Lecture_8_Kinetcs_BioIn

Lecture_8_Kinetcs_BioIn - Chem 310 Lecture Module 8...

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Chem 310 Lecture Module 8 Reactions, Kinetics/Mechanisms & Bio-Inorganic Concerns
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Reaction mechanisms Substitution reactions in square planar complexes: Replacement of one ligand A with a ligand B : three different pathways, as illustrated below: a) Nucleophilic substitution : nucleophilic attack of N 3 - ; details RE mechanism see below. b) Electrophilic substitution : electrophilic attack of Hg II on the Pt-C bond. c) Oxidative addition , followed by reductive elimination : oxidation of Pt II to Pt IV with multaneous addition of 2 Cl - followed by simultaneous addition of 2 Cl ; followed by elimination of methyl chloride H 3 C-Cl. Industrial relevance: Wacker process utilizes [PdCl 4 ] 2- as a homogeneous catalyst for the oxidation of ethylene to acetaldehyde: cis-platin is an antitumor agent. The biological activity involves coordination of the DNA to Pt, after hydrolysis and transfer of the agent from the blood system to the tumor cells:
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a) Nucleophilic substitution Y : nucleophilic ligand, which replaces X ; and T Four different substitution reaction types: 1. Associative, A. The M-Y bond is completely formed before M-X starts to break. pg , p ; is in trans position to X / Y . , py 2. Interchange associative, I a . M-X starts to break before M-Y is complete; but bond making is more important than bond breaking. 3. Dissociative, D. M-X is completely broken before the M-Y bond starts to form. It h d i it i I ik 2 b t b d ki i l i t t th b d b ki 4. Interchange dissociative, I d . Like 2., but bond making is less important than bond breaking. ould be first order reaction, and cond order Quite often, solvent molecules art Kinetics of nucleophilic substitution: D should be first order reaction, and A second order. Quite often, solvent molecules S start the reaction as nucleophiles, and then it looks like first order because [S] never changes. Trends. In nucleophilic solvents, A is preferred (with S attacking due to its high concentration). Generally, A is preferred in square planar complexes. In all observed reactions, Y goes where X was at the beginning (quite a difference to organic S N 2).
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The Trans Effect: In both cases, the second ligand is removed from the ans osition of a Cl - trans position of a Cl ligand due to its " trans-directing capability ". Must be a kinetic effect, for we observe two different products depending on the starting material. Explanation? Knowledge of mechanism will help.
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Carrying out more reactions helps to sort out the ligands according to their trans effect (compare, to the of π acceptor property series of ligands): This results in a trans-directing series: CN - , CO, NO, C 2 H 4 > PR 3 , H - , > CH 3 - , C 6 H 5 - , SR 2 > NO 2 - , I - , SCN - > Br - > Cl - > py > RNH 2 , NH 3 > OH - > H 2 O. cis-effect possible, but much weaker [as determined for the substitution of H 2 O for Cl - in [PtCl 3 L] n- ]:
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Mechanism: Attack of Y towards the empty p z orbital to form a square pyramid, followed by transformation to a trigonal bipyramid with Y , T , and X equatorial. With X leaving, the angle T-M-Y increases (from 120° to 180°) [again SP]: The TBP could either be an activated complex (a) or a true intermediate (b) which may be detected/isolated in some cases. The difference can be seen in the energy profiles:
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This note was uploaded on 02/28/2011 for the course CHEM 310 taught by Professor Nazar during the Fall '09 term at Waterloo.

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Lecture_8_Kinetcs_BioIn - Chem 310 Lecture Module 8...

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