Phosphine

Phosphine, the simplest phosphorus hydride, is a colorless and extremely toxic gas. Some people think it smells like rotting fish; it reminds others of the odor of garlic. In any case, pure phosphine is actually odorless; an impurity, diphosphane (P₂H₄), is responsible for its foul scent.

Phosphine is formed naturally via the anaerobic decay of phosphorus-containing organic matter. The earliest reported preparation was made by Antoine Lavoisier’s student Philippe Gengembre in the late 18th century.

Phosphine is made industrially from white phosphorus by hydrolysis with an alkali metal hydroxide or an aqueous acid–catalyzed disproportionation reaction. The industrial product is normally shipped as liquefied gas.

Although phosphine’s molecular structure is analogous to that of ammonia, the 3d orbital of its phosphorus atom interacts with its hydrogen atoms to reduce its ability to hydrogen bond. Thus, whereas ammonia is completely miscible in water, the aqueous solubility of phosphine is very low.

The hazard information box shows that phosphine is dangerous in several ways on Earth. But recent findings indicate that its presence in outer space may have had a key role in the beginnings of life.

Ralf I. Kaiser at the University of Hawaii at Manoa (Honolulu) and his colleagues there and in Taiwan and France simulated ices found in space by combining phosphine, carbon dioxide, water, and oxygen at near–absolute zero temperatures. They then bombarded the ices with high-energy electrons (typical of those found in space) to produce several phosphorus oxyacids, including some with the same phosphorus oxidation state that exists in DNA, ATP, and other important biomolecules.

The theme of this year’s National Chemistry Week (which begins today) is “Chemistry Is Out of This World”. Celebrate it by recognizing extraterrestrial phosphine’s potential contribution to life.