From: Richard Feinman (rumford$##$onebox.com)
Date: Wed Feb 20 2002 - 11:45:14 EST
The deltaG0, standard free energy, of hydrolysis of thioesters is about
-8 kcal as compared to -(3-4) kcal for oxygen esters. (As we teach in
biochemistry, thioesters are"high energy" compounds.) this is what I
meant by "thioesters are thermodynamically unstable relative to oxygen
esters." According to the Hammond (or Hammond-Leffler) postulate, thermodynamic
instability generally correlates with kinetic reactivity, that is, the
transition state resembles the reactant that it is closest to on a free
energy diagram. so, an endergonic reaction must get up to a high TS
at which point it resembles the (unfavorable) product. This rule is
generally followed for acyl derivatives, e.g. esters are more reactive,
kinetically AND thermodynamically than amides. The exception is supposed
to be thioesters which hydrolyze at the same rate as oxygen esters.
I wondered how general this was. There are, of course, two transitions
states, to and fromthe tetrahedral intermediate. The second step is surely
faster with thioester since thiol is a better leaving group than OR.
Is there a difference in rate determining step ? I know this is sort
of a basic question but obvious references (Jencks) don't seem to address
it explicitly.
-- Richard Feinman rumford$##$onebox.com - email (718) 404-3901 x6280 - voicemail/fax---- zabicky$##$bgumail.bgu.ac.il (Jacob Zabicky) wrote: > Hello Richard, > > As far as I understand it, the rate of reaction depends on the energy > difference between the ground and the transition states of the substrate, > assuming that the energy levels of the products are much lower (otherwise > complications with reversible reaction kinetics pop up). Therefore, > the > relative thermodynamic stability of the different substrates is irrelevant. > BTW, what do you mean quantitatively by "thioesters are thermodynamically > unstable relative to oxygen esters"? > > All the best, > > Jacob > > At 19:51 14/02/2002, Richard Feinman wrote: > >I don't know about questionable but mechanistic problems that I am > interested > >in which I'm having a little trouble getting a grip on (although it > may > >be elementary) are: > > > >1. It's usually assumed that although thioesters are thermodynamically > >unstable relative to oxygen esters, the rates of hydrolysis are similar. > > I wonder about the evidence for this. Fedor & Bruice studied trifluoroacetate > >esters and I wonder if this is representative. > >2. Along the same lines, it is also generally stated that thioesters > >are like ketones (due to poor electronegativity of S and limited > >delocalization). > > Is there comparative data on rates of hydrate formation of ketones > relative > >to formation of tetrhedral intermediates in ester and thioester hydrolysis > >? > > > >-- > >Richard Feinman > >rumford$##$onebox.com - email > >(718) 404-3901 x6280 - voicemail/fax > > > > > > > >---- "G. Robert Shelton" <rshelton$##$chem.ufl.edu> wrote: > >> orglist-ers, > >> > >> Just wanted to throw out a general question to the active people > of > >> the list, and some of those not so active like myself who are waiting > >> for phyiscal organic question to be asked. And from there maybe > start > >> some threads of conversation that would be of interest to some of > us > >> that are not solely organic synthesis people. Hopefully this is > the > >> place for this question. So In your opion, what are the most questionable > >> chemical mechanisms? > >> > >> -Robert > >> > > > >__________________________________________________ > >FREE voicemail, email, and fax...all in one place. > >Sign Up Now! http://www.onebox.com > > > >__________________ > > > >ORGLIST - Organic Chemistry Mailing List > >Website / Archive / FAQ: http://www.orglist.net/ > >List coordinator: Joao Aires de Sousa (jas$##$mail.fct.unl.pt) > > x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x > Prof. Jacob Zabicky > Institutes for Applied Research > Ben-Gurion University of the Negev Private: POB 12366, Beer-Sheva > 84863 > POB 653, Beer-Sheva 84105, ISRAEL Tel. +972-8-6496792 > http://profiler.bgu.ac.il/site/main.cfm > Tel. +972-8-6461271/6461062/6472754 > Fax. +972-8-6472969 > x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x > > >
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