Subject: Re: CCL:conformers and rate

From: Per-Ola Norrby <pon(at)kemi.dtu.dk>
Date: Fri, 9 Jul 2004 08:37:11 +0200

>2004 July 8
>
>The rate of formation of products Pa and Pb from quickly interconverting
>conformers Ca and Cb is treated with the the Curtin/Hammett and
>Winstein-Holness equations (the Curtin-Hammett principle, in particular,
>is well-known): J. I Seeman, Chem Rev, 1983, 83, 83; E. L. Eliel, "From
>Cologne to Chapel Hill", ACS, 1990, pp. 31-33.
>
>Question: has anyone a reference to a discussion of the case of rapidly
>interconverting conformers Ca and Cb, where Ca, although the minor
>conformer, is the only one that can react with X to form a product CaX,
>through transition state Ta; intuition aside, has someone explicitly
>addressed whether the _experimental_ activation E is measuring the Ta-Ca
>energy difference, or the Ta-Cb difference?

	Provided that the interconversion is fast relative to the 
final reaction, the free energy of activation should be relative to 
the ensemble energy (which is slightly lower than the lowest of Ca 
and Cb).  The apparent activation energy is taken from an expression 
where you consider the total concentration of reactant.  The sole 
effect of the pre-equilibrium is to make the effective concentration 
lower, with a concominant lowering of the rate which corresponds 
exactly to the free energy difference between the reacting conformer 
and the free energy of the ensemble.  You can get the latter by 
assuming a virtual species corresponding to the total concentration 
of the reagent, and use the standard Boltzmann expression.

	The above wasn't perfectly clear (I haven't digested my 
coffee yet), but it should be correct.  The math is available in the 
Ph.D. Thesis of Torben Rasmussen (Copenhagen 2001), and he took much 
of it from H Maskil, "The Physical Basis of Organic Chemistry", 
Oxford 1985.

	/Per-Ola
-- 
Per-Ola Norrby, Assoc. Professor, http://organisk.kemi.dtu.dk/PON/
Technical University of Denmark, Department of Chemistry
Building 201, Kemitorvet, DK-2800 Kgs. Lyngby, Denmark
Email: pon(at)kemi.dtu.dk  tel +45-45252123,  fax +45-45933968