Phosphorus oxychloride (molecular formula: POCl3), also known as phosphorus oxychloride, is a colorless liquid at room temperature. It fumes in moist air and hydrolyzes into phosphoric acid and irritating hydrochloric acid droplets. Industrially, it is prepared by reacting phosphorus trichloride with oxygen or phosphorus pentoxide, and is mainly used to produce phosphate esters such as tricresyl phosphate.
Structure
The phosphorus atom in phosphorus oxychloride is in a tetrahedral configuration, containing three P-Cl bonds and one P=O double bond. The P=O bond dissociation energy is approximately 533.5kJ/mol Japanese Kao. According to the data on bond length and electronegativity, the Schomaker-Stevenson rule states that the double bond type contributes more to the following resonance formulas, but among POF3 of the same family, the charge separation type structure contributes more. The P=O double bond is also not similar to the carbonyl π bond in ketones. In the past, textbooks believed that the phosphorus d orbital overlapped with the oxygen p orbital. Now most people believe that the π bond in the P-O bond is anti-bonded with the σ* of the P-Cl bond. Orbitals are related, and the role of d orbitals is not considered.
Preparation
Phosphorus oxychloride can be prepared by reacting phosphorus trichloride with oxygen at 20-50 °C (using air is less efficient):
2 PCl3 + O2 → 2 O=PCl3
Or antioxidant manufacturers use the reaction between phosphorus pentachloride and phosphorus pentoxide. However, the reactants are all solid and the reaction effect is not good. Chlorine the mixture of PCl3 and P4O10 with chlorine gas, and the generated PCl5 will react with P4O10 at the same time, and the effect will be better. The POCl3 produced can itself be used as the reaction solvent:
6 PCl3 + 6 Cl2 → 6 PCl5
6 PCl5 + P4O10 → 10 POCl3
POCl3 will also be produced when phosphorus pentachloride is hydrolyzed, but there are many side reactions and the reaction is difficult to control.
Purpose
Phosphorus oxychloride is most commonly used in the production of triaryl phosphates (such as triphenyl phosphate and tricresyl phosphate), which can be used as flame retardants and plasticizers for polyvinyl chloride. Trialkyl phosphates such as tributyl phosphate (prepared from 1-butanol through a similar reaction) are solvents for liquid-liquid extraction and are used in industries such as nuclear fuel reprocessing.
In the semiconductor industry, POCl3 is also used as a source of phosphorus in the diffusion process to make N-type silicon semiconductors by doping.
POCl3 is used as a water-losing agent in the laboratory to convert amides into nitriles. Certain amides can be cyclized through the Bischler-Napieralski reaction to generate dihydroisoquinoline derivatives:
If the R-C(=NH)-Cl (imidoyl chloride) intermediate passed by the above reaction is stable enough, the reaction can stay at this stage. For example, POCl3 can convert pyridones and pyrimidinones into the corresponding chlorinated products, which are important precursors in the pharmaceutical industry. At 140 °C, phosphorus oxychloride reacts with barbituric acid to form 2,4,6-trichloropyrimidine:
In the presence of phosphorus oxychloride, the activated aromatic ring undergoes Vilsmeier-Haack reaction to obtain aromatic aldehydes or aromatic ketones. The acylation reagent in the reaction is usually DMF or N-phenyl-N-methylformamide, and the intermediate product imine salt is easily hydrolyzed to generate aldehyde. For example, using anthracene as a reactant to obtain 9-anthracenecarboxaldehyde:
