl In the absence of base, however, with excess alcohol, phosphorus trichloride converts to diethylphosphite:

PCl3 + 3 EtOH → (EtO)2P(O)H + 2 HCl + EtCl Secondary amines (R2NH) form aminophosphines. For example, bis(diethylamino)chlorophosphine, is obtained from direct reaction of diethylamine and PCl3. Thiols (RSH) form P(SR)3. An industrially relevant reaction of PCl3 with amines is phosphonomethylation, which employs formaldehyde:

R2NH + PCl3 + CH2O → (HO)2P(O)CH2NR2 + 3 HCl The common herbicide glyphosate is produced this way. The reaction of PCl3 with Grignard reagents and organolithium reagents is a useful method for the preparation of organic phosphines with the formula R3P (sometimes called phosphanes) such as triphenylphosphine, Ph3P.

3 RMgBr + PCl3 → R3P + 3 MgBrCl Triphenylphosphine is produced industrially by the reaction between phosphorus trichloride, chlorobenzene, and sodium:

PCl3 + 3 PhCl + 6 Na → PPh3 + 6 NaCl, where Ph = C6H5 Under controlled conditions or especially with bulky R groups, similar reactions afford less substituted derivatives such as chlorodiisopropylphosphine.

Conversion of alcohols to alkyl chlorides Phosphorus trichloride is commonly used to convert primary and secondary alcohols to the corresponding chlorides. As discussed above, the reaction of alcohols with phosphorus trichloride is sensitive to conditions. The mechanism for the ROH →RCl conversion involves the reaction of HCl with phosphite esters:

P(OR)3 + HCl ⇌ HP(OR)+3Cl− HP(OR)+3Cl− → RCl + HOP(OR)2. HOP(OR)2 + HCl ⇌ H2OP(OR)+2Cl− H2OP(OR)+2Cl− → RCl + (HO)2P(OR) (HO)2P(OR) + HCl ⇌ H(HO)2P(OR)+Cl− H(HO)2P(OR)+Cl− → RCl + (HO)3P The first step proceeds with nearly ideal stereochemistry but the final step far less so owing to an SN1 pathway.

Redox reactions Phosphorus trichloride undergoes a variety of redox reactions:

3PCl3 + 2 CrO3 → 3POCl3 + Cr2O3 PCl3 + SO3 → POCl3 + SO2 3 PCl3 + SO2 → 2POCl3 + PSCl3

PCl3 as a nucleophile Phosphorus trichloride has a lone pair, and therefore can act as a Lewis base, e.g., forming a 1:1 adduct Br3B-PCl3. Metal complexes such as Ni(PCl3)4 are known, again demonstrating the ligand properties of PCl3. This Lewis basicity is exploited in the Kinnear–Perren reaction to prepare alkylphosphonyl dichlorides (RP(O)Cl2) and alkylphosphonate esters (RP(O)(OR')2). Alkylation of phosphorus trichloride is effected in the presence of aluminium trichloride give the alkyltrichlorophosphonium salts, which are versatile intermediates:

PCl3 + RCl + AlCl3 → RPCl+3 + AlCl−4 The RPCl+3 product can then be decomposed with water to produce an alkylphosphonic dichloride RP(=O)Cl2.

PCl3 as a ligand PCl3, like the more popular phosphorus trifluoride, is a ligand in coordination chemistry. One example is Mo(CO)5PCl3.

Uses PCl3 is important indirectly as a precursor to PCl5, POCl3 and PSCl3, which are used in the synthesis of herbicides, insecticides, plasticisers, oil additives, and flame retardants. For example, oxidation of PCl3 gives POCl3, which is used for the manufacture of triphenyl phosphate and tricresyl phosphate, which find application as flame retardants and plasticisers for PVC. PCl3 is the precursor to triphenylphosphine for the Wittig reaction, and phosphite esters which may be used as industrial intermediates, or used in the Horner-Wadsworth-Emmons reaction, both important methods for making alkenes. It can be used to make trioctylphosphine oxide (TOPO), used as an extraction agent, although TOPO is usually made via the corresponding phosphine. PCl3 is also used directly as a reagent in organic synthesis. It is used to convert primary and secondary alcohols into alkyl chlorides, or carboxylic acids into acyl chlorides, although thionyl chloride generally gives better yields than PCl3.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Phosphorus trichloride

Start with the simplest possible case. Write down what Phosphorus trichloride 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 Phosphorus trichloride 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 Phosphorus trichloride 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 Phosphorus trichloride

In research: Phosphorus trichloride 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 Phosphorus trichloride 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: Phosphorus trichloride is common in secondary-school and first-year university syllabi. It links to neighbouring topics Foul-smelling chemicals, Inorganic phosphorus compounds, Phosphorus(III) compounds, so understanding it makes those chapters shorter.
In everyday life: Look for Phosphorus trichloride 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 Phosphorus trichloride in 20 minutes

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

Frequently asked questions

What is Phosphorus trichloride in simple terms?

Why does Phosphorus trichloride 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 Phosphorus trichloride?

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 Phosphorus trichloride.