chapter 6 chemical reactivity and reaction...
TRANSCRIPT
Chapter 6
Chemical Reactivity and
Reaction Mechanisms
Chemical Reactivity
Enthalpy
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 = +BDE A· + Y· → A-Y ∆Hº = -BDE B· + Z· → B-Z ∆Hº = -BDE
BDE= Bond dissociation energy
∆Hºrxn =∑∆Hºbonds broken-∑∆Hºbonds formed
Homolytic Bond Dissociation Energies(∆Hº): Y−Z → Y· + Z·
∆Hºrxn =∑∆Hºbonds broken-∑∆Hºbonds formed
For the process: A-B + Y-Z → A-Y + B-Z
Bond Dissociation Energies
H
CH3C
H
H Br Br
H
CH3C
H
Br H Br++
423 kJ/mol 192 kJ/mol 294 kJ/mol 366 kJ/mol
423 kJ/mol 192 kJ/mol 294 kJ/mol 366 kJ/mol
∆Hºrxn =∑∆Hºbonds broken-∑∆Hºbonds formed
∆Hºrxn = 423 +192-294-366
= -45 kJ/mol
H
CH3C
H
H Br Br
H
CH3C
H
Br H Br++
423 kJ/mol 192 kJ/mol 294 kJ/mol 366 kJ/mol423 kJ/mol 192 kJ/mol 294 kJ/mol 366 kJ/mol
Enthalpy (H)
Reaction coordinate
An Exothermic Reaction
∆H0 =⊖
A + B
C + D
Enthalpy (H)
Reaction coordinate
And Endothermic Reaction
∆H0 =⊕
A + B
C + D
Chemical Reactivity
Entropy (a measure of the disorder associated with a system)
Entropy
Entropy change is favorable when the result is a more
random system.
This is called a Positive Entropy Change.
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 of movement movement
fewer particles more particles
Chemical Reactivity
Gibbs Free Energy Change
ΔG = ΔH - TΔS
Enthalpy Factor Entropy Factor
Free energy
(G)
Reaction coordinate
∆G0 =⊖
Exergonic (Spontaneous) process
A + B C + D
Free energy
(G)
Reaction coordinate
∆G0 =⊕
Endergonic (non-Spontaneous) Process
A + B C + D
Chemical Reactivity Equilibria
!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 Note: As Keq increases, i.e., ∆G becomes more negative
and the reaction becomes more favorable.
ΔG < 0 for a spontaneous processΔG > 0 for a nonspontaneous processΔG = 0 for a process at equilibrium
ΔG = ΔH - TΔS
ΔH ΔS Low Temperature
High Temperature
- + Spontaneous Spontaneous
+ - Non- Spontaneous
Non- Spontaneous
- - Spontaneous Non- Spontaneous
+ + Non- Spontaneous Spontaneous
∆Gº (kJ/mol) Keq% product at equilibrium
-17 1000 99.9%
-11 100 99%
-6 10 90%
0 1 50%
+6 0.1 10%
+11 0.01 1%
+17 0.001 0.1%
Relationship Between ∆G and Keq
Chemical Reactivity
Kinetics and Rates
Potentialenergy
Reaction coordinate
Thermodynamics vs. Kinetics
Reactants
Products
thermodynamics initial and final states
spontaneity
kinetics intermediate states speed of reaction
Eactivation
First-Order Kinetics Second-Order Kinetics
Rate of a Reaction
What Affects the Rate of a Reaction?
Energy of Activation Temperature
Steric Considerations
Potentialenergy
Reaction coordinate
Reactants ➝ Products
Reactants
Products
Influence of a Catalyst
uncatalyzed reaction
Ea
Ea
catalyzed reaction
(g) (l) (l)
∆Hº ≈ −20 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 ≈ −95 kJ/mol
HC C
H
H
HBr Br H C
Br
C
Br
H H
H+
HC C
H
H
HH H H C
H
C
H
H H
H+
(g) (l) (g)
∆Hº ≈ −43 kJ/mol
(g) (l) (l)
∆Hº ≈ −20 kJ/mol
H
C C
H
H
H
Br Br H C
Br
C
Br
H H
H+
Ni
H C
H
C
Br
H H
HH
C C
H
H
HH Br+
(l) (g) (g)
∆Hº ≈ +86 kJ/mol
HC C
H
H
BrH C C H H Br+
(l) (g) (g)
∆Hº ≈ +98 kJ/mol
Chemical Reactivity
Transition States vs. Intermediates
Potentialenergy
Reaction coordinate
Reactant
Product
Transition State High-energy state Cannot be isolated
Bonds being broken or formed
Potentialenergy
Reaction coordinate
Reactant
Product
Transition State Transition
State
Intermediate
Longer-lived Bonds not being
broken or formed
R
R' R"
LGNu
- R
R'R"
Nu LG -LGR
R'R"Nu:
-
Some reactions proceed through a single transition state.
