ChE 452 Lecture 12 Mechanisms Continued 1. The Idea Of Computing A Mechanism 2.

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Presentation transcript:

ChE 452 Lecture 12 Mechanisms Continued 1

The Idea Of Computing A Mechanism 2

Today More complex reactions When does the method fail 3

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

Additional Reactions 5 (5.64) (5.63)

Additional Reactions Continued 6 (5.65) (5.66)

Additional Rules: The most important initiation reaction will be the reaction which breaks the weakest bond in the reactants (5.67) During propagation reactions if everything else is equal, it is usually easier to transfer atoms than to transfer molecular ligands (5.68) Should include the reverse of the initiation reactions to avoid explosions 7

Example: Predict A Mechanism For CH 3 CHO  CH 4 +CO 8 (5.70)

Next: Need A Catalytic Cycle Lets start with a CH 3 Radical Possible cycle 9 (5.71) (5.72)

Is This Feasible? Consider CH 3 +CH 3 COH  CH 4 +CH 3 CO H f CH 3 COH = CH 4 = CH 3 = CH 3 CO = -2.9 Kcal  H r = -17.9–2.9– = E a = x(-15.1)=7.5kcal/mole 10 NIST Table 5.4

Intrinsic Barriers and Transfer Coefficients for Different Types of Neutral Species 11 Table 5.4 Return

Complete The Catalytic Cycle (Regenerate The Methyl Group) 12 Feasible for (5.74) NIST Table 5.4

Next Consider Other Reactions 13 (5.71) H f =CH 3 OH=-40.8 CH 4 =-17.9 CH 3 =35.1 CH 2 COH=~10  H r = (-40.8)-35.1=-2.2 E a = (-2.2)=11.3kcal/mole Important for T>11.3/0.07=162K NIST Table 5.4

Other Possible Reactions 14 (5.73) NIST Table 5.4

Next: The Formyl (HCO) Radical Produced Still Need To Consider The Reaction 15 Procedure should continue for H atoms H 2 CO NIST Table 5.4

When Does Procedure Fail? 16

Reactions With No Initiation- Propagation Mechanisms

Reactions With High Barriers Example: 18 (High barrier)

Association Reactions 19 + X

Unimolecular Reactions 20 (5.102) (5.103)

Could CH 3 NC  CH 3 CN Go By An Initiation/Propagation Mechanism? 21 Middle step has very high barrier Simple rearrangement has lower barriers

Lindemann Mechanism Methyl isocyanide + X  excited methyl-isocyanide + X excited isocyanide  methylcyanide 22 (5.107)

Concerted Eliminations 23 (5.108) (5.109)

Summary Of General Rules For Radical Reactions 24

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