Phosphorus is a critical ingredient in
fertilizers, pesticides, detergents, and other industrial and household
chemicals. Once phosphorus is mined from rocks, getting it into these products
is hazardous and expensive, and chemists have been trying to streamline the
process for decades.
MIT chemistry professor Christopher
Cummins and one of his graduate students, Daniel Tofan, have developed a new
way to attach phosphorus to organic compounds by first splitting the phosphorus
with ultraviolet light. Their method, described in an online edition of Angewandte Chemie, eliminates the need
for chlorine, which is usually required for such reactions and poses health
risks to workers handling the chemicals.
Guy Bertrand, chemistry professor at the
University of California
at Riverside,
says the beauty of the discovery is its simplicity. “It is amazing to realize
that nobody thought earlier about such a simple approach to incorporate
phosphorus into organic molecules,” he says. “Such a synthetic approach to
organophosphorus compounds is indeed urgent, since the old (chlorine)-based
phosphorus chemistry has a lot of undesirable consequences on our environment.”
While the new reaction cannot produce
the quantities needed for large-scale production of phosphorus compounds, it
opens the door to a new field of research that could lead to such industrial
applications, says Bertrand, who was not involved in the research.
Extracting phosphorus
Most natural phosphorus deposits come from fossilized animal skeletons, which
are especially abundant in dried-up seabeds. Those phosphorus deposits exist as
phosphate rock, which usually includes impurities such as calcium and other
metals that must be removed.
Purifying the rock produces white
phosphorus, a molecule containing four phosphorus atoms. White phosphorous is
tetrahedral. Known as P4, white phosphorus is the most stable form
of molecular phosphorus.
For most industrial uses, phosphorus has
to be attached one atom at a time, so single atoms must be detached from the P4
molecule. This is usually done in two steps. First, three of the molecules in P4
are replaced with chlorine, resulting in PCl3.
Those chlorine atoms are then displaced
by organic (carbon-containing) molecules, creating a wide variety of
organophosphorus compounds such as those found in pesticides. However, this
procedure is both wasteful and dangerous; so chemists have been trying to find
new ways to bind phosphorus to organic compounds without using chlorine.
A new reaction
Cummins has long been fascinated with phosphorus, in part because of its
unusual tetrahedral P4 formation. Phosphorus is in the same column
of the periodic table as nitrogen, whose most stable form is N2, so
chemists expected that phosphorus might form a stable P2 structure. However,
that is not the case.
For the past few years, Cummins’
research group has been looking for ways to break P4 into P2
in hopes of attaching the smaller phosphorus molecule to organic compounds. In
the new study, Cummins drew inspiration from a long overlooked paper, published
in 1937, which demonstrated that P4 could be broken into two molecules
of P2 with ultraviolet light. In that older study, P2
then polymerized into red phosphorus.
Cummins decided to see what would happen
if he broke apart P4 with UV light in the presence of organic
molecules that have an unsaturated carbon-carbon bond. After 12 hours of UV
exposure, he found that a compound called a tetra-organo diphosphane had
formed, which includes two atoms of phosphorus attached to two molecules of the
organic compound.
This suggests, but does not conclusively
prove, that P2 forms and then immediately bonds to the organic
molecule. In future studies, Cummins hopes to directly observe the P2
molecule, if it is indeed present.
Cummins also plans to investigate what
other organophosphorus compounds can be synthesized with ultraviolet light,
including metallic compounds. He has already created a nickel-containing
organophosphorus molecule, which could have applications in electronics.
SOURCE
