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

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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

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(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

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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

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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:

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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

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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

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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

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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.