Silber spiegel - SwissEduc

© c+b 3/04
Mechanisms
A theoretical Analysis
of Mechanisms
for the
Maleic-Fumaric
Isomerization
Abstract
A theoretical study of the isomerization mechanisms for the maleic to fumaric
acid interconversion is presented in this work. In earlier papers, that process has
been proposed as an academic topic with an established pathway. One of those
papers is about a student’s experiment involving the acid catalyzed isomerization
from maleic acid to fumaric acid. The experiment was designed to encouraged
student structural deductions based on traditional physical evidence (melting
points, water solubility and acidity). A second paper described modern spectroscopic
data for the two acids (IR, 1 H-NMR, 13 C-NMR and Mass spectra). The
present paper adds some basic theoretical aspects to this topic. Computations
allow one to select the best mechanism from three possible ones. Transition
states, reactant, product and possible intermediates have been characterized
using Gaussian 98.
1. Introduction
In this article, we present a theoretical study of the maleic–fumaric
isomerization to support experimental results given in
earlier papers (see Figure 1). A comparison of the effectiveness
of results obtained by the Gaussian 98 package [1] permits
one to consider the importance of the chemist’s criterion for
proposed mechanisms.
Gaussian 98 has become a successful chemical computational
software which allows one to study several kinds of chemical
species. However, it is wise to keep relevant experimental results
in mind when carrying out computational studies.
Perhaps undergraduate students think this software package
will conveniently solve chemical problems. Such an expectation
is unrealistic. The initial conditions should be carefully established
by the Gaussian user.
39
3/04