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7. Reaction Mechanisms, Pathways,
Bioreactions and Bioreactors
o Active Intermediates and Free Radicals
- Pseudo-steady-state-hypothesis
- Reaction pathway
o Enzymes
- Michealis-Menten enzyme kinetics
- Enzyme inhibition
o Bioreactors
o Pharmacokinetics
o Polymerization
1. Active Intermediates / Free Radicals
(PSSH) I
Apr/25 2011 Spring 2
o Active intermediate
- A molecule that is in a highly energetic and reactive
state
- It is short lived as it disappears virtually as fast as it
is formed (ca. 10-14s)
- Very low concentrations ☞ the net rate of reaction of
an active intermediate, A*, is zero
⇒ Pseudo Steady State Hypothesis (PSSH)
1. Active Intermediates / Free Radicals
(PSSH) II
Apr/25 2011 Spring 3
o Active intermediate 2
- The active intermediates reside in the trough of the
reaction coordinate
1. Active Intermediates / Free Radicals
(PSSH) III
Apr/25 2011 Spring 4
o Active intermediate 3 - Example
- The rate law for the reaction A → B + C
is found from experiment to be
Suggest a mechanism consistent with the rate law.
Sol)
- Two A molecules collide and energy is transferred
from one A molecule to the other molecule making it
highly reactive.
1. Active Intermediates / Free Radicals
(PSSH) IV
Apr/25 2011 Spring 5
o Active intermediate 4 - Example
- This activated molecule (A*) can do one of two things
• It (A*) can collide with another molecule to become
deactivated (A)
• or (2) the activated molecule, A* can decompose to
form B and C
1. Active Intermediates / Free Radicals
(PSSH) V
Apr/25 2011 Spring 6
o Active intermediate 5 - Example
- For reactions with active intermediates, the reaction
coordinated has trough in it and the active
intermediate, A*, sit in this trough
1. Active Intermediates / Free Radicals
(PSSH) VI
Apr/25 2011 Spring 7
o Active intermediate 6 - Example
- Rate laws
- Relative rates
- Net rates = Rate of formation of product
1. Active Intermediates / Free Radicals
(PSSH) VII
Apr/25 2011 Spring 8
o Active intermediate 7 - Example
- Use PSSH to solve for CA*
- Solving for CA*
- Substituting for CA* to get the rate of formation of B
1. Active Intermediates / Free Radicals
(PSSH) VIII
Apr/25 2011 Spring 9
o Active intermediate 8 - Example
- Relative rates overall
- For high concentrations of A, we can neglect k3 with
regard to k2CA, i.e.,
- The rate law becomes
1. Active Intermediates / Free Radicals
(PSSH) IX
Apr/25 2011 Spring 10
o Active intermediate 9 - Example
- For low concentrations of A, we can neglect k2CA
with regard to k3, i.e.,
- The rate law becomes
- Dividing by k3 and letting k’ = k2/k3 and k = k1
2
A1
2
A
3
13A CkC
k
kkr
Produce an active
intermediate
- Collide with active intermediate
- Spontaneous decomposition
1. Active Intermediates / Free Radicals
(PSSH) X
Apr/25 2011 Spring 11
o Active intermediate 10 - Temperature dependence
- The reaction 2NO + O2 → 2NO2
has an elementary rate law
However... the rate law decrease with increasing
temperature, why?
