Early budding period
The term "organic chemistry" was first coined in 1806 by Betzerius, the "father of organic chemistry". At that time, it was named as the antithesis of "inorganic chemistry". Due to scientific limitations, the object of organic chemistry research can only be organic matter extracted from natural animal and plant organisms. Therefore, many chemists believe that organic compounds can only be produced due to the existence of so-called "vitality" in living organisms, and cannot be synthesized from inorganic compounds in the laboratory.
In 1824, the German chemist Wühler hydrolyzed cyanate to produce oxalic acid, and in 1828, he inadvertently used heating to convert ammonium cyanate into urea. Both cyanide and ammonium cyanate are inorganic compounds, while oxalic acid and urea are both organic compounds. The results of Wüller's experiments gave the doctrine of "life force" its first impact. Since then, organic compounds such as acetic acid have been synthesized from carbon, hydrogen and other elements, and the theory of vitality has gradually been abandoned.
Due to the improvement and development of synthesis methods, more and more organic compounds are being synthesized in the laboratory, and most of them are synthesized under conditions that are very different from those in living organisms. The doctrine of "vitality" was gradually abandoned, but the term "organic chemistry" is still used today.
From the beginning of the 19th century to 1858, when the concept of valence bond was proposed, it was the embryonic period of organic chemistry. During this period, many organic compounds had been isolated, some derivatives had been prepared, and they were qualitatively described, and the properties of some organic compounds were recognized.
French chemist Lavoisier discovered that when organic compounds are burned, carbon dioxide and water are produced. His research work laid the foundation for the quantitative analysis of organic compounds. In 1830, the German chemist Liebig developed the analysis method of carbon and hydrogen, and in 1833, the French chemist Dumas established the analysis method of nitrogen. The establishment of these organic quantification methods enables chemists to find experimental formulas for a compound.
At that time, it was very difficult to solve the problem of how the atoms in the molecule of organic compounds were arranged and combined. Originally, organic chemistry used binary theory to solve the structural problem of organic compounds. The dualistic theory holds that the molecules of a compound can be divided into positively charged and negatively charged, and the two are held together by electrostatic force. Early chemists believed that in terms of certain chemical reactions, organic compound molecules were bound by electrostatic forces of a group that remained unchanged in the reaction and a group that changed in the reaction according to the electrostatic force of the opposite charge. But this doctrine itself is very contradictory.
The typology theory was established by the French chemists Gérard and Laurent. This theory denies that organic compounds are composed of positively charged and negatively charged groups, and that organic compounds are derived from some parent compounds that can be substituted, and can therefore be classified according to these parent compounds. The typology classifies many organic compounds into different types, according to which not only some properties of the compounds can be explained, but also some new compounds can be predicted. But typology fails to answer the question of the structure of organic compounds. This question has become a mystery that has plagued people for many years.
Period of classical organic chemistry
It was from the establishment of the valence bond theory in 1858 to the introduction of the electronic theory of valence bonds in 1916 that this insoluble mystery was solved, and this period was the period of classical organic chemistry.
In 1858, the German chemist Kekuler and the British chemist Cooper proposed the concept of valence bonds, and for the first time used the dash "-" to represent the "bond". They believe that organic compound molecules are formed by the bonds of the atoms of which they are composed. Since of all known compounds, a hydrogen atom can only bind to an atom of another element, hydrogen is chosen as the unit of valence. The valence of an element is the number of hydrogen atoms that can bind to one atom of that element. Kekuler also proposed the important concept that carbon atoms can bind to each other in a molecule.
In 1848, Pasteur isolated two tartaric acid crystals, one with a half-hemiphadral crystal to the left and one hemiphalic crystal to the right. The former rotates the plane-polarized light to the left, while the latter rotates it to the right at the same angle. A similar phenomenon has been encountered in the study of lactate. To this end, in 1874, the French chemist Lebel and the Dutch chemist Van Torf respectively proposed a new concept: isomers, which satisfactorily explained this isomerism.
They believe that the molecule is a three-dimensional entity, that the four valence bonds of carbon are symmetrical in space, pointing to the four vertices of a tetrahedron, and that the carbon atom is located in the center of the tetrahedron. When a carbon atom is linked to four different atoms or groups, a pair of isomers is produced, which are physical and mirror images of each other, or left-handed and right-handed chiral relationships, and this pair of compounds is optical isomers or enantiomers of each other. This phenomenon is called enantiomer phenomenon. These two molecules that are mutually physical and mirror images, enantiometric but not completely overlapping are called chiral molecules. The theories of Lebel and Vantorf are the basis of stereochemistry in organic chemistry. [3]
In 1900, the first free radical, the triphenyl free radical, was discovered, which is a long-lived free radical. The presence of unstable free radicals was also confirmed in 1929.
During this period, great progress was made in the determination of the structure of organic compounds, as well as in the reaction and classification. But the valence bond is only a concept derived by chemists from practical experience, and the essence of the valence bond has not yet been resolved.
The period of modern organic chemistry
On the basis of physicists' discovery of electrons and elucidation of atomic structure, American physical chemist Lewis and others proposed the electronic theory of valence bonds in 1916.
They believe that the interaction of the outer electrons of each atom is what binds the atoms together. An ionic bond is formed when the interacting outer electrons are transferred from one atom to another, and a covalent bond is formed if two atoms share the outer electrons. Through the transfer or sharing of electrons, the outer electrons of the interacting atoms are all acquired in the electronic configuration of the noble gas. Thus, the dash "-" used to represent the valence bond in the pictorial representation of the valence bond is actually a pair of two atoms sharing electron pairs.
After 1927, Heitler and London used quantum mechanics to deal with molecular structure problems, established the valence bond theory, and proposed a mathematical model for chemical bonds. Later, Malikon used the molecular orbital theory to deal with the molecular structure, and the results were generally consistent with the electronic theory of valence bonds, and because of the ease of calculation, many problems that could not be answered at the time were solved.