Physical organic chemistry is the study of the interrelationships between structure and reactivity in organic molecules. It can be seen as the study of organic chemistry using tools of physical chemistry such as chemical equilibrium, chemical kinetics, thermochemistry, and quantum chemistry. The term "physical organic chemistry" is commonly attributed to Louis Hammett, who used it as a title for a book in 1940.
The two main themes in physical organic chemistry are structure and reactivity. The study of structure starts from chemical bonding, with special emphasis on the stability of organic molecules due to factors such as steric strain and aromaticity. Other topics in structure include stereochemistry and conformational analysis. Supramolecular structure is also considered in terms of intermolecular forces including hydrogen bonding. Finally, the acid-base chemistry of the molecules is studied in terms of structure, based on resonance and inductive effects and through the use of linear free-energy relations.
The study of reactivity focuses on the mechanisms of organic reactions. It uses chemical kinetics, spectroscopy, isotope effects, and quantum chemistry to determine the sequence of elementary steps involved in a reaction. These elementary steps can be classified in a few major classes: addition, elimination, substitution, and pericyclic reactions. The mechanisms are commonly expressed in terms of "electron pushing" and potential energy surfaces. Other major topics are photochemistry, the effect of light on the reactivity of organic molecules, and solvent effects on organic reactions.
Structure and reactivity are both involved in the study of reaction intermediates—the transient species involved in reaction mechanisms. The main types of intermediates of interest are carbocations, carbanions, free radicals, and carbenes. Usually, these intermediates are not isolated, but their presence is inferred from stereochemical evidence, spectroscopy, or through the use of chemical traps. In some cases, however, it is possible to isolate these types of molecules at very low temperatures (cryochemistry) or via matrix isolation. It is also possible to create specific derivatives that are stabilized through chemical means such as resonance, as in the case of the triphenylmethyl radical.
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