CHBE 452 Lecture 30 Tying It All Together 1

Object For Today We previously stated that there are 5 principle causes for activation barriers Now we stated that there 8 ways catalysts work. How does it all fit together? 2

Exam Question What are the 5 principle causes of barriers to reaction? How do catalysts affect them? Give examples of each of the effects. 3

Recall From Before: Barriers To Reaction Are Caused By Uphill reactions Bond stretching and distortion. Orbital distortion due to Pauli repulsions. Quantum effects. Special reactivity of excited states. 4

Mechanisms Of Catalyst Action Catalysts can initiate reactions Catalysts can stabilize intermediates Catalysts can hold the reactants in close proximity. Catalysts can hold the reactants in the right configuration to react. Catalysts can block side reactions Catalysts can sequentially stretch bonds and otherwise make bonds easier to break. Catalysts can donate and accept electrons. Catalysts can act as efficient means for energy transfer. 5

How Do Catalysts Affect The Following Uphill reactions Bond stretching and distortion. Orbital distortion due to Pauli repulsions. Quantum effects. Special reactivity of excited states. 6

Uphill Reactions Catalysts can initiate reactions Catalysts can stabilize intermediates 7

Bond Stretching and Distortion Catalysts can sequentially stretch bonds and otherwise make bonds easier to break 8

Bond Stretching and Distortion 9 Figure The active site for reaction (12.91) on a palladium catalyst.

Lysozome Acts Like A PacMan To Help Bonds Stretch 10 Figure A Picture of Lysozyme 161L. This figure was generated using a program called RASMOL, using data in the protein data base from an x-ray diffraction spectrum generated by Weaver and Matthews[1987]

Catalytic Antibodies Modify Bond Stretching Energy 11 (12.94)

Transition Metals – Weaken Bond By Attaching To Antibonding Orbitals 12 Figure A diagram of the key interactions during the dissociation of hydrogen on platinum.

Orbital Distortion Due to Pauli Repulsions Catalysts can hold the reactants in close proximity. Catalysts can hold the reactants in the right configuration to react. Catalysts can donate and accept electrons. Catalysts can otherwise make bonds easier to break. 13

Acid Catalysts Reduce Pauli Repulsions 14

Acid Catalysts Aid Isomerization 15 Filled Empty

Catalyst Block Side Reactions By Increasing Pauli Repulsions 16 Figure A diagram of propylene polymerization in a Ziegler-Natta catalyst.

Holding Reactants In The Right Configuration Modifies Pauli Repulsion 17 Figure A cartoon of the reaction of ethanol and NAD + on the active site of liver alcohol dehydrogenase. Adapted from Oppenheimer and Handlon (1992) (In the Enzyme, vol 20 (1992) 453. (12.92)

Example Enzymes Modifying Pauli Repulsions 18

Influence Of Catalysts On Quantum Effects Catalysts can donate or accept electrons Electrons can be of the opposite symmetry 19

Catalysts Can Eliminate Barriers Due To A Need For Excited States Catalysts can stabilize intermediates Can be an excited state of reactants Di & tri radicals can be stable on catalysts 20 N

Summary Catalysts can reduce barriers due to: Uphill reactions Bond stretching and distortion. Orbital distortion due to Pauli repulsions. Quantum effects. Special reactivity of excited states. 21

Query What did you learn new in this lecture? 22