Cacodyl

Cacodyl, also known as dicacodyl or tetramethyldiarsine, (CH₃)₂As–As(CH₃)₂, is an organoarsenic compound that constitutes a major part of "Cadet's fuming liquid" (named after the French chemist Louis Claude Cadet de Gassicourt). 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 (CH₃)₂As.

Preparation

A mixture of dicacodyl and cacodyl oxide ((CH₃)₂As–O–As(CH₃)₂) 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 (mass balance) corresponding to the oxide formation is the following:

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A more efficient synthesis was later developed which started from the dimethyl arsine chloride and dimethyl arsine:

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History

Robert Wilhelm Bunsen coined the name kakodyl (later modified to cacodyl in English) for the dimethylarsinyl radical, (CH₃)₂As, from the Greek κακώδης kakōdēs ("evil-smelling") and ὕλη hylē ("matter").

It was investigated by Edward Frankland and (for over six years) by Robert Bunsen and is considered the earliest organometallic compound discovered (even though arsenic is not a true metal). 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.