Chemical & Chemical Engineering News (80th Anniversary Issue), Vol. 81, No. 36, 2003, Sept. Edited by X. Lu Introduction

SELENIUM AT A GLANCE Name: From the Greek selene, moon. Atomic mass: 78.96.

History: Discovered in 1817 by Swedish chemist Jöns Jakob Berzelius after analyzing an impurity that was contaminating the sulfuric acid being produced at a factory in Sweden.

Occurrence: Occurs naturally in the rare minerals eucairite, crooksite, and clausthalite. Obtained commercially as a by-product of copper refining.

Appearance: Exists as two allotropes: red and gray. Red selenium, the less stable of the two, is an amorphous powder; gray selenium is a silvery metal.

Behavior: Burns in air, but is unaffected by water. Although ordinarily a poor conductor of electricity, gray selenium is a photoconductor, meaning it becomes an excellent conductor in the presence of light.

Uses: Used in photoelectric cells, photocopiers, solar cells, and semiconductors. Because it is a powerful photoconductor, gray selenium is valuable as a light sensor and is used in robotics, light-switching devices, and light meters. Compounds containing selenium are useful in controlling dandruff and are often added to shampoos.

TELLURIUM

DONALD C. DITTMER, SYRACUSE UNIVERSITY

The discovery of tellurium by Franz J. Müller in 1782 almost went unnoticed. He was examining the gold-containing ores of Transylvania, from one of which, aurum paradoxum, a supposedly pure sample of antimony (a neighbor of tellurium in the periodic table) had been obtained. After many experiments, the possibilities--antimony, bismuth, or an alloy of both--were eliminated. Müller called the new element, for that was what he had found, "metallum problematicum." He published his findings in an obscure journal, Physikalischen Arbeiten der einträchtigen Freunde in Wien, of which only two volumes were issued. The title of his work was equally obscure: "An Investigation of the Supposed Native Antimony from the Mariahilf Mine in the Facebaj Mountains Near Zalatna." In 1798, Müller's discovery was rescued from oblivion by Martin H. Klaproth, who examined the "problematical metal" and named it tellurium.

Elemental tellurium exists mainly in right- and left-handed helical chains of atoms, and reduction yields colored anions, Te_n^2–, in addition to colorless Te^2–. The first organic compound of tellurium, diethyl telluride, was reported in 1840 by Friedrich Wöhler. Since then, many chemists have investigated organic tellurium chemistry, with most activity being concentrated in the past 50 to 60 years.

GOLDEN TOUCH Gold telluride, or calaverite, is a rare mineral but an important source of gold. This photomicrograph shows calaverite crystals in a quartz matrix.

Müller was made Baron von Reichenstein in 1795 for his skills in mining administration, and he no doubt would be interested in the roles that tellurium is playing in recent times, for example, in the conversion of solar energy into electricity via CdTe, in the possible coating of the highlands of the planet Venus, in an episode of the 1950s TV science fiction program "Space Patrol," in triggering and catalyzing chemical reactions, and in alloys. The retail cost of tellurium varies from about $60 to $560 per mole, depending on form and purity.

Elemental tellurium originally was put on the list of "extremely hazardous substances," but a review of the study by the Environmental Protection Agency indicated that sodium tellurate was used instead of tellurium. The LD₅₀ is now given as 5,000 mg per kg for mice. Ingestion of tellurium by humans causes garlic-like "tellurium breath" because of the formation of dimethyl telluride. Tellurium compounds in microgram amounts occur fairly widely in plants (for example, onions, peas, and tea leaves), and larger quantities (31–73 µg per g) are found in garlic buds.

