Phosphoryl chloride

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Phosphoryl chloride
Phosphoryl chloride
Phosphoryl-chloride-3D-vdW.png Phosphoryl-chloride-3D-balls.png
POCl3.jpg
Identifiers
CAS number 10025-87-3 YesY
PubChem 24813
ChemSpider 23198 YesY
EC number 233-046-7
UN number 1810
ChEBI CHEBI:30336 YesY
RTECS number TH4897000
Jmol-3D images Image 1
Properties
Molecular formula POCl3
Molar mass 153.33 g/mol
Appearance Clear, colourless liquid, fumes in moist air
Density 1.645 g/cm3, liquid
Melting point 1.25 °C (274.4 K)
Boiling point 105.8 °C (379.0 K)
Solubility in water Reacts
Structure
Molecular shape tetrahedral
Dipole moment 2.54 D
Hazards
MSDS External MSDS
EU Index 015-009-00-5
EU classification Very toxic (T+)
Harmful (Xn)
Corrosive (C)
R-phrases R14, R22, R26, R35, R48/23
S-phrases (S1/2), S7/8, S26, S36/37/39, S45
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 4: Very short exposure could cause death or major residual injury. E.g., VX gas Reactivity code 2: Undergoes violent chemical change at elevated temperatures and pressures, reacts violently with water, or may form explosive mixtures with water. E.g., phosphorus Special hazard W: Reacts with water in an unusual or dangerous manner. E.g., cesium, sodiumNFPA 704 four-colored diamond
Related compounds
Related compounds Thiophosphoryl chloride
Phosphorus oxybromide
Phosphorus trichloride
Phosphorus pentachloride
Supplementary data page
Structure and
properties
n, εr, etc.
Thermodynamic
data
Phase behaviour
Solid, liquid, gas
Spectral data UV, IR, NMR, MS
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
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Infobox references

Phosphoryl chloride (commonly called phosphorus oxychloride) is a colourless liquid with the formula POCl3. With the freezing point of 1 °C and boiling point 106 °C, the liquid range is very similar to water. It hydrolyses in moist air releasing phosphoric acid and choking fumes of hydrogen chloride. It is manufactured industrially on a large scale from phosphorus trichloride and oxygen or phosphorus pentoxide. It is mainly used to make phosphate esters such as tricresyl phosphate.

Structure[edit]

Like phosphate, phosphoryl chloride is tetrahedral in shape. It features three P-Cl bonds and one strong P=O double bond, with an estimated bond dissociation energy of 533.5 kJ/mol. On the basis of bond length and electronegativity, the Schomaker-Stevenson rule suggests that the double bond form is very dominant (in contrast with POF3). The P=O bond does not utilize the d-orbitals on phosphorus as is commonly described in older textbooks as quantum chemical calculations have shown that d-orbitals are not involved in main group chemical bonding (see Hypervalent molecule). More modern texts favour a description which involves the donation of the lone pair electrons from oxygen p-orbitals to the antibonding phosphorus-chlorine bonds thus constituting π bonding.[1]

POCl3 structure.png

where pm = picometers

Chemical properties[edit]

POCl3 reacts with water and alcohols to give hydrogen chloride and phosphoric acid or phosphate esters, respectively:

O=PCl3 + 3 H2O → O=P(OH)3 + 3 HCl

If the water is replaced by an alcohol, the trialkyl phosphate esters result. Such reactions are often performed in the presence of an HCl acceptor such as pyridine or an amine. If POCl3 is heated with an excess of a phenol (ArOH) in the presence of a Lewis acid catalyst such as magnesium chloride, a triaryl phosphate ester such as triphenyl phosphate is formed. For example:

3 C6H5OH + O=PCl3 → O=P(OC6H5)3 + 3 HCl

POCl3 can also act as a Lewis base, forming adducts with a variety of Lewis acids such as titanium tetrachloride:

Cl3P+-O + TiCl4 → Cl3P+-O-TiCl4

The aluminium chloride adduct (POCl3·AlCl3) is quite stable, and so POCl3 can be used to remove AlCl3 completely from reaction mixtures at the end of a Friedel-Crafts reaction. POCl3 reacts with hydrogen bromide in the presence of AlCl3 to produce POBr3.

