Molecular Structure
The notion that molecules might have specific geometric structures arose in the second half of the nineteenth century. Prior to that time, much debate existed among chemists as to whether molecules had definite shapes that could be determined experimentally. For example, French chemist Charles Gerhardt (1816-1856) emphasized his belief that they did not by writing different formulas for the same compound. He wrote BaO,SO3; BaS,O4; and BaO2, SO2, for barium sulfate.
Friedrick Kekulé was one of the first chemists to attack the problem of molecular structure. In 1857, he suggested that the carbon atom was tetravalent, that is, it could bond with four other atoms. He developed the tool of structural formulas to illustrate this concept, although the formulas created by Archibald Couper (1831-1892) at about the same time were far simpler and more efficient to use than were those of Kekulé. Neither Kekulé nor Couper carried their analysis of molecular architecture much beyond the two-dimensional diagrams that could be drawn on paper. Couper, for example, showed the bonds on a carbon atom directed at the four corners of a square.
In 1874, Jacobus Van't Hoff and Joseph Le Bel (1847-1930) independently developed a three-dimensional concept of the carbon atom. They proposed that the four carbon bonds were directed towards the corners of a tetrahedron. When they constructed models of organic compounds using tetrahedral atoms, they were able to explain a number of phenomena, including the existence of optical isomer s, first discovered by Louis Pasteur in 1848. Other chemists were unimpressed by the ideas of Van't Hoff and Le Bel. Adolph Kolbe, for example, doubted the reality of atoms, molecules, and chemical bonds, and warned that thinking of them in concrete, structural terms was carrying theory too far.
The great German chemist Hermann Helmholtz expressed similar concerns. The work of Van't Hoff and Le Bel was justified, however, because of its successes in explaining many physical phenomena. Other chemists began to explore other consequences of molecular structure. The technique soon had spectacular success in the field of organic chemistry, where many puzzling experimental results were eventually explained. Attempts to describe the molecular structure of inorganic compounds were less successful. In fact, there was limited progress in this field in the twenty years following the work of Van't Hoff and Le Bel.
At first, structures for simple compounds, such as binary salts, could be drawn. But a number of more complex compounds escaped explanation. Then, in 1893, German chemist Alfred Werner successfully applied structural theory to inorganic compounds. Waking early one morning, Werner had arrived at a fully formed solution in his sleep. He began writing what turned out to be his most important scientific paper on the spot and had finished it by the next afternoon. His theory of coordination compounds showed that the structure of molecules could be understood in terms of geometry, rather than simply in terms of chemical bonds between atoms. In working with organometallic compounds, for example, he placed the metal atom at the center of a geometric figure (such as a cube) and surrounded it with other atoms, ions, and groups of atom at the corners of the figure. He suggested that the metallic atom was attached to the surrounding groups by "secondary valences." Werner's suggestion was enormously fruitful. He used the theory, for example, to predict the existence of geometric isomers, two forms of a compound that differ from each other only in the position of a single atom, ion, or group of atoms. In 1907, he synthesized a pair of geometric isomers that confirmed his predictions.
The theory of molecular structure reached its highest development in the work of Linus Pauling in the 1920s and 1930s. Pauling applied the theory of quantum mechanics to electrons and showed how the formation of chemical bonds resulted in more stable configurations than existed in free atoms. Pauling's work not only provided a clearer understanding of the chemical bond, but also explained the structures of molecules. He demonstrated that the most stable configuration for some molecules was some intermediary structure between two other structures. The bonds in a benzene molecule, for example, are neither pure single nor pure double bonds, but some kind of " compromise" that results from the shifting of electrons back and forth between the two possibilities. This theory of resonance explained a number of phenomena that had previously been a mystery.
In the 1940s, Pauling applied these ideas to the complex molecules that make up living systems, especially proteins and nucleic acids. One approach he found highly productive was model-building. Beginning with tinker-toy-type equipment, he constructed molecular structures that seemed reasonable based on available experimental evidence. Then he refined the models until he could produce a structure that accounted for all existing data. In the early 1950s, he used this technique to demonstrate the helical structure of proteins. He very nearly achieved similar success in unraveling the structure of nucleic acids. This work--using the physical structure of chemical molecules to explain biological phenomena--led to the development of the science now known as molecular biology.
In the 1980's, computer technology had improved to the point where it could be used to construct graphic models of molecular structures. This led to the development of a new science known as computational chemistry. In this area of chemistry, scientists input data about compounds such as molecular composition or chemical behavior, and the computer displays a picture of the molecular structure. This technology will be an important part of the development of synthetic drugs and new catalysts.
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