Kinetics 3

1
Fred J. Grieman
Kinetics (3) Mechanism consists of Elementary
Steps Rate Law ? Mechanism via Collision
Complex Rate Determining Step Pre-equilibria Steps
Division 1 Rugby Claremont Colleges 27
UCLA 7
Claudia Lucero, Candidate for visiting organic
professor tomorrow at 400 PM in SN 202
2
For elementary step Rate IS given by
reaction Rate ? reactant
Mechanism made of Elementary Steps 3 types -
molecularity
ku unimolecular A ? X
kb bimolecular A
B ? Y
kt termolecular A B
C ? X
rate ku A rate kb A B rate kt
ABC very rare
Overall reaction S elementary steps Over all
rate law determined from elementary
steps Example NO2(g) CO(g) ? CO2(g)
NO(g) rate -dNO2/dt kNO22 (from
experiment) Mechanism?
k1 NO2 NO2 ? NO3
NO k2
NO3 CO ? CO2
NO2 NO2 CO ? CO2 NO v
Rate? Assume 1st step is rate determining
step (RDS) Rate k1 NO2 NO2 k1 NO22
slow
fast
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NO2 CO ? CO2 NO v v
? any unreasonable conditions or species
v Does NO3 occur? NO2/NO2 crossed beams ? NO
NO3

• Is this the mechanism?
• sums to overall reaction
• gives resulting rate law
• chemically and physically reasonable
• experimental proof
• Rate Law must be consistent w/ mechanism but only
  gives clues
• Rate Law gives Collision Complex (C.C.) of RDS
• exponents in rate law become factors
• rate kNO22 ? 2N 2(2O) N2O4
  collision complex
• C.C. is some combination of molecules and/or
  atoms during collision in RDS
• In our last example NO2???NO2 ? N2O4

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Another Example 2NO(g) O2(g) ? 2NO2(g)
rate kNO2O2 (from exp.) C.C.
? C.C. N2O4 What rate determining step?
No one step will work. Rate Law rule
non-integral powers, negative powers, overall
order 3 then RDS in
NOT first step How might we form C.C.? If
start with O2, then O2 N2O2 would give C.C.

k1 Possible mechanism NO NO ?
N2O2 dimer formation

k -1
k2
N2O2 O2 ? 2
NO2 Rate Law from mechanism rate k2 O2
N2O2 from rate determining step From
pre-equilibrium k1NO2 k-1N2O2
solving for N2O2 k1NO2/k-1 Subbing in
rate k2O2 k1NO2/k-1 k2k1 NO2O2

k-1 Which agrees with
exp. rate law with kexp k2k1/k-1
?N?O?- ? ? ?-O?N?
If so, back rxn occurs ? fast pre-equilibrium
fast pre-eq.
?
slow, RDS
Intermediate!!!
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Example with Charge and Negative Power Hg22
Tl3 ? Tl 2Hg2 rate k
Tl3Hg22 C.C. ?

Hg2 C.C. Tl2Hg-Hg32-2
TlHg3 Proposed Mechanism
k1 Hg22 ? Hg Hg2 Pre-eq.
k -1 k2 Hg Tl3
? Tl Hg2 RDS
rate k2 HgTl3 (from rate determining
step) pre-eq. k1Hg22 k-1HgHg2
rate k2k1Hg22 Tl3
k -1 Hg2
? Hg k1Hg22
k-1Hg2
Does it agree With rate law?
? with k k2 k1/k -1
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Example with fractional power Cl2 CHCl3 ?
HCl CCl4 rate k Cl21/2
CHCl3 C.C. ?
C.C. ½(2Cl)CH3Cl
CHCl4 Proposed Mechanism
k1 Cl2 ? Cl Cl
pre-eq. k -1
k2 Cl
CHCl3 ? CCl3 HCl RDS
CCl3 Cl ? CCl4 fast Cl2
CHCl3 ? HCl CCl4
Rate k2 ClCHCl3 from rate determining
step pre-eq. k1Cl2 k-1Cl2 rate
k2k11/2 Cl21/2 CHCl3 k -11/2
Does it agree with rate law?
? with k k2 ( k1/k -1)1/2
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Summary of Mechanism from Rate Law with Rate
Determining Step

• Rate law gives Collision Complex Formula Charge
• Overall order ? 3, negative fractional power
  indicates 1 or more pre-equilibria before RDS
• If stoichiometric coefficient gt order in rate
  law, then intermediate is formed that is consumed
  later.
• After RDS, rate law gives no information about
  mechanism.
• Next understand rate constant k(T)