chapter 6 chemical reactivity and reaction...
TRANSCRIPT
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Chapter 6
Chemical Reactivityand
Reaction Mechanisms
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Chemical Reactivity
Enthalpy
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A simple chemical reaction can be broken down into bond creating and
bond breaking components:
A-B + Y-Z → A-Y + B-Z
A-B → A· + B· ∆Hº = +BDE Y-Z → Y· + Z· ∆Hº = +BDEA· + Y· → A-Y ∆Hº = -BDEB· + Z· → B-Z ∆Hº = -BDE
BDE= Bond dissociation energy
∆Hºrxn =∑∆Hºbonds broken-∑∆Hºbonds formed
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Homolytic Bond Dissociation Energies(∆Hº): Y−Z → Y· + Z·
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∆Hºrxn =∑∆Hºbonds broken-∑∆Hºbonds formed
H
CH3C
H
H Br Br
H
CH3C
H
Br H Br++
423 kJ/mol 192 kJ/mol 294 kJ/mol 366 kJ/mol
For the process:A-B + Y-Z → A-Y + B-Z
BDE=
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∆Hºrxn =∑∆Hºbonds broken-∑∆Hºbonds formed
H
CH3C
H
H Br Br
H
CH3C
H
Br H Br++
423 kJ/mol 192 kJ/mol 294 kJ/mol 366 kJ/mol
∆Hºrxn = 423 +192-294-366
= -45 kJ/mol
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Enthalpy(H)
Reaction coordinate
An Exothermic Reaction
∆H0 =⊖
A + B
C + D
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Enthalpy(H)
Reaction coordinate
And Endothermic Reaction
∆H0 =⊕
A + B
C + D
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Chemical Reactivity
Entropy(a measure of the disorder associated with a system)
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Entropy
Entropy change is favorable when the result is a more
random system.
This is called a Positive Entropy Change.
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The Concept of Entropy(The Degree of Randomness)
A change in degree of randomness is a change in the number of ways of arranging particles.
More order Less Order
solid liquid gas
solid + liquid ions in solution
solid + solid gases + ions in solution
Fewer degrees of More degrees ofmovement movement
fewer particles more particles
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Chemical Reactivity
Gibbs Free Energy
ΔG = ΔH - TΔS
Enthalpy Factor Entropy Factor
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Freeenergy
(G)
Reaction coordinate
∆G0 =⊖
A + B
C + D
Exergonic process
A + B C + D
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Freeenergy
(G)
Reaction coordinate
∆G0 =⊕
A + B
C + D
Endergonic Process
A + B C + D
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Chemical ReactivityEquilibria
!G under Nonstandard Conditions
"!!G = !G° only when the reactants and products are in their standard states
"! there normal state at that temperature
"!partial pressure of gas = 1 atm
"!concentration = 1 M
"!under nonstandard conditions, !G = !G° + RTlnQ
"!Q is the reaction quotient
"! at equilibrium !G = 0
"!!G° = !RTlnK
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ΔG < 0 for a spontaneous processΔG > 0 for a nonspontaneous processΔG = 0 for a process at equilibrium
ΔG = ΔH - TΔS
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!G under Nonstandard Conditions
"!!G = !G° only when the reactants and products are in their standard states
"! there normal state at that temperature
"!partial pressure of gas = 1 atm
"!concentration = 1 M
"!under nonstandard conditions, !G = !G° + RTlnQ
"!Q is the reaction quotient
"! at equilibrium !G = 0
"!!G° = !RTlnK
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∆Gº (kJ/mol) Keq% product at equilibrium
-17 1000 99.9%
-11 100 99%
-6 10 90%
0 0 50%
+6 0.10 10%
+11 0.01 1%
+17 0.001 0.1%
Relationship Between ∆G and Keq
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Chemical Reactivity
Kinetics and Rates
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First-Order Kinetics Second-Order Kinetics
Rate of a Reaction
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What Affects the Rate of a Reaction?
Energy of ActivationTemperature
Steric Considerations
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Thermodynamics vs. Kinetics Thermodynamics vs. Kinetics
Eactivation
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Faster Slower
∆Eact
∆Eact
∆Eact
∆Eact
Exe
rgo
nic
En
de
rgo
nic
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Effect of a Catalyst
∆E
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(g) (l) (l)
∆Hº ≈ −108 kJ/mol
H C
H
C
H
H H
H + Br Br H C
H
C
Br
H H
H + H Br
(g) (l) (l) (g)
∆Hº ≈ −53 kJ/mol
HC C
H
H
HBr Br H C
Br
C
Br
H H
H+
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HC C
H
H
HH H H C
H
C
H
H H
H+
(g) (l) (g)
∆Hº ≈ −105 kJ/mol
(g) (l) (l)
∆Hº ≈ −108 kJ/mol
H
C C
H
H
H
Br Br H C
Br
C
Br
H H
H+
Ni
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H C
H
C
Br
H H
HH
C C
H
H
HH Br+
(l) (g) (g)
∆Hº ≈ +57 kJ/mol
HC C
H
H
BrH C C H H Br+
(l) (g) (g)
∆Hº ≈ +198 kJ/mol
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Chemical Reactivity
Transition States vs.Intermediates
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Potentialenergy
Reaction coordinate
Reactant
Product
Transition State
High-energy stateCannot be isolated
Bonds being broken or formed
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Potentialenergy
Reaction coordinate
Reactant
Product
Transition State Transition
State
Intermediate
Longer-livedBonds not being
broken or formed
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R
R' R"
LGNu
- R
R'R"
Nu LG -LGR
R'R"Nu:
-
Some reactions proceed through a single transition state.
