Cacodyl

  Cacodyl, dicacodyl, tetramethyldiarsine, alkarsine or minor part of the "Cadet's fuming liquid" (after the French chemist Louis Claude Cadet de Gassicourt) (CH₃)₂As—As(CH₃)₂ is a poisonous oily liquid with a garlicky odor. Cacodyl undergoes spontaneous combustion in dry air.

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Preparation

Cacodyl oxide is prepeared by the reaction of potassium acetate with arsenic trioxide.

4KCH₃COO + As₂O₃ → As₂(CH₃)₄O + 4K₂CO₃ + CO₂

A subsequent reduction or disproportionation of the substance at the reaction condition is leading to a mixture of several methylated arsenic compounds.

A far better synthesis was developed which started from the dimethyl arsine chloride and dimethyl arsine.

As(CH₃)₂Cl + As(CH₃)₂H → As₂(CH₃)₄ + HCl

History

Jöns Jakob Berzelius coined the name kakodyl (later changed to cacodyl) for the dimethylarsinyl radical, (CH₃)₂As, from the Greek kakodes (evil-smelling) and hyle (matter).^[1]

It was investigated by Edward Frankland and Robert Bunsen and is considered one of the earliest organometallic compounds ever discovered. It was originally made from arsenic distilled with potassium acetate.

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.

Use

Cacodyl was used to proof the radical theory of Berzelius, which resulted in a wide use of cacodyl in research laboratories. After the radical theory was established the interest in the toxic substance with the literally evil smell decreased. During World War I the use of cacodyl as poison gas was considered, but it was never used in the war. The inorganic chemistry discovered the properties of cacodyl as ligand for transition metals.