Inorganic chemistry can be defined as “the chemistry of all the elements of the periodic table,” but as such, the field is impossibly broad, encompassing everything from organic chemistry to materials science and enzymology. One way to gain insight into and appreciate the rapidly moving and diverse field of inorganic chemistry is to view the science from the perspective of the elements themselves, since they are the basic ingredients for assembling molecules or materials–and indeed, all matter, living or in animate.
Although phosphorus may be less celebrated than carbon or hydrogen, it joins those elements (along with nitrogen, oxygen, and sulfur) to constitute the six “biogenic elements” (those needed in large quantities to make living organisms). Let us take a look at some of the issues that arise in inorganic chemistry from the perspective of phosphorus, illustrating in the process the notion that each element has its own story to tell. Many phosphorus-containing chemical compounds are commercially valuable and have interesting or important applications. Lithium hexafluoro – phosphate, for example, is the electrolyte in common lithium-ion batteries, which are used in consumer electronics (such as laptops) and automotive applications. So how is it made? The synthesis route begins with the white form of elemental phosphorus, a simple molecular form of the element consisting of tetrahedral P4 molecules. White phosphorus is combined with elemental chlorine in order to bring the phosphorus to the correct oxidation state (+5), and then, in a second step, chloride is re placed by fluoride.
This process is also frequently used to synthesize many organo-phosphorus compounds that are important components of catalysts used in the chemical industry. In these applications, again, white phosphorus is first oxidized using chlorine, and then the chloride provides the basis for the formation of carbon-phosphorus bonds. But notably, elemental chlorine is hazardous to use and ship, and environmental groups have called for an outright ban on it. So why use chlorine to oxidize phosphorus if chlorine is not even present in the products, such as lithium hexafluoro – phosphate, that are the target of synthesis? These industry standard processes suggest there is room for improvement: if manufacturers eliminated the use of chlorine in the synthesis of important phosphorus compounds in which chlorine is absent, both hazards and waste would be significantly reduced.
Research has shown that it is indeed possible to derive organo-phospho rus compounds directly from white phosphorus, this is an opportunity for inorganic chemistry to improve the safety and efficiency of the manufacturing process. In one advance, it was shown that phosphorus-carbon bonds can be generated by using white phosphorus together with a source of organic radicals. Each of the six phosphorus-phosphorus bonds present in a molecule of white phosphorus absorbs two organic radicals in the process of being broken; each P4 tetrahedron is broken completely apart, and each phosphorus atom becomes incorporated into a freshly formed organo-phosphorus compound.
The method for developing this new process was derived from basic inquiries into phosphorus’s relationship to the elements neighboring it on the periodic table. Phosphorus is immediately beneath nitrogen on the periodic table, suggesting that these elements should have some similarities in their chemical properties. Then why was it the case that, while Earth’s atmosphere consists mainly of triply-bonded N2 molecules, a similar di-atomic molecular form of phosphorus is neither prevalent nor even particularly stable? Part of the answer is that nitrogen is unusual because the stability of its multiple bond far exceeds that of the sum of an equivalent number (three) of its single bonds. So the only stable form of elemental nitrogen is the diatomic molecular form floating innocuously about in the atmosphere we breathe; in contrast, phosphorus (like its diagonal relative, carbon) exists in a wide variety of structural arrangements, all of which are networks exclusively based upon phosphorus-phosphorus single bonds, three for every phosphorus node. The variant known as red phosphorus, for example, has cages of phosphorus atoms connected into linear tubes which in turn are cross-linked together to form a polymeric net work.
Can we design and synthesize a molecule that would be prone to a fragmentation reaction wherein one of the fragments produced would be the diatomic molecule P2? If we could, we would have the opportunity to study the properties and chemical characteristics of an all-phosphorus molecule structurally analogous to the main constituent of Earth’s atmosphere. In the first attempt to produce it, the selected design incorporated a feature patterned after the reaction used to inflate an automobile airbag in the event of a collision, a process that rapidly generates nitrogen gas from a solid precursor. The target molecule embedded a di-phosphorus moiety into the stabilizing environment of a niobium complex (niobium is a transition metal; it forms complexes by arranging sets of molecules or ions–called ligands–around itself ), from which it could be released by a stimulus of mild heating.Carrying out the fragmentation reaction in the presence of other molecules permitted the mapping of the reactivity patterns of diatomic phosphorus. One important result was the discovery that P2 easily undergoes addition to unsaturated organic molecules, such as 1,3- cyclohexadiene.
If diatomic molecular phosphorus is indeed capable of direct combination with organic molecules, then the means of its generation should not matter. Could there be a way to access the P2 molecule by starting from a stable form of the element, rather than from an exotic niobium complex? The researchers found the suggestion in a lightly cited 1937 paper that the photochemical conversion of white phosphorus into the red form of the element may occur with P2 as the key intermediary, which is initially generated and subsequently polymerizes. The addition of methyl isoprene to a solution of white phosphorus during irradiation both inhibits the production of red phosphorus and yields molecules in the same class of organo-phosphorus compounds that was studied earlier in connection with niobium-mediated access to di – phosphorus molecules. Hence, in effect and in principle, it has been shown that in certain cases the hazardous and wasteful use of chlorine in the synthesis of organo-phosphorus com pounds can be replaced with a process relying on ultraviolet radiation.
