Classification by reaction mechanism

In modern organic chemistry organic reactions can be classified by their mechanisms. A detailed description of the changes in structure and bonding that take place in the course of a reaction, and the sequence of such events is called the reaction mechanism. A reaction mechanism should include a representation of possible electron reorganization, as well as the identification of any intermediate species that may be formed as the reaction progresses. In studying the reaction mechanisms, we ascertain the order and the way old chemical bonds are broken and new ones are made in the course of a reaction. When we classify reactions by their mechanisms, our attention is attracted above all by the way a covalent bond in a reacting molecule is cleaved Since chemical reactions involve the breaking and making of bonds, a consideration of the movement of bonding ( and non-bonding ) valence shell electrons is essential to this understanding. If a covalent single bond is broken so that one electron of the shared pair remains with each fragment, this bond-breaking is called homolysis. If the bond breaks with both electrons of the shared pair remaining with one fragment, this is called heterolysis. The products of bond breaking are not stable in the usual sense, and cannot be isolated for prolonged study. Such species are referred to as reactive intermediates.

Reaction mechanisms provide details on how atoms are shuffled and reassembled in the formation of products from reactants.

Chain reactions. Chain reactions occur in a sequence of steps, in which the product of each step is a reagent for the next. Chain reactions generally involve three distinct processes: an initiation step that begins the reaction, a series of chain-propagation steps, and, eventually, a termination step.

Polymerization reactions are chain reactions, and the formation of Teflon from tetrafluoroethylene is one example. In this reaction, a peroxide (a compound in which two oxygen atoms are joined together by a single covalent bond) may be used as the initiator. Peroxides readily form highly reactive free-radical species (a species with an unpaired electron) that initiate the reaction.

Photolysis reactions. Photolysis reactions are initiated or sustained by the absorption of electromagnetic radiation. One example is the decomposition of ozone to oxygen in the atmosphere. Another example is the synthesis of chloromethane from methane and chlorine, which is initiated by light. This reaction, coincidentally, is also a chain reaction. It begins with the endothermic reaction of a chlorine molecule (Cl₂) to give chlorine atoms, a process that occurs under ultraviolet irradiation. When formed, some of the chlorine atoms recombine to form chlorine molecules, but not all do so. If a chlorine atom instead collides with a methane molecule, a two-step chain propagation occurs. The first propagation step produces the methyl radical (CH₃). This free-radical species reacts with a chlorine molecule to give the product and a chlorine atom, which continues the chain reaction for many additional steps. Possible termination steps include combination of two methyl radicals to form ethane (CH₃CH₃) and a combination of methyl and chlorine radicals to give chloromethane.