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Stories About Chemistry

INDEX

61. A Third Possibility

It was thought that the element carbon was a union of three substances. Scientists call this “triune” allotropy. In other words one and the same element can exist in three allotropic modifications.

The three carbon substances were diamond, graphite, and carbon black They differ greatly from one another: the “king of hardness”, diamond, soft scaly graphite, and dull carbon black. The difference between them is due to dissimilar arrangement of the carbon atoms in their molecules.

In diamond they occupy the peaks of a geometrical figure called a tetrahedron and are bound very strongly. That is what makes diamond so hard

In graphite, on the contrary, the carbon atoms are arranged in planes and the bonds between the In diamond and graphite the chains of carbon atoms are closed, though arranged differently in space. Now could the carbon atoms be made stretch out in a long linear chain? In other words, is it possible to produce a polymeric molecule consisting only of carbons arranged in a straight line?

The first thing required to prepare any chemical product is the starting material. The only raw material that could serve for the preparation of “carbon No. 3” is acetylene, a compound of two carbon atoms and two hydrogen atoms, C2H2.

Why acetylene? Because in its molecule the carbon atoms are tied up with the least possible number of hydrogen atoms. Extra hydrogens would be an obstacle to the synthesis.

Acetylene has another important feature: it is very reactive, as the chemists say. The carbon atoms in its molecule are held together by three chemical bonds (H–C☰C–H), and two of them are comparatively easy to break, and can be used subsequently to connect the carbons with the atoms of other molecules, for instance, with molecules of the same acetylene. Thus the first step in the planned operation was to prepare the polymer polyacetylene from monomeric acetylene.

This was not the first attempt. In the 19th century the German chemist Baeyer tried to accomplish this reaction. But the best he could produce was tetraacetylene, a combination of four acetylene molecules. But even so this compound proved very unstable. The same path was tried by other chemists in various countries. But all their efforts ended in disappointment Only the powerful present-day methods of organic synthesis have finally made it possible to produce polyacetylene. Soviet scientists produced a new class of organic compounds known as polyynes. These new-born substances immediately found practical usage, because they turned out to be excellent semiconductors.

Now the second step had to be made towards synthesis of the third variety of carbon. This was to exclude the hydrogen atoms from the polyacetylene molecules, and to exclude them in such a manner as to preserve a chain consisting only of carbon atoms.

In the language of chemistry the process by means of which the hydrogen atoms were to be excluded bears the long and tedious name of oxidative dehydropolycondensation. There is no point in trying to explain the essentials of this process, in laboratory logs the description of the process took up scores of pages, because the removal of hydrogen from polyacetylene proved no easy task.

Nevertheless, Soviet scientists scored a brilliant success.

...An unattractive black powder resembling soot. Chemical analysis showed 99 per cent pure carbon. But ninety-nine is not one hundred.

Strictly speaking, one more step remains to complete victory. The notorious last per cent of hydrogen has to be got rid of. It is this per cent that prevents the carbon atoms from falling into straight-line formation, from arranging themselves in parallel chains. It is the last obstacle in the way to “carbon No. 3.”

Chemists call this synthesized “almost third” variety of carbon carbyne. It has already demonstrated some remarkable qualities. It is an excellent semiconductor, possesses photoelectric properties and its heat resistance is breathtaking: fifteen hundred degrees means nothing to it.

We are confident that “one hundred per cent” carbyne will become a reality in the very near future.


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