After conceptualizing the beautiful tetrahedral molecular form of elemental phosphorus , one might wonder whether this arrangement of phosphorus is unique to this particular element. Arsenic (As) lies just below phosphorus on the periodic table, separated from it by the stair-step line dividing the metals from the nonmetals. Once again, the periodicity of chemical properties suggests that molecular arsenic might adopt a similar tet rahedral struc ture to that of phosphorus. Indeed, it does, but only in the gas phase where the molecules are well isolated from one an other, or in solution at low temperature and in the dark.
To generate gas phase As4 molecules, one heats grey arsenic (which has a layered sheet structure reminiscent of graphite or black phosphorus) to about 550 degrees while flowing a carrier gas over it. The As4 molecules, en trained in the carrier gas, can be led into a solvent and used for reaction chemistry before re-polymerization to grey arsenic can take place. If condensed to a solid on a cold surface, the As4 condensate is “yellow arsenic,” but it cannot be kept. Warming to room temperature or exposure to light brings about a facile return to the grey form.
Phosphorus and arsenic lie on either side of the divide separating the metals from the non-metals. White phosphorus is stable enough that it can be stored as a pure liquid above its melting point of 44 degrees C and pumped into tank cars for shipping; while, conversely, samples of yellow arsenic are evanescent. It is legitimate to wonder would it be possible to synthesize a stable substance whose tetrahedral molecules would be composed of a mixture of phosphorus and arsenic To test this idea, researchers made a niobium complex carrying a P3 3− unit, and combined this with a source of arsenic (3+), effectively knitting together the neutral AsP3 molecule in a selective fashion. The new sub stance turned out to have a waxy appearance much like that of white phosphorus, and it could be purified by sublimation, wherein the pure material is condensed on to a cold probe. Because of the volatile nature of AsP3, the researchers determined its properties by a variety of techniques, including electron diffraction, microwave spectroscopy, and photo-electron spectroscopy.
Obtaining gas-phase property data on a simple molecule containing a heavy element (arsenic) provides a benchmark for theorists working on the apriori prediction of properties; heavy elements pose the greatest challenge in this regard. The elements in the AsP3 molecule are packaged together in a 1:3 ratio at the molecular level; and now this substance is readily avail able as a starting material. Substitution of a single nitro gen atom into the P4 tetrahedron has scarcely been considered; one possibility involves stabilization inside a recently dis covered spherical B80 molecule that is analogous to Buck min sterfullerene (C60).
To ask why diatomic phosphorus is neither stable nor prevalent is really to ask a larger question: why is elemental phosphorus not found on Earth as a pure substance uncombined with other chemical elements? It is because elemental phosphorus is especially prone to oxidation, a process encouraged by Earth’s atmosphere at this point in history. Elements that form very stable oxides (such as aluminum, phosphorus, and silicon) are not found in uncombined form on our planet unless they can be formed by biological or geological processes taking place under anaerobic conditions (as in the case of volcanic sulfur, or carbon in the form of coal and diamond). If we cannot obtain phosphorus in pure form directly by digging it out of the ground, where do we get it?
Phosphate rock (also known by its mineral name apatite) is essentially the bones and teeth of ancient marine organisms formed into concentrated deposits where long-evaporated seas once stood.It is extracted through strip mining and forms the basis for the phosphorus fine chemicals industry. One of the principal methods for white phosphorus production is the “thermal process,” which involves use of an electric arc furnace, carbon in the form of coke as a reducing agent, and silica to absorb the oxide ions liberated in the heating process. The elemental phosphorus is thus extracted from the rock in what is essentially an expensive purification process.
Note that most phosphorus-containing commercial chemicals contain phosphorus in the +5 oxidation state: the same as is found in phosphate rock when it is dug out of the ground. The typical purification process reduces phosphorus’s oxidation state from +5 to zero; however, when it is converted to other chemicals, chlorine is often used to return the phosphorus to its highest oxidation state (zero back to +5). (This method of making white phosphorus is, in fact, reminiscent of the one used by the alchemist Hennig Brand, who made phosphorus the thirteenth element to be obtained in pure form. In search of the philosopher’s stone, the alchemist collected great quantities of human urine, which he concentrated to a paste and subjected to reductive distillation.)
Phosphate rock is not only the basis for the fine chemicals industry of phosphorus; it is also the starting point for the (much larger) phosphorus side of the fertilizer industry. The “wet process” of purification uses sulfuric acid to generate phosphoric acid from phosphate rock, after which it can be made into critical fertilizers such as mono ammonium phosphate, or map. Around 1940, the human population of our planet began to rise more rapidly than it had previously .This critical rise in population growth coincided with two important developments in the fertilizer industry: the worldwide commercial de-ployment of the Haber-Bosch ammonia synthesis (whereby ammonia for agricultural applications is obtained by direct combination of the elements hydrogen and nitrogen); and the large-scale mining of phosphate rock deposits, mainly for fertilizer applications.
