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 reaction forming the oxide is something like:
A far better synthesis was later developed which started from the dimethyl arsine chloride and dimethyl arsine:

History

coined the name kakodyl for the dimethylarsinyl radical, 2As, from the Greek kakodes and hyle.
It was investigated by Edward Frankland and by Robert Bunsen and is considered the earliest organometallic compound ever discovered.
From it other compounds were made, such as cacodyl fluoride, chloride, et cetera. One compound, cacodyl cyanide, was particularly awful. 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 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.