Cacodyl

Cacodyl, also known as dicacodyl or tetramethyldiarsine, ₂As–As₂, is an organoarsenic compound that constitutes a major part of "Cadet's fuming liquid". It is a poisonous oily liquid with an extremely unpleasant garlicky odor. Cacodyl undergoes spontaneous combustion in dry air.
Cacodyl is also the name of the functional group or radical ₂As.

Preparation

A mixture of dicacodyl and cacodyl oxide ₂As–O–As was first prepared by Cadet by the reaction of potassium acetate with arsenic trioxide. A subsequent reduction yields a mixture of several methylated arsenic compounds including dicacodyl. The global reaction corresponding to the oxide formation is the following:
A more efficient synthesis was later developed which started from the dimethyl arsine chloride and dimethyl arsine:

History

Robert Wilhelm Bunsen coined the name kakodyl for the dimethylarsinyl radical, ₂As, from the Greek κακώδης kakōdēs and ὕλη hylē.
It was investigated by Edward Frankland and by Robert Bunsen and is considered the earliest organometallic compound discovered. Bunsen began studying it in the 1830s and had determined its chemical composition by 1843; during this time Bunsen discovered cacodyl's flammability and extreme unpleasant odor. As this was prior to the invention of the fume hood, to avoid the unpleasant odor of the chemical Bunsen worked quickly and breathed through a long glass tube that extended to the outside of his laboratory. Despite these precautions, Bunsen learned the hard way just how dangerous and toxic cacodyl could be: he nearly died of arsenic poisoning and lost sight in one of his eyes after a glass vial of cacodyl exploded in close proximity to his face.
From cacodyl, other compounds were made, such as cacodyl fluoride, cacodyl chloride, et cetera. One compound, cacodyl cyanide, was particularly dangerous. In Bunsen's words "the smell of this body produces instantaneous tingling of the hands and feet, and even giddiness and insensibility...It is remarkable that when one is exposed to the smell of these compounds the tongue becomes covered with a black coating, even when no further evil effects are noticeable".
Work on cacodyl led Bunsen to the postulation of "methyl radicals" as part of the then-current radical theory.

Applications

Cacodyl was used to try to prove the radical theory of Jöns Jacob Berzelius, which resulted in a wide use of cacodyl in research laboratories. Afterward interest in the toxic, malodorous compound decreased. During World War I the use of cacodyl as a chemical weapon was considered, but it was never used in the war. Inorganic chemists discovered the properties of cacodyl as a ligand for transition metals.