§10.5 catalytic reaction out-class extensive reading: levine, p.577 17.16 catalysis 17.17 enzyme...
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§10.5 Catalytic reaction
Out-class extensive reading:
Levine, p.577
17.16 Catalysis
17.17 enzyme catalysis
5.1 Catalysts and catalysis
catalyst Substance that changes the rate of a chemical reaction without themselves undergoing any chemical change.
catalysis
The phenomenon of acceleration or retardation of the speed of a
chemical reaction by addition of small amount of foreign substances
to the reactants.
5.2 type of catalysis
Heterogeneous catalysis is going to be discussed in Surface Chemistry.
Types Definition Examples
1) Homogeneous catalysis
the catalyst is present in the same phase as the reactant.
Hydrolysis of sucrose with inorganic acid.
2) Heterogeneous catalysis
the catalyst constitutes a separate phase from the reaction system
Haber’s process for ammonia
synthesis; contact oxidation of
sulphur dioxide; Hydrogenation
of alkene, aldehyde, etc.
3) Biological catalysis / enzyme catalysis
Reaction catalyzed with biological catalysts: enzyme
Hydrolysis of starch in stomach
5.3 General characteristics of catalyzed reactions
1) Catalyst takes part in the reaction.
(CH3)3COH (CH3)2C=CH2 + H2O
without catalyst:
k = 4.8 1014 exp(-32700/T) s-1
with HBr as catalyst:
kc = 9.2 1012 exp(-15200/T) dm3mol-1s-
1
23
14
12
101.432700
exp108.4
15200exp102.9
T
Tk
kc
with HBr as catalyst:
2) t-Bu-Br (CH3)2C=CH2 + HBr
1) t- Bu-OH + HBr t-Bu-Br + H2O
1
1
2
A C A C
A C + B A B + C
k
k
k
1 2
1
[A][B][A][B]
k kr k
k
1,2,1,, aaaappa EEEE
By altering reaction path, catalyst lower activation energy of the overall
reaction significantly and change the reaction rate dramatically.
2) No impact on the thermodynamic features of the reaction
(1) Cannot start or initiate a thermodynamically non-spontaneous
reaction;
(2) Can change the rate constant of forward reaction and backward
reaction with the same amplitude and does not alter the final
equilibrium position.
Catalyst can shorten the time for reaching equilibrium.
e
e
ln ( )( )
xk k t kt
x x e
e
ln ( )( )
xk k t kt
x x
(3) Is effective both for forward reaction and backward reaction.
Study on the catalyst for formation of formic acid can be done with
easy by making use of the decomposition of formic acid.
.
2 2HCOOH CO HCat
3) Selectivity of catalysts
(1) The action of catalyst is specific. Different reaction calls for
different catalyst.
Hydrogenation? Isomerization?
(2) The same reactants can produce different products over different
catalysts.
CH2Ag
200~300 oCCH2 CH2
O
+21
O2CH2
CH2200~300 oC
+21
O2CH2
PdCl2 CuCl2CH3 C
O
H
(1) The chemical composition of catalyst remains unchanged at the
end of the reaction;
(2) Only a small amount of catalyst is required;
(3) Catalyst has optimum temperature;
(4) Catalyst can be poisoned by the presence of small amount of
poisons; anti-poisoning.
(5) The activity of a catalyst can be enhanced by promoter;
(6) catalyst usually loaded on support with high specific area , such
as activated carbon, silica.
4) Other characteristics:
5.4 kinetics of homogeneous catalysis
1 2
1 2
[S][C][S][C] '[S]
k kr k k
k k
For homogeneous reaction, the reactant is usually named as substrate.
S C M P C1 2
1
k k
k
' [C]k k
0 2 4 6 8 10
-2
-1
0
1
2
3
log
k a
- lgKa
C12H22O11 + H2O C6H12O6 + C6H12O6
612 22 11 2[C H O ][H O] [H ]r k
When C is acid, rate constant is
proportional to dissociation constant
(Ka) as pointed out by Brønsted et al.
in the 1920s:
Dehydration of acet-aldehyde catalyzed by different acids.
Where Ga and is experimental constants.
ranges between 0 ~ 1.
aaa KGk lglglg aaa KGk
In aqueous solution, the acid may be H+ or H3O+ but in general it
may be any species HA capable of being a proton donor (Brønsted
acid) or a electron acceptor (Lewis acid).