Prior to the mid-twentieth century, humankind had been largely limited to locally available nutrients for crop production. Now, ammonia can be had in essentially limitless supply by combining the atmosphere’s inexhaustible supply of nitrogen with hydrogen (which is currently derived from natural gas by steam reforming). Can phosphorus keep up?
Stars are the element factories.They consist mainly of our universe’s lightest and most abundant elements: hydrogen and helium. Red giants are more evolved stars with an onion-like layered structure; the most abundant metallic elements, iron and nickel, make up their core, and layers of progressively lighter elements surround them, moving outward to the surface. Elements heavier than iron and nickel are formed by neutron capture when a massive star explodes in a supernova, and these (including the precious gold sought by the alchemist) are of minimal cosmic abundance. It is one of the peculiarities of nuclear physics that nuclei of odd atomic number (odd Z) are generally less stable and less abundant than those of even Z.
The only stable isotope of phosphorus is 31P(Z= 15), and the 31Pnucleus is the product of an extremely improbable sequence of nuclear reactions (the final reaction in the sequence converts 31Si into 31P by pro – ton capture), only taking place during an explosive neon burning phase in the core of massive, hot stars.Accordingly, the cosmic abundance of phosphorus is lower –by orders of magnitude–than that of the other five biogenic elements. Indeed, to quote astro biologist Douglas Whittet: “The only biogenic element present in the human body (and in biological tissue generally) at a concentration substantially above its solar abundance is P. If one were to attempt to place an upper limit on the total biomass present in the Universe at large, on the basis of cosmic abundances, then the critical element would be phosphorus.” This is in keeping with the observation that, in many of the ecosystems on Earth, phosphorus is life-limiting. This means that the addition of phosphorus (usually in the form of phosphate) will bring about an abrupt bloom of life, since the absence of phosphorus was all that was holding it back.
Our land reserves of phosphorus are finite. And given the ongoing depletion of phosphate rock reserves, it is natural to ask what is left, where it is, and how long it will last. The U.S. Geological survey indicates that roughly three-quarters of the available reserves are concentrated in Morocco and Western Sahara.Mining locations in Florida and Idaho contain the most significant amount of phosphate rock in the United States, but these constitute a small percentage of global reserves. And Central Florida’s mines have been largely exhausted, leaving behind a legacy of radioactive phospho gypsum stacks and collapsing sinkholes. The term “peak phosphorus” is now used with reference to the point in time when phosphate rock production (mining) will inevitably begin to taper off. Current estimates place peak phosphorus some time later in the twenty- first century.
Off the coast of Brittany, France, there are sometimes blooms of marine algae vast enough to be visible from space. Brittany is a livestock-producing region where large amounts of phosphate from feed is transferred to the ground water and ultimately to the ocean. This perfectly illustrates two consequences of the large-scale mining of phosphate rock and industrialized agricultural activity: first, we are depleting the concentrated reservoirs of this key nutrient; second, its dispersal into the world’s oceans can have negative effects on marine ecosystems, chiefly by causing eutrophication through overgrowth of certain species of phytoplankton.
What can we do to mitigate the movement of phosphorus from land to sea? Efforts are being directed at optimizing the separation and recovery of phosphorus from waste water, which is an important direction.In some countries (such as India and Sweden), the use of toilets that separate liquid from solid waste is being adopted; phosphate can then be recovered from urine as the crystalline mineral struvite, while solid waste is composted.Pigs cannot digest plant-derived phosphate because of the phytic acid form in which plants store it so researchers at the University of Guelph in Canada developed the Enviropig. This genetically engineered pig secretes the enzyme phytase in its saliva, enabling the pig to digest the plant phosphate, whereupon its excreta are phosphate-poor, leading to an improvement in waste water quality. While the meat of the Enviropig is the same as that of an unmodified pig, concerns about this creative kind of genetic engineering have effectively blocked its adoption thus far.
The chemistry of an element is a fascinating thing, and we have explored several of the issues that flow naturally from asking questions about where an element comes from, what we use it for, and how we might gain an improved understanding of it. We have come to appreciate the vital role played by this relatively precious element that forms the inorganic backbone of DNA, the energy currency of ATP, and the main component of bones and teeth. We now have the ability to identify ways of using this limited resource that minimize waste, but have to acknowledge our limited ability to grapple with the consequences of enormous demand for phosphorus–a markedly limited resource– stemming from a rapidly rising human population. Phosphorus, therefore, is interesting not only for its chemistry but also in light of the rich texture of its larger story, only one of the many stories that emerge when we view inorganic chemistry from the perspective of a single element.
By Ashoka Jahnavi Prasad