Electronic Theories in Organic Chemistry

theories that deal with the structure, physical properties, and reactivity of organic compounds in terms of the electron density distribution of atoms and molecules and the shifts in electron density that occur during chemical reactions.

The first electronic theories appeared at the turn of the 20th century, soon after the discovery of the electron. In the earliest of these theories, the belief that electrostatic bonds existed in inorganic compounds was applied to nonpolar organic compounds. Such theories could not explain many experimental observations in organic chemistry, however, and were replaced by theories based on the concept of covalent bonds. The idea of the covalent bonds was developed by the German scientist J. Stark between 1908 and 1915 and by the American chemist G. Lewis between 1916 and 1923. Lewis defined a covalent bond as one that is formed between two atoms by the sharing of an electron pair (doublet). The formation of covalent bonds was subsequently interpreted within the framework of quantum mechanics as the overlap of electron densities of interacting atoms (seeCHEMICAL BOND and VALENCE).

The concept of the covalent bond proved to be most fruitful in organic chemistry. The electronic theories proposed in the 1920’s and 1930’s on the basis of the concept explained the structure of many organic compounds and the relationship between the structure and properties of these compounds. The development of these theories was facilitated by the formulation of quantum-chemical concepts of the various types of covalent bonds (seeSIGMA AND PI BONDS and SEMIPOLAR BOND.)

The most popular electronic theories of the 1920’s and 1930’s are still accepted today; they were developed by L. Pauling and the English chemists T. Lowry, R. Robinson, and C. Ingold. The concepts these scientists introduced concerning shifts of electron densities within molecules—inductive, resonance, anomeric, and electronic effects—are commonly used to explain and sometimes to predict the properties and reactivities of various organic compounds. The English chemists also proposed a system for classifying organic reactions by shifts in electron density and by the electrostatic orientation of the reagents during a reaction—that is, by the nucleophilic, electrophilic, and radical mechanisms (seeORGANIC CHEMISTRY; CONJUGATION OF BONDS; RESONANCE; and NUCLEOPHILIC AND ELECTROPHILIC REAGENTS).

Using the theory of shifts in electron densities, scientists were able to explain the equalization of bonds in conjugated chains—in particular, in benzene. They were also able to account for the transmission of substituent effects along a system of conjugated bonds, the order of replacement in aromatic rings with substituents (seeAROMATIC COMPOUNDS and ORIENTATION RULES), and such relationships as the Markovnikov rule and the El’tekov rule, which have been experimentally proved in organic chemistry.

The development of electronic theories in organic chemistry was linked with the development of the classical theory of chemical structure and quantum chemistry, which is the basis of all modern electronic theories.