Sol)
- Mechanism:
1. Active Intermediates / Free Radicals
(PSSH) XI
Apr/25 2011 Spring 12
o Active intermediate 11 - Temperature dependence
- Pseudo Steady State Hypothesis (PSSH)
• net rate of species A* (in this case, NO3*) is zero
- All specific reaction rates are defined with respect to
NO*3
- Assume that all reactions are elementary reactions
1. Active Intermediates / Free Radicals
(PSSH) XII
Apr/25 2011 Spring 13
o Active intermediate 12 - Temperature dependence
- The net reaction rate for NO*3
- The PSSH assumes that the net rate of NO*3 is zero
- Solving for NO3*
1. Active Intermediates / Free Radicals
(PSSH) XIII
Apr/25 2011 Spring 14
o Active intermediate 13 - Temperature dependence
1. Active Intermediates / Free Radicals
(PSSH) XIV
Apr/25 2011 Spring 15
o Gas phase chain reaction involving free radicals 1
- Hydrodealkylation
• in the gas phase at high temperature and involves free radical with the sequence
• specific reaction rates k1 and k4 are defined w.r.t. H2
• rC6H6 = ?
1. Active Intermediates / Free Radicals
(PSSH) XV
Apr/25 2011 Spring 16
o Gas phase chain reaction involving free radicals 2
- For the elementary equation, the rate of formation of
benzene is
- We need to eliminate the concentration of the free
radical by expressing it in terms of the
concentrations of toluene and hydrogen
- Under psuedo-steady-state hypothesis, we set the
rates of formation of and equal to zero, i.e.
1. Active Intermediates / Free Radicals
(PSSH) XVI
Apr/25 2011 Spring 17
o Gas phase chain reaction involving free radicals 3
- Adding the above 2 equations
- Solving for yields
- Solving for yields
1. Active Intermediates / Free Radicals
(PSSH) XVII
Apr/25 2011 Spring 18
o Gas phase chain reaction involving free radicals 3
- Eliminate
1. Active Intermediates / Free Radicals
(PSSH) XVIII
Apr/25 2011 Spring 19
o Reaction pathway
2. Enzymes I
Apr/25 2011 Spring 20
o Enzymes are protein like substances with catalytic
properties
- A pathway for the substrate to proceed at a faster
rate. The substrate, S, reacts to form a product P.
2. Enzymes II
Apr/25 2011 Spring 21
2. Enzymes III
Apr/25 2011 Spring 22
o A given enzyme
can only catalyze
only one reaction
2. Enzymes IV
Apr/25 2011 Spring 23
o Artificial kidney
2. Enzymes V
Apr/25 2011 Spring 24
o Artificial kidney 2
- The corresponding mechanism
- Michaelis-Menten Equation
2. Enzymes VI
Apr/25 2011 Spring 25
o Derive: Michaelis-Menten Kinetics
2. Enzymes VII
Apr/25 2011 Spring 26
o Michaelis-Menten Kinetics 2
- Inverting yields
(or double reciprocal plot)
2. Enzymes VIII
Apr/25 2011 Spring 27
o Michaelis-Menten Kinetics 3
- Lineweaver-Burk plot
2. Enzymes IX
Apr/25 2011 Spring 28
o Types of Enzyme Inhibition 1
- Competitive
2. Enzymes X
Apr/25 2011 Spring 29
o Types of Enzyme Inhibition 2
- Competitive 2 - Derive the rate
• competitive inhibition
• rate laws
2. Enzymes XI
Apr/25 2011 Spring 30
o Types of Enzyme Inhibition 3
- Competitive 2 - Derive the rate
-
2. Enzymes XII
Apr/25 2011 Spring 31
o Types of Enzyme Inhibition 4
- Competitive 2 - Derive the rate
- Total enzyme
2. Enzymes XIII
Apr/25 2011 Spring 32
o Types of Enzyme Inhibition 5
- Competitive 2 - Plot
2. Enzymes XIV
Apr/25 2011 Spring 33
o Types of Enzyme Inhibition 6
- Uncompetitive
2. Enzymes XV
Apr/25 2011 Spring 34
o Types of Enzyme Inhibition 7
- Uncompetitive 2
2. Enzymes XVI
Apr/25 2011 Spring 35
o Types of Enzyme Inhibition 8
- Mixed inhibition 1
2. Enzymes XVII
Apr/25 2011 Spring 36
o Types of Enzyme Inhibition 9
- Mixed inhibition 2