Silber spiegel - SwissEduc

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3/04
Mechanisms
Figure 5:
Reactant, intermediaries,
product and transition
state for mechanism 1.
Fig.5
Mechanism 3:
As Figure 4 shows, this mechanism is very similar to the mechanism
1. So one could think that the intrinsic activation barrier
will be similar too, but that is not true. A sensible explanation
could be that a chemical species with resonance is produced in
the fi rst elementary step. Thus reordering its structure should
involve a higher energy for the intrinsic activation barrier than
that intrinsic activation barrier belonging to mechanism 1. This
is shown in Table 3 where the intrinsic activation barrier value
is 69.1 kcal mol -1 which is higher than that belonging to the
one in mechanism 1. But the other energetic quantities are, as a
rule, higher than those ones for mechanism 2. We note that the forward reaction
energy is lower than the forward reaction energies given by mechanisms 1 and 2.
That is due to the way the intermediate I 3 has for eliminating the HCl molecule
in the last two elementary steps (see Figure 4). The energy difference between the
fumaric acid obtained in this mechanism compared to the fumaric acid obtained
from mechanisms 1 and 2 is different because of the conformation presented
by the lower COOH group in structure P ( compare with the corresponding
COOH group in structure P given in mechanisms 1 and 2).
© c+b 3/04