Potentialenergy
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.
Potential
energy
Effect of a Catalyst
Energy
Reaction coordinate
Eact
Eact
Intermediate
Kinetic product
Thermodynamic product
Reaction 1
Product #1
Reaction 2
Product #2
Potentialenergy
Reaction coordinate
In an exergonic reaction, the transition state is closer to the energy and structure
of the reactant(s).
Ea
The Hammond Postulate
Changing the energy of the products does very little to influence the rate of the
reaction.
Products
Reactants
Products
Potentialenergy
Reaction coordinate
The Hammond Postulate
In 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.
Products
Ea
Reactants
Products
Chemical Reactivity
Nucleophiles and Electrophiles
Nucleophilic Centers An 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+
Electrophilic Centers An 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
Patterns of Chemical Reactions
Nucleophilic Attack Loss of a Leaving Group
Proton Transfer Carbocation Rearrangements
Patterns of Chemical Reactions Nucleophilic Attack
BrH
O
H
Br O
H
H
Patterns of Chemical Reactions Loss of a Leaving Group
Br O Br
O
Br++ Br
Patterns of Chemical Reactions Proton Transfer
H
H
Patterns of Chemical Reactions Proton Transfer
OH
HO
H+
O
HO
H
H+
CC
O
HO H
CC
O
H
O H
Patterns of Chemical Reactions
Nucleophilic Attack Loss of a Leaving Group
Proton Transfer Carbocation Rearrangements
Examples
H3CCH2
H
CO
O
CH3C
H3CCH2
HC
O
O
CH3CH2
H3CCH2
H3C
1a
1b
H3CCH2
CH
CH3
HO
CH3
H3CCH2
HCCH3
HO
CH3
1c HS
O
O
CH3
O
H
SO
O
CH3
O
1d
1e C
C
O
H3C
H3CH
H
OH
H
H
C
C
O
H3C
H3CH
H
OH
H
H
C C
O
H3C CH3
HH
OCH2
CH3
C C
O
H3C CH3
HH
OCH2
CH3
3C
C
O
H3C
H3CH
HC
C
OH
H3C
H3CH
H
HOH2O, HO-
C
C
O
H3C
H3CH
H
δ-δ+
H O C
C
O
H3C
H3CH
H
HO
H
O
Hδ+δ-
H
O
4
C
C
O
H3C
H3CH
HC
C
OH
H3C
H3CH
H
HOH2O, H3O+
δ-
C
C
O
H3C
H3CH
H
H O
H
H
C
C
OH
H3C
H3CH
H
OH
HOH
H
O
H
HH
C
C
O
H3C
H3CH
HO
H
HH
C
C
O
H3C
H3CH
H
HO
H
H
Patterns of Chemical Reactions Carbocation Rearrangements
Where do carbocations come from ?
C LG C LG+
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
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.
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
Patterns of Chemical Reactions Carbocation 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.
Patterns of Chemical Reactions Carbocation 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 !!!
Patterns of Chemical Reactions Carbocation Rearrangements
H3CH3C CH3
HH
H3CH3C CH3
H3C
H3CH3C CH3
H3CH3C CH3
CH3
hydride shift
methyl shift
2º carbocation 3º carbocation
2º carbocation 3º carbocation
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 Reactions Carbocation Rearrangements
Examples
4aCH3
CH3
H
CH3
CH3
4b
H3C C
CH3
C
CH3
CH3
OH
H3C C
CH3
C
CH3
CH3
OH
H3C C
OH
C
CH3
CH3
CH3
5a
H3C C
CH3
CH
C
CH3
CH3
H
H
5bH
H
H
H
H
5c
O H
H
H
H3C C
CH3
CH
C
CH3
CH3
H
H
H
H
H
H
H
O H
H
HO
H
OH
H
H
H
H
H