§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%.