Chapter 4' Chemical Reactions

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Chapter 4. Chemical Reactions
Definitions Mechanism - detailed step by step
account of how a reaction occurs from reactants
to products. Kinetics - how fast a reaction
occurs as expressed by the rate of a reaction as
a function of the concentrations of reactants
(or products). Thermodynamics - the energy
difference between reactants and products.
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A. Chlorination of methane this is not a
useful reaction
1. overall reaction
2. mechanism
(just the first stage of the rxn)
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the mechanism is an example of a radical chain
reaction process
a. chain initiation
note single-headed arrows!
Cl atoms are reactive intermediates
b. chain propagation
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c. chain termination - radicals react with
themselves and interrupt the propagation cycle
also reaction against walls, disproportionation
reactions, etc
But there is more to the reaction mechanism -
there are also energetics to explore...
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B. Thermodynamics and Equilibrium Constants
1. Equilibria -
C, etc. concentration of C, etc
?????????????????????????????? - irreversible
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?????????????????
????DG Gibbs free energy -RTlnKeq
change in free energy on going from reactants to
products - kcal/mol
????????????????????????????????????????? T
temperature - K ln(x) 2.303log(x)
note that this is potential energy not kinetic
energy
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b. DG DH - TDS
DH enthalpy change on going from reactants to
products - difference in how much energy is
required to break bonds in reactants compared to
how much energy is gained by forming bonds in
products - kcal/mol
DS entropy change on going from reactants to
products - measures the change in randomness or
disorder - cal/mol-K
c. DG, DH and DS can be () or (-) lets look at
some examples
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DG, DH (-) K gt 1.0
DG, DH () K lt 1.0
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a multi-step reaction
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d. chlorination of methane - chain propagation
steps will control the overall thermodynamics of
the reaction
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C. Kinetics 1. rate of reaction - how fast
products form or reactants go away
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2. energetics
DG -RTln(k) and DG DH - TDS
measures reactants relative to transition state
in one step
DG Gibbs free energy of activation DH
enthlapy of activation DS entropy of activation
or in terms of the Arrenhius equation
Ea -RTln(k) A
Ea activation energy DH A frequency factor
? DS
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kinetic energy - normally from heat - provides
the energy to get over the reaction barrier
associated with the transition state
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3. rate determining step - each step has its own
rate in a multi-step reaction - the slowest
determines the rate of the overall reaction
note difference in forward and reverse reaction!
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rate does not depend on Nu used!
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for the chlorination of methane we have -
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D. The Relationship Between Kinetics and
Thermodynamics
1. linear free energy relationships
General Rule the more stable the product is, the
lower the activation energy is leading to it.
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2. But there is more to this reaction - consider
the chlorination of propane
? 2 H is abstracted 4.5 times faster than 1 H
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Furthermore, in isobutane
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3. reactivity versus selectivity - consider the
bromination of methane it has the same mechanism
this step was endothermic by 1 kcal/mol
for chlorination!
The rate determining first step is now much, much
slower than it is in chlorination.
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selectivities (relative rates) halogen 1 2
3 F2 1.0 1.2 1.4 Cl2 1.0 3.9 5.1 Br2 1.0 82 16
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This illustrates the reactivity- selectivity
principle the more reactive a reagent is, the
less selective it will be in a reaction.
Guys in bar analogy...
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4. The Hammond postulate - the transition state
structurally resembles most the molecule -
reactant or product - closest to it in energy.
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or, in general...
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5. Free radicals and aging.
X. cell membranes ? foreign cells ? destroyed
by immune system
various molecules including those formed by hn in
Cancun
A number of molecules can be used as radical
traps vitamin C A E...
Vitamin E
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very stable radical- will not damage cells
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E. Reactive Intermediates
1. carbocations - carbonium ions/ carbenium ions
a. structure
6 e-s around carbon - electron deficient -
very strong acid/ electrophile
b. stability
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• There are two effects at work here
• i. inductive effects - methyl groups donate
  electron density
• ii. resonance effects - hyperconjugation

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The more hyperconjugation resonance
structures there are, the more stable the
carbocation is.
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An even more stable carbocation is the allyl
cation
2. Free radicals - not the jazz/rock group!
a. structure
7 electrons around carbon
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b. stability - same as before

• inductive effects
• resonance effects - hyperconjugation

allyl radicals are also very stable
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3. carbanions
a. structure
8 electrons around carbon - too much electron
density - very strong base / nucleophile
b. stability - exactly opposite as before -
primarily due to inductive effects of methyl
groups
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an allyl anion is very stable again because of
resonance
In reality any CC will stabilize these
carbocations, radicals or carbanions via
resonance.
benzyl cation
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pentadienyl carbanion
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4. carbenes
a. structure
6 electrons around carbon, but...
there are really two kinds of carbenes
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b. history...
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• F. Summary
• mechanism
• kinetics
• a. rate constants
• b. activation energies (DG, DH, DS)
• c. rate determining step
• thermodynamics
• a. equilibrium constants
• b. energies (DG, DH, DS)
• c. endothermic/ exothermic
• linear free energy relationships
• reactivity versus selectivity
• the Hammond principle

• 5. mechanism of halogenation reactions
• a. initiation, propagation, termination events
• b. energetics - how to compute them from BDEs
• c. selectivities

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6. reactive intermediates a. carbocations,
carbanions, radicals b. stabilities - resonance,
inductive effects c. drawing resonance
structures, allyl, benzyl, etc. d. carbenes