Methylphosphonyl difluoride (DF), also called methyl difluorophosphite, methylphosphonic difluoride, and difluoromethylphosphine oxide, is a chemical weapon precursor. Its chemical formula is CH₃POF₂. It is a Schedule 1 substance in the sense of the Chemical Weapons Convention. It is used for production of sarin and soman as a component of binary chemical weapons; an example is the M687 artillery shell, where it is used together with a mixture of isopropyl alcohol and isopropyl amine, producing sarin.

Methylphosphonyl difluoride can be prepared by reacting methylphosphonyl dichloride with hydrogen fluoride (HF) or sodium fluoride (NaF).

Methylphosphonyl difluoride is both reactive and corrosive. It is absorbed through skin and cause burns and mild nerve agent symptoms. It reacts with water producing HF fumes and methylphosphonic acid as a result. It is also capable of corroding glass. It has a boiling point of only 55.4 °C at normal atmospheric pressure, and thus a fairly high vapor pressure at room temperature.

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