Brief History
Recent Milestones
There were many problems left to solve. The structural theory showed certain limitations at that time, among which it is worth highlighting:
1) Many simple structures such as vinyl alcohol or those that contain two OH or NH2 groups on the same carbon atom have never been isolated.
2) Pasteur, in his classic works on the optical activity of salts of tartaric acids, was able to isolate two different crystalline forms of sodium ammonium tartrate, one mirror image of the other, whose composition, melting and boiling points, solubilities and chemical reactions were identical. However, they deflected the polarized light to different sides even in solution, where the crystalline structure was lost.
3) According to Kekulé, an orthosubstituted benzene could be written in two different ways although these possible isomers had never been able to be separated.
4) Some isolated and relatively stable chemicals did not fit the formulas of structural theory. The most significant example is the isolation in 1900 by Gomberg of the free radical triphenylmethyl.
Louis Pasteur (1822-1895): Chemist and bacteriologist; professor from Lille and Paris. He carried out crystallographic studies and research on fermentation, laid the foundations of bacteriology, and introduced inoculation against diseases (smallpox vaccine) and the method of sterilization of milk that bears his name.
In 1873 van't Hoff spent several months in Kekulé's laboratory and, based on Wislicenus's work on lactic acids, published the idea that by placing four different substituents on the vertices of a tetrahedron, two different formulas could be written that only They differed in that one was the mirror image of the other. This idea was published, almost at the same time and completely independently, by le Bel.
Jacobus Henricus van't Hoff (1852-1911): Professor of Chemistry in Amsterdam and Berlin. With le Bel he established the foundations of organic stereochemistry. He also studied electrolytic dissociation, chemical equilibrium and reaction rate, and made detailed analyzes of the Stassfurt mineral deposits in Germany. He received the Nobel Prize in Chemistry in 1901.
Johannes Adolf Wislicenus (1835-1902): Professor of Chemistry from Zurich in Switzerland and from Würzburg and Leipzig in Germany. His research helped establish the concepts of organic stereochemistry.
Josep Achille Le Bel (1847-1930): Chemist from Paris. He studied the stereochemistry of carbon and nitrogen compounds, fermentation processes, and the crystalline forms of platinum salts.
The solution to the other problems had to wait a few more years, until the valence bond was replaced by a pair of electrons and the behavior of electrons was understood through quantum mechanics. The names of the main researchers in the development of resonance theory are Arndt (Germany), Ingold and Robinson (England) and Pauling (USA).
Linus Carl Pauling (1901-1994): Professor of Chemistry and laboratory director at the California Institute of Technology. President of the American Chemical Society (1949). He has been recognized for his work on the introduction of quantum mechanics in atomic chemistry and on the structure of molecules and chemical bonds. His most important work, The Nature of the Chemical Bond (1939), is considered capital throughout the world. Nobel Prize in Chemistry (1954), Nobel Peace Prize (1962) and Lenin Peace Prize (1970).
Some structural problems, such as the non-planar shape of cyclohexane and the possibility of it having axial and equatorial substituents, were not resolved until 1950 by Barton.
Sir Derek Harold Richard Barton (1918-1998): Professor of Organic Chemistry of London and Glasgow in the United Kingdom and of Gif-sur-Yvette in France. Nobel Prize in Chemistry (1969), shared with O. Hassel, for his contribution to the creation of conformational analysis, which studies the deformations that molecules can experience depending on the interactions between neighboring atoms or groups.
Purcell's and Bloch's research groups on the Harvard and Stanford campuses, respectively, developed Nuclear Magnetic Resonance (NMR) spectroscopy in the late 1940s. Both researchers shared the Nobel Prize in Physics in 1952 for their pioneering work. in the application of NMR to problems of chemical structures. The development of Infrared, Raman, UV-vis and, above all, NMR spectroscopic techniques, together with mass spectrometry and x-ray diffraction, have given an exponential turn to progress since the middle of the 20th century. of Chemistry and, especially, Organic Chemistry.
Edward Mills Purcell (1912 – 1997) was an American physicist who shared the 1952 Nobel Prize in Physics with Felix Bloch for their independent research (1946) on the measurement of magnetic fields in the atomic nucleus. Nuclear magnetic resonance (NMR) has become widely used to study the molecular structure of pure materials and the composition of mixtures. Shortly after beginning his work as a professor at Harvard University (1949), he detected microwaves emitted by hydrogen in interstellar space, radiation that allows astronomers to locate hydrogen clouds in galaxies and measure the rotation of the Milky Way.
Felix Bloch (Zürich, Switzerland, October 23, 1905 – September 10, 1983) was a Swiss physicist who worked primarily in the United States and who won the Nobel Prize in Physics in 1952. Born in Zurich (Switzerland), he studied there. , at the Federal Polytechnic School of Zurich. Although he first studied engineering, he soon switched to physics studies. After 1927, he continued his studies in physics at the University of Leipzig, obtaining his doctorate in 1928. He remained in Germany, where he studied with Werner Heisenberg, Wolfgang Pauli, Niels Bohr and Enrico Fermi. In 1933, he left Germany, and went to Stanford University in 1934. He adopted American citizenship in 1939. During World War II, he worked on nuclear energy issues at the Los Alamos National Laboratory, before resigning to join the project. radar at Harvard.
