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Re: EDTA, catalysis

> From: Greg Morin <greg at seachem_com>
> Subject: Re: EDTA
> So would you say that actually an EDTA complex should be relatively pH 
> insensitive because the entropy advantage of the chelate insures that even 
> a protonated amine will eventually equilibrate to yield a N->metal 
> coordination?

	In the case of amines, even the entropy advantage isn't required,
amines are good co-ordinating ligands even when not chlelating.  They
are very good when they are.
> Something else to consider is that the the pKas listed are for the 
> uncomplexed tetraacid; the presence of a suitable Lewis acid such as a 
> metal should tend to lower the pKas of each carboxylic acid as the ionized 
> state (carboxylate) becomes more stabilized due to the availability of a 
> metal to chelate. 

	Once something like co-ordination starts, the pKa's will certainly
change a bit (look at the way the pKa's of the four initially identical
acid groups differ, simply because of the increasing charge on the
ion as each one ionises).  What you are talking about, though, is no
longer a pKa, because the carboxylate is co-ordinated, not free.

> So basically the metal acts as an ionization catalyst by 
> lowering the energy of the ionized form (which is metal chelated) thus 
> yielding an "apparent" pKa that is lower than that of the acid alone.

	A catalyst changes the _rate_ of a reaction without changing
the equilibrium position.  You are talking about changing the equilibrium,
so this isn't catalysis.  

Paul Sears        Ottawa, Canada