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Omar A. El Seoud*
Instituto de Quimica, Universidade de Sao Paulo, C.P. 26077, 05599-970, Sao Paulo, SP, Brazil
Keiko Takashima
Departamento de Quimica, UEL, C.P. 6001, 86051-970, Londrina, PR, Brazil
Introduction
The largest body of information about reaction pathways has and still does come from kinetic studies (1-3). As a result, the mechanistic use of chemical kinetics is introduced early in any introductory organic chemistry course. Examples are the use of reaction order to distinguish between the S^sub N^1 and S^sub N^2 reaction pathways and the protium/deuterium kinetic isotope effect to probe the mechanism of electrophilic aromatic substitution (4-6). Consequently, the determination of a reaction rate constant is a very important experiment, and it is worthwhile to make it as interesting and informative as possible.
Kinetic experiments have the following objectives: (i) to provide experience in obtaining and processing kinetic data, (ii) to give an idea of the scope and variety of experimental techniques that can be used for rate measurement, and (iii) to illustrate how kinetic results can provide information about the mechanism of a reaction. Introduction of microcomputers into the undergraduate physical chemistry laboratory has made data acquisition and subsequent rate constant calculations relatively easy. Although organic chemistry textbooks treat point iii above in sufficient depth (4-6), objective ii has not received much attention and is usually discussed in more specialized literature (7-9). A quiz given to our chemistry students showed that they have some difficulty in choosing among different experimental techniques for studying a particular reaction and in explaining the reason for their choice. Consequently, we use the spontaneous hydrolysis of methyl chloroformate (hereafter referred to as the "ester") to address this particular problem.
Results and Discussion
In aqueous solution, ethyl hydrogen carbonate decomposes very rapidly, with the production of carbon dioxide and ethanol (lOb). By analogy, methyl hydrogen carbonate (eq 2) can be considered to be a reactive intermediate. Application of the steady-state approximation to equations 1 and 2 leads to equation 3:
That is, this solvolytic reaction is a pseudo-first-order one, as indicated by the linear plots of Figures 1A-C.
This experiment is ideal for the undergraduate physical chemistry laboratory because the reaction involved is simple; the ester is inexpensive, and its stock solution in acetonitrile can be safely...