My interest in tellurium came as the result of an attempt to prepare telluracyclobutene, a four-membered cyclic compound with one tellurium atom and a carbon-carbon double bond, to be used as an electron donor in semiconductor formation. Treatment of epichlorohydrin (chloromethyloxirane) with sulfide ion gave 3-hydroxythiacyclobutane, a precursor of thiacy-clobutene (which does form a semiconducting material), but treatment with telluride ion gave allyl alcohol and elemental tellurium. This represents a nucleophilic reduction in which the powerful nucleophilic telluride ions, Te^2– or Te_n^2–, are oxidized and the organic compound is formally reduced. The first nucleophilic reduction by telluride ion, reported by W. V. Farrar and J. Masson Gulland (J. Chem. Soc. 1945, 11–14), was the unexpected conversion of 1,2-dibromoethane to ethylene by sodium telluride, with elemental tellurium also being produced.

The driving force for these reactions is caused by the high nucleophilicity of Te^2– and the thermodynamic instability of Te^2– with respect to elemental tellurium. Nucleophilic attack by telluride ion on an organic compound with an electrophilic site or sites opens a pathway whereby oxidation to the element occurs with concomitant transformation of the original organic compound to a new compound.

My coworkers found that Sharpless-Katsuki asymmetric epoxidation of primary allylic alcohols; conversion of the alcohol function to an electrophilic site with a good leaving group, typically a tosylate or mesylate; followed by treatment with telluride ion obtained by reduction of elemental tellurium produce new chiral secondary and tertiary allylic alcohols [J. Org. Chem., 58, 718 (1993)]. Similar results for the synthesis of allylic amine derivatives have been obtained via O-tosylates of aziridinemethanols or oxazolidinonemethanols [J. Org. Chem., 62, 7920 (1997); Tetrahedron Lett., 40, 2255 (1999); and 42, 5789 (2001)].

Müller's discovery is an interesting element whose unique properties, which may be inferred from its position in the periodic table, make it useful in a variety of organic chemical transformations as well as in the construction of alloys and other solid-state materials such as semiconductors and solar cells.

Donald C. Dittmer is a professor emeritus at Syracuse University. His research interests include telluride-induced nucleophilic reductions

TELLURIUM AT A GLANCE

Name: From the Latin tellus, earth.

Atomic mass: 127.60.

History: Discovered in Transylvania in 1782 by Franz J. Müller but forgotten until mentioned in a 1798 paper by German chemist Martin H. Klaproth. Klaproth named the element, but gave full credit for its discovery to Müller.

Occurrence: Occasionally found native, but more often found in calaverite or other minerals. Recovered commercially as a by-product of copper refining.

Appearance: Silvery white as a crystal, exhibits a metallic luster when pure, and is usually obtained as a dark gray powder.

Behavior: Very brittle, with low conductivity. It burns in air or oxygen.

Uses: Primarily used as an alloying agent with copper, steel, or lead. It is also used in blasting caps and ceramics

POLONIUM

CHERYL HOGUE, C&EN WASHINGTON

Were these scientists making a Polish joke? I was an adolescent with my nose deep into a book on heroes and heroines, soaking up a story about Marie Curie. I was learning how this extremely smart scientist spent hours in mind-numbing labor stirring a boiling concoction as she processed pitchblende in a quest to figure out exactly what in this uranium ore made it more radioactive than uranium itself. On July 18, 1898, she and husband Pierre discovered a new element. They named it polonium, after Marie's homeland, Poland.

"Poland!" I shouted to my book. Of all the nations in the world, how could the Curies name an element after a country whose citizens were the butt of all the jokes making the rounds at my school? Wasn't her name Marie? So very French! And did she not earn her degrees, work, marry, and die in France?

But I soon learned how much Marie, born Marya Sklodowska, loved her native country and yearned to return there, though she never did move back. A crack appeared in my adolescent outlook on life, allowing the thought to slip in that just maybe there was more to Poland than jokes.

Polonium, Marie's dear polonium, number 84 on the periodic chart, was the first element discovered via its radioactivity. In their work with pitchblende, the Curies next identified radium (named after the Latin word for ray). In 1899, André Debierne, a colleague of the Curies, teased out a third radioactive element from pitchblende--actinium.

