Analysis of mixtures based on rates of reaction

TLDR: In this article, the authors proposed a method based on the differences in rates of reactions of the components of the mixture with a given reagent, which can be used for the analysis of mixtures of homologous organic compounds.

Abstract: The analysis, by conventional means, of mixtures of two constituents that have similar chemical properties is often difficult or impossible. Mixtures of two organic compounds that contain the same functional group (e.g., two esters or two aldehydes) are frequently encountered and present special difficulties to the analyst. For example, mixtures of homologous organic compounds or of certain orthoand para-isomers usually cannot be analyzed by classical chemical methods. Methods that depend on physical properties of the compounds are usually not suitable either. In illustration of this point, ultraviolet spectrophotometry cannot be used to analyze a mixture of homologous ketones because the spectrum of each ketone is determined chiefly by the carbonyl group and is affected only slightly by the nature of the alkyl group substituents. Infrared spectrophotometry, which is capable of distinguishing between homologous organic compounds, requires expensive equipment and is frequently not applicable if the sample is available only as a dilute solution in water or in certain other common solvents. (On the other hand, infrared analysis is ideal if the apparatus is available and if the sample is available in a suitable form.) I t frequently happens that such mixtures of organic compounds can be analyzed by a method based on differences in rates of reactions of the components of the mixture with a given reagent. The effect of a substituent on the rate of reaction of a reactive group with a particular reagent is usually pronounced. Propylene, for example, reacts with perbenzoic acid a t only one-thirtieth the rate a t which 2-butylene reacts (see below). Methyl alcohol reacts with hydrobromic acid nearly six times as rapidly as ethyl alcohol does (solvent phenol, SO').' It should be pointed out, however, that, if the substituents of the functional group are quite similar, as for example, n-butyl and n-amyl, the method of analysis based on reaction rate cannot be applied. Furthermore, the method is generally limited to mixtures of two components. Highly specific methods for the analysis of mixtures based on rates of reaction have been described previously in the literature?-' Most of these methods were developed for the investigation of specific physical chemical problems and are not convenient as general analytical procedures. They differ greatly from the proposed general method. In order to understand the principles underlying the proposed method of analysis, consider two compounds, A and B, that react with a reagent R a t different rates. Suppose that the reaction of A with R is faster than the reaction of B with R, a t a given constant temperature. Mixtures of A and B can be analyzed by the following procedure. A reaction solution is prepared in which the the total molar concentration of A + B is specified and in which the con-