PS7 - Recommended reading: 23, 29 Ch 102 Problem Set 7 Due:...

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1 Recommended reading: 23 , 29 Ch 102 Problem Set 7 Due: Tuesday, May 24, before class Problem 1 (20 points) Square planar, d 8 platinum complexes have been the focus of much research for several decades due to their involvement in many industrial and pharmaceutical processes. One of the chief players in platinum chemistry is cisplatin ( 1 ), a simple platinum coordination complex which causes DNA crosslinking and, ultimately, cell death in cancer cells. Cl Pt NH 3 Cl NH 3 cisplatin, 1 The use specifically of the cis -isomer of diamminedichloroplatinum(II) is crucial as the trans - isomer does not display the same kind of biological activity. Therefore, cis -/ trans - isomerism in d 8 , square-planar platinum complexes has been extensively investigated. Platinum(II) species 2 has received some attention with regards to its isomerization. Two mechanisms have been suggested for this process. One proposal invokes dissociation of bromide to yield a 14e - , three-coordinate intermediate ( 3a ) followed by isomerization and trapping by bromide. A second postulated mechanism proceeds by an associative mechanism involving a solvent molecule to give solvent-coordinated species 3b which then undergoes isomerization. P Pt R P Br P Pt R P k a k -a P = PEt 3 k b R Pt P P + Br - fast R Pt P P Br 2 3a 4a 5 P Pt R P Br P Pt R P k c k -c P = PEt 3 k d R Pt P P + Br - fast R Pt P P Br 2 3b 4b 5 S S = solvent + Br - + Br - S Dissociative pathway: Associative pathway:
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2 a) For each proposed reaction pathway, derive a rate law describing formation of the isomerized product 5 . Give expressions for k obs for the two mechanisms. Note that since bromide coordination to generate 5 from 4 is a very fast process you do not need to include this last step in your analysis. For each reaction use the steady-state approximation on the product of the first, reversible step (intermediates 3a and 3b , respectively). Based on the derived expressions for k obs , do you expect to be able to distinguish these two mechanisms using concentration-dependence kinetics experiments? b) The observed isomerization rates were recorded upon variation of bromide concentration in the reaction solution. The recorded data is shown in the table below. 10 4 [Br - ], M 10 4 k obs , s -1 R = p -MeC 6 H 4 0.0 38.40 1.0 12.60 2.0 7.25 4.0 3.71 6.0 2.42 8.0 2.03 10.0 1.45 20.0 0.76 30.0 0.58 R = o -MeC 6 H 4 0.0 6.95 0.6 3.91 0.8 3.34 2.0 1.74 4.0 0.96 8.0 0.58 10.0 0.38 R = 2,4,6-Me 3 C 6 H 2 0.0 2.13 2.0 1.05 4.0 0.69 6.0 0.53 8.0 0.43 Analyze the above data by utilizing a double-reciprocal plot, and determine relevant rate
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This note was uploaded on 01/03/2012 for the course CH 102 taught by Professor Hill,m during the Fall '08 term at Caltech.

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PS7 - Recommended reading: 23, 29 Ch 102 Problem Set 7 Due:...

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