Preparation[edit]

Phosphoryl chloride can be prepared by the reaction of phosphorus trichloride with oxygen at 20–50 °C (air is ineffective):

2 PCl3 + O2 → 2 O=PCl3

An alternative synthesis involves the reaction of phosphorus pentachloride (PCl
5
) and phosphorus pentoxide (P
4
O
10
). Since these compounds are both solids, a convenient way of performing the reaction is to chlorinate a mixture of PCl3 and P4O10, which generates the PCl5 in situ. As the PCl3 is consumed, the POCl3 becomes the reaction solvent.

6 PCl3 + 6 Cl2 → 6 PCl5
6 PCl5 + P4O10 → 10 POCl3

Phosphorus pentachloride also forms POCl3 by reaction with water, but this reaction is less easily controlled than the above reaction.

Uses[edit]

The most important use for phosphoryl chloride is in the manufacture of triarylphosphate esters (as described above) such as triphenyl phosphate and tricresyl phosphate. These esters have been used for many years as flame retardants and plasticisers for PVC. Meanwhile trialkyl esters such as tributyl phosphate (made similarly from butan-1-ol) are used as liquid-liquid extraction solvents in nuclear reprocessing and elsewhere.

In the semiconductor industry, POCl3 is used as a safe liquid phosphorus source in diffusion processes. The phosphorus acts as a dopant used to create N-type layers on a silicon wafer.

In the laboratory, POCl3 is widely used as a dehydrating agent, for example the conversion of primary amides to nitriles. Similarly, certain aryl amides can be cyclised to dihydroisoquinoline derivatives using the Bischler-Napieralski reaction.

Two uses for phosphorus oxychloride in organic chemistry

Such reactions are believed to go via an imidoyl chloride; in certain cases where it is stable, the imidoyl chloride is the final product. For example pyridones and pyrimidones can be converted to chloro- derivatives of pyridines and pyrimidines, which are important intermediates in the pharmaceutical industry.[2]

Another common laboratory use for POCl3 is in the Vilsmeier-Haack reaction, where it reacts with amides to produce a "Vilsmeier reagent," a chloro-iminium salt, which subsequently reacts with electron-rich aromatic compounds to produce aromatic aldehydes upon aqueous work-up.[3]

References[edit]

  1. ^ D. B. Chesnut (1999). "The Electron Localization Function (ELF) Description of the PO Bond in Phosphine Oxide". Journal of the American Chemical Society 121 (10): 2335–2336. doi:10.1021/ja984314m. 
  2. ^ R. C. Elderfield, Heterocyclic Compounds, Vol. 6, p 265
  3. ^ Charles D. Hurd and Carl N. Webb (1925), "p-Dimethylaminobenzophenone", Org. Synth. ; Coll. Vol. 1: 217 

Further reading[edit]

  1. N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, 2nd ed., Butterworth-Heinemann, Oxford, UK, 1997.
  2. Handbook of Chemistry and Physics, 71st edition, CRC Press, Ann Arbor, Michigan, 1990.
  3. J. March, Advanced Organic Chemistry, 4th ed., p. 723, Wiley, New York, 1992.
  4. The Merck Index, 7th edition, Merck & Co, Rahway, New Jersey, USA, 1960.
  5. A. D. F. Toy, The Chemistry of Phosphorus, Pergamon Press, Oxford, UK, 1973.
  6. L. G. Wade, Jr., Organic Chemistry, 6th ed., p. 477, Pearson/Prentice Hall, Upper Saddle River, New Jersey, USA, 2005.
  7. B. J. Walker, Organophosphorus chemistry, p101-116, Penguin, Harmondsworth, UK, 1972.
  8. CDC - NIOSH Pocket Guide to Chemical Hazards