Nature of organic reactions

Nature of organic reactions

Much work has been done on reaction mechanisms, i.e., the actual steps by which a reaction takes place. A chemical equation indicates the initial and final products of a reaction; rarely does it indicate how the reaction proceeds. Many reactions take place via intermediates which may or may not have been isolated. When the products of a reaction are formed by a single collision of the reactant molecules, i.e., the reaction proceeds without any intermediates, the reaction is said to be a one-step (or elementary) reaction. Most reactions, however, are complex, i.e., they occur via a number of reaction steps.

Mechanisms are, in general, theories that have been devised to explain the facts which have been obtained experimentally. Mechanistic students also include the nature of the transition state leading to intermediates (real or postulated) and to the products. The interpretation of experimental data may not be clear-cut; several mechanisms may appear to fit equally well. In any case, the acceptability is strengthened when a particular mechanism can be used to make predictions which are borne out in practice. Many methods are used to elucidate mechanisms, some of the commoner ones being :

• Kinetics, Kinetic studies are concerned with rates of reactions and provide the most general method for determining reaction mechanisms.
• The identification of all the products of a reaction.
• The detection, or better still (if it is possible), the isolation of intermediates. A special method o detection of intermediates is the trapping experiment.
• The effect on reaction rates of changing the structure of the reactants.
• The effect on reaction rates of changing the solvent.
• Stereochemical evidence.
• The use of isotopes. This method is particularly useful for tracing the part played by a particular atom in a reaction.
• The use of crossed-experiments.