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ChE 553 Lecture 19

ChE 553 Lecture 19. Mechanisms Continued. The Idea Of Computing A Mechanism. Today. More complex reactions When does the method fall. More Complex Reactions. Same process works Need additional reactions Need additional and  - Betas Need additional rules. Additional Reactions.

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ChE 553 Lecture 19

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  1. ChE 553 Lecture 19 Mechanisms Continued

  2. The Idea Of Computing A Mechanism

  3. Today • More complex reactions • When does the method fall

  4. More Complex Reactions • Same process works • Need additional reactions • Need additional and  - Betas • Need additional rules

  5. Additional Reactions

  6. Additional ReactionsContinued (5.65)

  7. Additional Rules: • The most important initiation reaction will be the reaction which breaks the weakest bond in the reactants (5.66) • During propagation reactions if everything else is equal, it is usually easier to transfer atoms than to transfer molecular ligands (5.67) • Should include the reverse of the initiation reactions to avoid explosions

  8. Example: Predict A Mechanism For CH3CHOCH4+CO

  9. Next: Need A Catalytic Cycle Lets start with a CH3• Radical Possible cycle

  10. Is This Feasible? ConsiderCH3•+CH3COHCH4+CH3CO• Hf CH3COH = -40.8 CH4 = -17.9 CH3• = +35.1 CH3CO = -2.9 Kcal Hf = -17.9–2.9–35.1+40.8= -15.1 Ea = 12+0.3x(-15.1)=7.5kcal/mole

  11. Complete The Catalytic Cycle (Regenerate The Methyl Group) Feasible for

  12. Next Consider Other Reactions (5.71) • Hf=CH3OH=-40.8 • CH4=-17.9 • CH3=35.1 • CH2COH=~10Kca • H=10+17.9-(-40.8)-35.1=-2.2 Ea=12+0.3+(-2.2)=11.3kcal/mole Important for T>11.3/0.07=162K

  13. Other Possible Reactions (5.72)

  14. Next: The Formyl (HCO) Radical Produced In Still Need To Consider The Initiation Procedure should continue for H atoms H2CO

  15. When Does Procedure Fail?

  16. Reactions With No Initiation-Propagation Mechanisms

  17. Reactions With High Barriers Example: (High barrier)

  18. Association Reactions + X + X

  19. Unimolecular Reactions

  20. Could CH3NCCH3CN Go By An Initiation/Propagation Mechanism? Middle step has very high barrier Simple rearrangement has lower barriers

  21. Lindemann Mechanism Methyl isocyanide + X  excited methyl-isocyanide + X excited isocyanide  methylcyanide

  22. Concerted Eliminations

  23. Summary Of General Rules For Radical Reactions Temperatures above 700 K (More reactions happen)

  24. Next: Reactions Of Ions • Gas phase reactions and reactions on metals generally go by radicals • Reactions in solution often go via ions • Ions more stable than radicals

  25. Reactions Of Ions Still Follow Initiation Propagation Mechanism • Form ion • Ion reacts • Ions recombine Not necessary cycles since ion concentrations high

  26. Some Examples Of Ion Reactions

  27. Some Examples Of Ion Reactions

  28. Key Features Of Ion Reactions • High concentrations of reactive species do not need cycles. • Ions have lower pauli repulsions: leads to additional reactivity

  29. Example: CH3CH2OHH2C=CH2+H2O H+ Initiation: Propagation: Termination:

  30. Ions Have Unusual Bonding: Changes Reactivity Example: Also can get intermolecular bonding

  31. Leads To Rich Chemistry

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