Subject: Re: CCL:Formula accounting for the catalyst effect on entrop

From: "Matthew Harbowy" <[email-masked]>
Date: 28 Aug 1996 21:19:59 +0100

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It is my firm belief that I cannot give your question a truly good treatment, 
but I will make a paltry attempt. I would refer you to Barry Carpenter's book, 
Determination of Organic Reaction Mechanisms, page 135-136 (section 7.2). To 
quote,

"For reactions of higher molecularity than unimoleclar, the sign and magnitude 
of delta_s depend on the implied standard state, which is defined by the units 
of the rate constant... If one has a reference bimolecular reaction of "known" 
mechanism then the comparison of its activation entropy to that of some other 
reaction can be useful...but under no circumstances does it make sense to 
compare activation entropies for reactions of different molecularity."

It would seem that addition of a catalyst might change the mechanism or 
molecularity of the reaction, and therefore your quest seems (to my limited 
scope) to be nonsensical. However, (also to my limited scope) it would seem that
a catalyst that brings two molecules into a complex

a + b + c ---> acb ---> (ab)c ts --->---> ab + c

potentially reduces the degrees of freedom in the reactants relative to the 
transition state which might accelerate the reaction. That is, it brings 
reactants closer together so they react better. In this case, it might be that 
the entropy should decrease for the reactants relative to the transition state. 
What I means to say is that the change in degrees of freedom for the reference 
reaction from reactants to transition state

a + b ---> ab (ts) ---> ab

relative to the activated complex

acb ---> (ab)c (ts) ---> abc

is different, that you lose a degree of freedom by having to bring the two 
together for the reference reaction, but in the activated complex with the 
catalyst, you have 'already' lost a degree of freedom. But that also doesn't 
make sense to me, since you will lose another degree of freedom during the 
rearrangement acb to abc in the formation of the cyclic (ab)c. 

If you follow the book, section 7.3 then goes on to discuss activation volumes, 
which seems more relevant to your quest that activation entropies.

It's hard enough for me to talk about this in the most general terms, so if you 
want to talk about putting numbers on any of this, it's beyond my scope. But I 
do recommend Carpenter's book as a good starting point for you.

matt
______________________________ Reply Separator _________________________________
Subject: CCL:Formula accounting for the catalyst effect on entropy.
Author:  [email-masked] at INTERNET
Date:    8/28/96 11:37 AM

Dear all,

Please let me have your idea on the following problem.

Consider the following reaction,

        +                                  +
      C-C-C  +  C=C-C  --->(TS)--->  C-C-C-C-C     (1)
                                       C

and focus on the entropy differences between reactants and the transition
state(TS) at 298.15K , i.e.,

                             +
      delta_S* = S(TS) - S(C-C-C) - S(C=C-C)       (2)

If reaction (1) occurs in a catalytic reaction sysem, there should be
another factor(s) that influences the value of delta_S*.

Under the condition that the entropy values for all the reactants and the
product in *GAS-PHASE* are available, but NOT for the transition state,
my question is,

        is it possible to construct a general formula which includes
        the factor(s) which can account for the catalyst effect?

More clearly, is it possible to relate the delta_S* for catalytic
reaction to gas-phase values of entropies for reactants and product,
and some unknown parameter? I.e.,

  delta_S* = function of
             [ S(reactants), S(product), parameter]

If so, what does this equation look like?  Should it be in a kind of
added form, i.e.,

              S(reac) +/- S(prod) +/- parameter(s)

or multiplied form, i.e.,

              parameter*S(reac) +/- parameter*S(prod) +/- ...

or something else??

Any suggestion/information(books, articles..)/advice will be
a great help for me.

                                Sincerely,

                                     Park, TAE-YUN
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