DETERMINING REACTION MECHANISMS

There are a number of commonly used methods for determining mechanisms. In most cases, one method is not sufficient and the problem is generally approached from several directions.

Identification of Products

Any mechanism proposed for a reaction must account for all the products obtained and their relative proportions. A proposed mechanism cannot be correct if it fails to predict the products in approximately the observed proportions.

Determination of Prescence of an Intermediate

1. Isolation of an Intermediate: An intermediate can be isolated from a reaction mixture by stopping the reaction after a short time or by the use of very mild conditions. If the isolated compound gives the same product when subjected to the reaction conditions and at a rate no slower than the starting compound, this gives a strong evidence that the reaction involves that intermediate.

2. Detection of an Intermediate: Intermediates can be detected by IR, NMR, or some other spectra. For example, the detection of NO2+ by Raman spectra was regarded as strong evidence that this is an intermediate in the nitration of benzene. Free radicals and triplet intermediates can be detected by ESR and by CIDNP.

3. Trapping of an Intermediate: In some cases, the intermediates may react with a certain compound in a given way. The intermediate can be trapped by running the reaction in the precsence of that compound. For exampe, benzyne react with dienes in the Diels-Alder reaction. The detection of the Diels-Alder adduct indicate that the benzyne was probably present.

Isotopic Labeling

Informations about the reaction mechanism can be obtained by using molecules that have been isotopically labeled and tracing the path of the reaction in this way. Radioactive isotopes as well as stable isotopes can be used as tracers. O-18 can be detected by mass spectrometry. D can be determined by IR and NMR spectra when used as a substitute for H. Also, C-13 which is non-radioactive can be detected by C-13 nmr.

Stereochemical Evidences

If the products of a reaction are capable of existing in more than one stereoisomeric form, the form that is obtained may give information about the mechanism. For example, Cis-2-butene when treated with KMnO4 gives meso-2,3-butane diol and not the racemic mixture is evidence that the two OH groups attack the double bond from the same side.

Kinetic Evidence

Several types of mechanistic informations can be obtained from kinetic studies such as the order of the reaction, the rate determining step etc. The rate constant obtained from kinetic data is most important since it tells the effect of changes in the structure of the reactants, the solvent, ionic strength, addition of catalyst etc. on the reaction rate.