Though polonium was the first highly radioactive element the Curies identified, radium became the star of their work. Marie, writing in December 1904, explained why: "Polonium, when it has just been extracted from pitchblende, is as active as radium, but its radioactivity slowly disappears." We know now that polonium's most stable isotope has a half-life of 138.39 days compared with the 1,620 years of radium's longest lived isotope. The Curies never isolated polonium, which is formed as one of the decay products of radium.

Though the scientific community was at first doubtful about the existence of polonium, the element was at last ensconced on the periodic table in 1905. All of its isotopes are radioactive.

Polonium has had a minor flare of celebrity in debate between creationists--who believe the universe, Earth, and the life upon it were created some 6,000 years ago during a seven-day period called Genesis Week--and those who subscribe to the Big Bang theory. At issue are "halos" of color found in granites. These halos are areas of damage in the crystalline structure of the rock.

Physicist Robert V. Gentry, a creationist, contends that these halos were formed through -particle emission. He says the size of these circles is linked to the amount of energy released during radioactive decay. He concludes that polonium provided the proper amount of energy to form these halos. Given polonium's half-live, measured in days, the radioactive decay of this element could form these circles of damage only if granites were created instantaneously, he argues.

	Marie Curie

This contrasts with the standard geological model for rock formation, which holds that the crystalline structure of granites developed though the slow cooling of magma deep within Earth over millions of years.

Not surprisingly, mainstream geologists refute Gentry's idea. They say these halos weren't caused by polonium. Instead, they say the circles of damage are probably due to the decay of other radioactive elements with a longer half-life--or that they aren't even created through radioactivity.

Neither the creationists or Big Bangers are budging. Polonium, meanwhile, is merrily forming and decaying.

With its short half-life, polonium isn't easy to come by. While the Curies processed pitchblende in a cast-iron basin to get polonium, the modern method, developed in 1934, involves bombarding bismuth-209 with neutrons to get polonium-210.

Oh, yes, polonium has its limited commercial uses. This metal is a source of -radiation and a heat source in space vehicles. It is also used in industrial equipment to eliminate static electricity.

Polonium is highly radioactive--it's hot. Its decay releases 140 W per g. This element should be handled with great care.

And that's no joke.


Cheryl Hogue is a senior editor who writes about environmental pollution for C&EN. She doesn't tell ethnic jokes but delights in limericks and puns.

POLONIUM AT A GLANCE

Name: Named for Poland, the native country of scientist Marie Curie.

Atomic mass: (209).

History: Discovered by Marie and Pierre Curie in 1898 while studying a material called pitchblende.

Occurrence: Formed chiefly through the decay of radioactive uranium and thorium.

Appearance: Silvery metal.

Behavior: Extremely toxic and radioactive. About half of a sample will evaporate within two days if kept at 55 ºC. Polonium dissolves readily in dilute acids but is only slightly soluble in alkalis.

Uses: Used in nuclear batteries, antistatic agents, and film cleaners. It is also used as a neutron source, as a lightweight heat supply for space satellites, and as a source of -radiation for research.

FLUORINE

NANCE DICCIANI,SPECIALTY MATERIALS, HONEYWELL

Fluorine is an element of many mosts: It is the most reactive of all of the elements, the most powerful oxidizing agent, and the most electronegative. But its true strength and the secret of its success lies in the compounds it forms--some of the most stable and inert substances known to man. These compounds have enabled a steady stream of scientific and commercial advances: Staying warm and dry in a downpour, cooking with the ease of a nonstick pan, and beaming a "cavity free" smile are all available thanks to the fluorine atom.

Fluorine occurs naturally as a mono-isotope ¹⁹F₉ and is not a rare element: About 0.07% of Earth's crust is comprised of fluorine, mostly as the minerals fluorapatite and fluorite (CaF₂), the main constituent of the ore fluorspar. Although very few naturally occurring organofluorine compounds are known to exist, many man-made fluorochemicals have been developed over the years, all of which have fluorspar as their starting point. The unique properties, pervasive influence, and lasting impact of these compounds have given fluorine a distinction as "the little atom that could."

LIFTOFF! The National Space Centre in Leicester, England, features a Rocket Tower composed of ethylene-tetrafluoroethylene copolymer.