Diphenylchloroarsine (DA) is the organoarsenic compound with the formula (C6H5)2AsCl. It is highly toxic and was once used in chemical warfare. It is also an intermediate in the preparation of other organoarsenic compounds. The molecule consists of a pyramidal As(III) center attached to two phenyl rings and one chloride. It was also known as sneezing oil during World War I by the Allies.
It was first produced in 1878 by the German chemists August Michaelis (1847–1916) and Wilhelm La Coste (1854–1885).[1][2] It is prepared by the reduction of diphenylarsinic acid with sulfur dioxide. An idealized equation is shown:[3]

Ph2AsO2H + SO2 + HCl → Ph2AsCl + H2SO4
The process adopted by Edgewood Arsenal, the "sodium process", for the production of DA for chemical warfare purposes employed a reaction between chlorobenzene and arsenic trichloride in the presence of sodium.[4][5]

The German process, used in the first war, applied at Hochstam-Main, used the Sandmeyer reaction between phenyldiazonium chloride and sodium arsenite. The acidified product was reduced and then neutralized. The salt was condensed again by the Sandmeyer reaction and reduced again, the final product was then acidified, resulting in DA.[5]

The structure consists of pyramidal As centre. The As-Cl distance is 2.26 A and the Cl-As-C and C-As-C angles are 96 and 105°, respectively.[6]
It is a useful reagent for the preparation of other diphenylarsenic compounds, e.g. by reactions with Grignard reagents:

RMgBr + (C6H5)2AsCl → (C6H5)2AsR + MgBrCl
(R = alkyl, aryl)
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