For base-catalyzed reaction there also exists:
bbb KGk
5.6 Enzyme catalysis
Enzymes are biologically developed catalysts, each usually having
some one specific function in a living organism.
Enzymes are proteins, ranging in molecular weight from about
6000 to several million. Some 150 kinds have been isolated in
crystalline form.
The diameter of enzyme usually ranges between 10 ~ 100 nm.
Therefore, the enzyme catalysis borders the homogeneous catalysis
and the heterogeneous catalysis.
( 1 ) Kinds of enzymes:
pepsin Hydrolysis of proteins
diastase Hydrolysis of starch
urease hydrolysis of urea
invertase hydrolysis of sucrose
zymase hydrolysis of glucose
maltase Hydrolysis of maltose
Important hydrolytic enzymes
oxidation-reduction enzymes
SOD(Superoxide Dismutase) Decomposition of superoxide (O2-)
Nitrogenase Dinitrogen fixation
1) hydrolytic enzymes
2) oxidation-reduction enzymes
(2) Kinetics of enzyme catalysis
A rather widely applicable kinetic framework for enzymatic action is that known as the Michaelis-Menten Mechanism (1913).
Enzyme-substrate complex
3
[P][ES]
dk
dt 1 2 3
[ES][E][S] [ES] [ES]
dk k k
dt
0[E] [E] [ES]
1 0 1 2 3
[ES][E] [S] [ES][S] [ES] [ES]
dk k k k
dt
1 3
2
S E SE P Ek k
k
?
Using stationary-state approximation
1 0
1 2 3
[E] [S][ES]
[S]
k
k k k
1 3 0
1 2 3
[E] [S][P]
[S]
k kd
dt k k k
3 0 3 0
2 3
1
[E] [S] [E] [S]
[S][S] M
k kr
k k kk
Michaelis constant
Discussion: 1) When [S] >> kM:
3 0[E]mr k
2) When [S] << kM:
30[E] [S]
M
kr
k
When [S] = kM:
3 0 3 0[E] [S] [E] 1
2[S] 2 2 m
k kr r
3 0[E] [S]
[S] M
kr
k
3 0[E]mr k
[S]
[S]m M
r
r k
1 1 1
[S]M
m m
k
r r r
Lineweaver-Burk plotSlope: S = kM/rm
intercept: I = 1/rm
Both rm and kM can be obtained by solving the equations.
Many enzyme systems are more complicated kinetically than the
foregoing treatment suggests.
There may be more than one kind of enzyme-substrate binding site;
sites within the same enzyme may interact cooperatively. Often, a
cofactor is involved.
http://en.wikipedia.org/wiki/Image:Luciferase-1BA3.png
Luciferase (荧光素酶 ) is a generic name for enzymes commonly used in nature for bioluminescence.
(2) Outstanding characteristics of enzyme catalysis
1) High selectivity:
substrate
enzyme
Lock and key
Even 10-7 mol dm-3 urease can catalyze the hydrolysis of urea
(NH2CONH2) effectively. However, it has no effect on CH3CONH2.
NH
N
O
O
H OO
OHN
Multiple optically active centers produced by imidase catalysis
OHHO
R2HH R1
O O
O O
NH
OH
O
R1
R2 R1 R2 R1 R2
R2R1R2
R1
HHH
H HOOH
Imidase
Chirality of enzyme catalysis
1975 Noble Prize
Great Britain 1917/09/07
for his work on the stereochemistry of enzyme-catalyzed reactions
John Warcup Cornforth
2) High efficiency
Activation energy of hydrolysis of sucrose is 107 kJ mol-1 in presence of H+, while that is 36 kJ mol-1 in presence of a little amount of saccharase, corresponding to a rate change of 1022.
A superoxide Dismutase can catalytically decompose 105 molecules
of hydrogen peroxide in at ambient temperature in 1 s, while
Al2(SiO3)3, an industrial catalyst for cracking of petroleum, can only
crack one alkane molecules at 773K in 4 s.
3) Moderate conditions
Nitrogenase in root-node can fix dinitrogen from dinitrogen and water at ambient pressure and atmospheric pressure with 100 % conversion. While in industry, the conversion of dinitrogen and dihydrogen to ammonia over promoted iron catalyst at 500 atm and 450 ~ 480 oC for single cycle is only 10~15%.