Potentialenergy
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LGR
R'R"
R
R' R"
+ R
R'R"
Nu
LGR
R'R"
R
R' R"
+Nu:-
Some reactions proceed through two 0r more reactive intermediates and transition states.
Pote
ntia
len
ergy
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Effect of a Catalyst
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Potentialenergy
Reaction coordinate
In an exergonic reaction, the transition state is closer to the energy and structure
of the reactant(s).
Ea
Reactants
Products
The Hammond Postulate
Changing the energy of the products does very little to influence the rate of the
reaction.
Products
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Potentialenergy
Reaction coordinate
Reactants
Products
The Hammond PostulateIn an endergonic reaction,
the transition state is closer to the energy and structure
of the product(s).
Ea
Changing the energy of the products lowers the energy of activation and increases
the rate of the reaction.
ProductsEa
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Chemical Reactivity
Nucleophiles and Electrophiles
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Nucleophilic CentersAn electron-rich atom which is capable of
donating a pair of electrons (i.e., a “Lewis base”)
May or may not have a negative charge:
Must consider electronegativity of atoms:
HO
H HO
CO
H
H
H
H
CLi
H
H
H
δ+....δ-
δ-....δ+
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Electrophilic CentersAn electron-deficient atom which is capable of
accepting a pair of electrons (i.e., a “Lewis acid”)
May or may not have a positive charge:
The electrophilic center may be an empty orbital:
CCl
H
H
HC
CH3
H3C
H3C
δ+....δ- C
CH3
H3C
CH3
≡
CCH3
H3C
CH3
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Patterns of Chemical Reactions
Nucleophilic AttackLoss of a Leaving Group
Proton TransferCarbocation Rearrangements
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Patterns of Chemical ReactionsNucleophilic Attack
BrH
O
H
Br O
H
H
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Patterns of Chemical ReactionsLoss of a Leaving Group
Br O Br
O
Br++ Br
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Patterns of Chemical ReactionsProton Transfer
H
H
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Patterns of Chemical ReactionsProton Transfer
OH
HO
H+
O
HO
H
H+
CC
O
HO H
CC
O
H
O H
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Patterns of Chemical ReactionsCarbocation Rearrangements
Where do carbocations come from ?
C LG C LG+
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H
C
H H
C HH
HC
H
HH
Two views of the methyl cation, CH3+. The carbon and three hydrogens all lie in the same plane and the carbon is sp2 hybridized. One unoccupieid, unhybridized p orbital remains perpendicular to the plane of the four atoms.
The orbitals of the three C-H bonds lie perpendicular to the
p orbital, resulting in zero overlap and zero
stabilization.
Stabilization by Hyperconjugation
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Two views of the ethyl cation, CH3CH2+. The carbon and three hydrogens of the positive carbon all lie in the same plane and the carbon
is sp2 hybridized. One unoccupied, unhybridized p orbital remains perpendicular
to the plane of the four atoms. The second carbon has the expected sp3 hybridization.
One of the three C-H bonds of the adjacent methyl group lies in the same plane as the unoccupied p orbital of of the positive carbon.
Some of the electron density flows from the sigma bond toward the
electron deficient carbon, resulting in stabilization of the carbocation.
Note: This stabilization is also present in two other conformers as
the C-C bond rotates.
H
C
H CH3
C CH
H
H
H
H
CH
HC
H
HH
Stabilization by Hyperconjugation
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Patterns of Chemical ReactionsCarbocation Rearrangements
Increasing Stability
Methyl Primary Secondary Tertiary
H
H H
H
C H
H
C C
C
C C
As more adjacent carbons with additional attached groups are added, additional
hyperconjugation and stabilization of the resulting carbocation occurs.
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Patterns of Chemical ReactionsCarbocation Rearrangements
Increasing Stability
Methyl Primary Secondary Tertiary
H
H H
H
C H
H
C C
C
C C
Carbocations may “rearrange” from 1º--->2º--->3º
to form more stable structures !!!
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Patterns of Chemical ReactionsCarbocation Rearrangements
H3CH3C CH3
HH
H3CH3C CH3
H3C
H3CH3C CH3
H3CH3C CH3
CH3
hydride shift
methyl shift
2º carbocations 3º carbocations
2º carbocation 3º carbocation
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H
C
C
C
CH3H3C
CH3
H H
C
C
C
CH3H3C
CH3
H
2º carbocation 3º carbocation
formation of a more stable resonance structure
Patterns of Chemical ReactionsCarbocation Rearrangements