MadSci Network: Chemistry |
My textbook says that the rings in ferrorcene are constantly
rotating, but all of the sources I have looked at indicated that the rings
are staggered or eclipsed. How can they be constantly rotating if the
ring orientation can be known? Also, according to the references I have
found, ferrocene has an internal rotation barrier of 4-10kJ/mol. Is this
barrier just for the change of staggered to eclipsed or is it the barrier
for the rings to be constantly rotating?
Please include any references used. |
The staggered structure of ferrocene is an energy minimum; the all-eclipsed version is a transition state which is not very much higher in energy than the minimum. The barrier for rotation is, as you have said, only a few kJ/mol; what this means is that at room temperature, in the gas phase and in solution, ferrocene's cyclopentadienyl rings are spinning like tops. In the crystal, ferrocene exists as a staggered molecule; interactions with neighboring molecules seem to damp down the spin enough for us to get good X-ray fixes on the ring carbon atoms. It's easy to estimate the rate for any reaction, for which the activation energy is known or estimated. One just uses the pseudo-Arrhenius relationship shown here (taken from Carey & Sundberg, Advanced Organic Chemistry, Part A, p.191-192), and plug in the temperature you're interested in. At 300 K and assuming 5 kJ/mol, I get a rate constant of 8´1011 per second, or pretty fast.
When I was in grad school I learned a rule of thumb that a reaction had to have an activation energy of 20 kcal/mol or less (about 85 kJ/mol) to have a reasonable rate at room temperature. For 85 kJ/mol I get a rate constant of 10-2 per second.
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