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. Determine/predict reaction enthalpy (H) or energy (G).
e.g., he
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 method for tracking/illustrating this:
Electron Pushing
No
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 O2N O OEt O2N O H/D OEt
+ (H/D)OH
kH = 4.6 kD
Just repla
Marcus Theory and Photosynthesis
Fundamental question: Photosynthesis converts light energy into potential energy of charge separation. How does photosystem prevent chargehole recombination?
periplasmic space cyt c
photosynthetic reaction center (RC) BCh
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
resonancestabilized
 +

resonancedestabilized
+ +

O
O

NH2
+
NO2
i
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 CH3 O O
H CH 3
H CH3
Expected that reaction was intramole
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 H3C D/H
Br

How should isotopic substitution affect rat
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 [AB], can the cartoon catalyze the reaction?
reaction
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 k12C ~ 1.07 k14C k14N ~ 1.03 k15N k16O ~ 1.03 k18O
Met
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 < Guncat
Ga(S)
P
reaction coordinate
Enthalpic Catalysis: Spec
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 around A
KAA
broken interactions between AA
KBB
same for B
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 course of hours to
acetonitrile (CH3CN), which condenses f
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. Fit integrated model law to data.
Hypothesis: Kinetic
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 entropy.
Ha = 1.5 kcal/mol TSa = 1.0 kcal/mol
but ther
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 2 [ A ][Br  ] t
k1
N Pt N Cl
P
fast
Br N Pt N Cl
A
+ C
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 SecondOrder Kinetic Data
Direct fitting of data to inte
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 systems.
A
EA
reaction coordinate
Transition State Theory
A
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, orbitals mix.
R HO H Cl H R HO H Cl H
Osp3
CCl
Molecula
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 transient and unmeasurable. How do we deal with [C]?
The S
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 obs ) = ln A R T
but large factor of error makes this
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
krxn
kBT  G / RT = e h
When we determine G, what are we
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 Postulate correct?
KineticThermodynamic Connections: Marcus Th