In What Ways Do We Characterize Reactions?
2 H2 + O2 2 H2 O
What can we know about this reaction?
In What Ways Do We Characterize Reactions?
2 H2 + O2 2 H2 O
Identify reactants and products. Determin
Chemical Reactions Involve
Changing Electronic Configuration
Electrons move (many orders of magnitude)
faster than atoms.
So, as atoms move, electrons & orbitals are
constantly responding.
One meth
Isotope Effects
Remarkably, replacing one isotope of an atom with another can substantially affect reaction equilibria and rates.
O H/D NO2 N O H/D NO2 N
K
toluene
+
NO2 NO2
+
KH = 1.4 KD
OH D/H H/D O
Marcus Theory and Photosynthesis
Fundamental question: Photosynthesis converts light energy into potential energy of charge separation. How does photosystem prevent chargehole recombination?
periplas
Linear FreeEnergy Relationships (Substituent Effects, LFERs, or Hammett Plots)
O OH O O
+ X X
H+
Qualitative question:
O O
How does substituent X affect acidity? As a reminder,
O O
resonancestabilize
Isotope Tracking
Easier way to use isotopes in mechanistic analysis is to simply track location of isotope in products. Classic Example: Eschenmoser's experiment.
O S O O H R R R base O S O O O S H CH
Kinetic Isotope Effects
Isotope effects more interesting when they tell us about character of ratedetermining transition state. Example:
EtO D/H EtO H H
H/D H
EtO(H/D) H3C H D/H H H Br
+
H3C D/H Br H
Entropic Catalysis: Bringing Reactants Together
Can catalysts function by improving only S?
A B P
E
Huncat TSuncat A B P
Guncat
A+B P Assuming our cartoon catalyst doesn't interact enthalpically with
HeavyAtom Isotope Effects
kH/kD provides large change in reduced mass, but other isotope effects also studied.
isotope effect typical maximum value
k35Cl Example: k37Cl
B
H R R
35/37
k12C ~ 1.04 k13C
Enthalpic Catalysis: Specific Interactions with Transition States
More common: Catalyst stabilizes ratedetermining transition state with specific, positive interactions.
E
Ga(TS)
S
EP ES Gcat < Gunca
Solvation and Weak Interactions
Like dissolves like: appropriate, but not sufficient.
Gains:
SA
entropy of molecular dispersion
KAB
weak interactions between A, B
Losses:
SB
solvent B is ordered aroun
Chemistry 4011/8011
Wednesday, September 27
Discussion Question
1. Although gaseous methyl isocyanide (CH3NC) can be stably isolated and
bottled at low temperature, at 273 K it isomerizes over the cou
Integrating Rate Expressions
1. Rewrite rate law in terms of one variable reactant ([A]). 2. Rearrange to put all [A] on one side, t on the other. 3. Integrate from [A]0 to [A]t and time t = 0 to t. 4
EnthalpyEntropy Compensation
Strategy of adding binding groups to increase ligand affinity works.
H H O
But, for additive weak interactions, increased binding enthalpy is compensated by decreased e
Pseudo FirstOrder Kinetic Data
H2O
+
H
Cl
HO H
+
HCl
Reactions With Multiple Mechanisms
Cl N Pt N Cl Br
+ Br
N Pt N Cl
+ Cl
Why not?
BrCl N Pt N Cl
k2
Br N Pt N Cl
+ Cl
A

Cl N Pt N Cl
[ A ] = k
First and SecondOrder Kinetic Data
Which plots are 1st order, and which are 2nd?
First and SecondOrder Kinetic Data
1 1 = + kt [ A ]t [ A ] 0
(2nd order)
ln[ A]t = ln[ A]0 kt
(1st order)
First and S
Transition State Theory
A
k1 k1
P
A
k
ETS
[TS]
Previously,
P
E
TS
nTS ( E TS E A ) / kBT  / kBT =e =e nA
But this is a microscopic equation. For thermodynamic analysis, need functions of whole syst
Molecular Orbital (MO) Theory
Molecular orbital mixing takes place at all times along reaction coordinate.
R HO (SN2) R HO H H
+
H
Cl H
+ Cl
R HO H Cl H
CCl
As HO approaches, and as Cl departs, or
Approximations in Complex Kinetics
k1
A + B
k1 k2
C
[ A ] = k1[ A ][B] + k 1[C] t [P] = k 2 [C] t [C] = k1[ A ][B]  k 1[C]  k 2 [C] t
C
P
All of these expressions contain [C], which is often tra
Arrhenius Relationship and Plots
k obs = Ae
Ea RT
Ea 1 ln(k obs ) = ln A R T
Can use lnk vs. 1/T plot to obtain A (intercept), Ea (slope)
Arrhenius Relationship and Plots
k obs = Ae
Ea RT
Ea 1 ln(k
Transition States in Multistep Kinetics
G
krxn
kBT  G / RT = e h
When we determine G, what are we actually measuring? For simple kinetics, answer is easy.
Transition States in Multistep Kinetics
G
kr
KineticThermodynamic Connections: Hammond Postulate
Variation in GO shows up in G. However, this isn't mathematical.
Can relationship between G and G0 be stated mathematically? Is Hammond's Postulat