ChE 452 Lecture 12 Mechanisms Continued 1. The Idea Of Computing A Mechanism 2.
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Transcript of ChE 452 Lecture 12 Mechanisms Continued 1. The Idea Of Computing A Mechanism 2.
ChE 452 Lecture 12
Mechanisms Continued
1
The Idea Of Computing A Mechanism
2
1) write down all possible reactions 2) Use rules to make sure no important
reactions are missing. must have initiation must have catalytic cycle should have termination
3) Use rules to eliminate excess reactions. Ignore identity reactions Ignore reactions with high barriers Allows H2 + Br2
2HBr to be predicted.
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
oAE
Additional Reactions
5
Chain Transfers
CH3 + CH3CH3 CH4 + CH2CH3
-scissions CH2CH2CH3 + X CH2=CH2 + CH3+X
Fragmentation reactions XCHOCCHXCOCH 32333
(5.62)
Association/Addition reactions
XCHCHCHXCHCHCH 223223 (5.63) (5.64)
(5.63)
Additional ReactionsContinued
6
0 2 3 43
4
20
34
20
34
20
(5.65)
Disproportionations
22332323 CH=CH+CHCHCHCHCHCH (5.64)
Isomerizations
(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 explosions7
Example: Predict A Mechanism For
CH3CHOCH4+CO
8
Initiation: Weakest bond breaks According to CRC the bond strengths are
)COCHCH(mole/kcal99CH 333 )COCHCH(mole/kcal65CC 33
mole/kcal96OCH Therefore C-C bond breaks
CH COH X CH HCO X3 3
(5.69)
• •
(5.70)
Next: Need A Catalytic Cycle
Lets start with a CH3• Radical
Possible cycle
9
COCHXCOCH
COCHCHCOHCHCH
33
3433
(5.71)
(5.72)
Is This Feasible? ConsiderCH3
•+CH3COHCH4+CH3CO•
Hf CH3COH = -40.8
CH4 = -17.9
CH3• = +35.1
•CH3CO = -2.9 Kcal
Hr = -17.9–2.9–35.1+40.8= -15.1
Ea = 12+0.3x(-15.1)=7.5kcal/mole
10
NIST
Table 5.4
Complete The Catalytic Cycle (Regenerate The
Methyl Group)
12
Feasible for
K25005.0
6.12T
XCOCHXCOCH 33
(5.73) From the NIST web book
mole/kcal9.2COCH3 mole/kcal4.26CO
mole/kcal1.35CH3 6.11)9.2()4.26(1.35H
mole/kcal6.1216.11Ea
(5.74)
NIST
Table 5.4
Next Consider Other Reactions
13
(5.71)
Hf=CH3OH=-40.8CH4=-17.9CH3=35.1CH2COH=~10Hr=10+17.9-(-40.8)-35.1=-2.2
Ea=12+0.3+(-2.2)=11.3kcal/mole
Important forT>11.3/0.07=162K
COHCHCHCOHCHCH 2433
NIST
Table 5.4
Other Possible Reactions
14
mole/kcal5.38mole/kcal23x5.050)72.5(Ea
mole/kcal23mole/kcal88mole/kcal65H
OCHCHCHOCHCHCH 3333
(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 H2CO
X+COHXHCO
(5.74)
3.154.104.261.52H
4.10HCO 1.52H 4.26CO mole/kcal3.1613.15Ea
NIST
Table 5.4
When Does Procedure Fail?
No initiation propagation mechanism High Barriers to key steps Reactions occuring with a single concerted
process Reaction usually slow
16
Reactions With No Initiation-Propagation Mechanisms
2Na Cl 2NaCl2
Na Cl NaCl Cl2
now what?
Na + Cl NaClwallwall
Very low rate
17
5.90
5.91
5.84
Reactions With High Barriers
Example:
18
2CH C H H4 2 6 2 CH CH H4 3
H CH CH H4 3 2 CH CH C H H3 4 2 6 H CH CH3 4 Very slow reactions: Usually not seen!!
(High barrier)
5.95
5.96
5.99
Association Reactions
NH HCl NH Cl3 4
NH HCl X NH Cl X3 4
19
C
C
CC
R
C
C
Y+
C
C
CC
R
C
C
Y
+ X+ X
5.92
5.93
5.94
Unimolecular Reactions
Cyclopropane Propene
(5.100) CH NC CH CN3 3
(5.101)
20
(5.102)
(5.103)
Could CH3NCCH3CN Go By An Initiation/Propagation
Mechanism?
CH NC CH NC3 3
CH NC CH CH NCCH3 3 3 3
CH NC CH CN3 3
21
Middle step has very high barrier Simple rearrangement has lower barriers
5.104
5.105
5.106
Lindemann Mechanism
Methyl isocyanide + X excited methyl-isocyanide + X
excited isocyanide methylcyanide
22
(5.107)
Concerted Eliminations
23
XCH2CHCHCHCHCHX 233223
(5.106)
X C H CH CHCHCH H X4 10 3 3 2
(5.107)
(5.108)
(5.109)
Summary Of General Rules For Radical Reactions
24
Table 5.5 Summary of the Initiation-Propagation mechanisms of radicals.
1) Initiation Step - Weakest bond in reactants break to yield radicals
2) Radical reacts via a catalytic cycle - Atoms transferred one atom at a time - Must be cycle
3) Termination Step - Radicals recombine Exceptions: No catalytic cycle where atoms are transferred one atom at a time.