i know that the trityl cation has a pKR of -7 that is somehow related
to the trityl cation stability.
what does pKR standsfor and what does it mean?
please answer as soon as possible. if it is possible answer tomorow by
e-mailing to samerrafikhanna@yahoo.com

Hello -

pKR is similar to pKa and pH - the 'p' indicates it's relationship to
the value of interest as a negative log - eg. pH = - log [H], pKa =
-log Ka, pKR = -log KR.  KR is the equilibrium constant for the
dissociation of a sidegroup - this, as tehuti suggests, can be applied
to amino acids where the amino acid has a carboxylic acid group (with
a pKa) and also can have an ionizable side chain with pKR relating to
the dissociation constant for a hydrogren ion bound to the side chain.

In the case of the trityl cation, the pKR is the equilibrium constant
for the reaction involving the abstraction/release of a hydride group
by the trityl cation.  The trityl cation is able to exist as a
somewhat stable species due to inductive and resonance stabilizing
effects of the three phenyl groups which surround the central carbon.

The pKR can be used to describe the first step of an E1 elimination or
SN1 substitution reaction in which the first step is the loss of a
leaving group - the rate of this loss is related to the stability of
the resulting carbocation and the rate constant is KR.

The following page, under the heading "Factors Influencing Carbocation
Stability", gives a description of the inductive and resonance
stabilizing effects upon carbocation stability.
http://www.cem.msu.edu/~reusch/VirtualText/special3.htm

The following page explains the use of the trityl cation as an
oxidizing agent (due to it's ability to remove a hydride):
http://www.science.uottawa.ca/CHM/eng/prof/Richeson/CHM%208343/group%2013%20activation.ppt


Google search terms "carbocation stability", "pkr carbocation
stability"

Please let me know if you have further questions.

synarchy

PKR might stand for "Protein Kinase Resource"
http://www.sdsc.edu/kinases/

Could it be "parallel kinetic resolution"?

I think you will find that PKR in this context stands for Parallel
Kinetic Resolution.

Explaining what means is not easy.  How good is your backgorund in
organic chemistry? How much do you know about Kinetic Resolution? Do
you know what an enantiomer is?

No, it's to do with side groups on amino acids, bit like pK1 and pK2.
However, I'm not au fait enough with the concepts to explain exactly
what it means.