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Chapter 6. Chemical Reactivity
and Mechanisms
Junha Jeon
Department of Chemistry
University of Texas at Arlington
Arlington, Texas 76019
Chem 2321, Fall 13
Can you remember?
Nature of Atomic Orbital Overlap: Two Theories: How is a covalent bond formed from the overlap of atomic overlap?
Valence bond theory
Molecular orbital theory
Valence Bond (VB) Theory
A bond: simply viewed as the sharing of electron density between two atoms as a
result of the constructive interference of their atomic orbitals.
For example,
sigma (") bond
In fact, all single bond are sigma (") bond.
H H H H H H
Valence Bond Theory
A bond: simply viewed as the sharing of electron density between two atoms as a
result of the constructive interference of their atomic orbitals.
For example,
sigma (") bond
In fact, all single bond are sigma (") bond.
H H H H H H
Molecular Orbital (MO) Theory
Valence bond theory is good, yet not perfect (only use constructive interference).
Molecular orbitals: mathematically combined atomic orbitals that extend over the
entire molecule. The mathematical method is called the linear combination of
atomic orbitals (LCAO).
MOs are a more complete analysis of bonds because they include both
constructive and destructive interference.
The number of MOs created must be equal to the number of atomic orbitals that
were used.
H2MOs
A Bond to Make
stabilizationH H
HH
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A Bond to Break
destabilizationH H
HH
A Bond to Break
destabilization
: need kinetic energy
H H
HH
6.1 Enthalpy
Enthalpy (#Hor q)
!The kinetic energy exchange between the reaction and its surroundings at
constant pressure! (q = heat)
!The amount of energy necessary to break the bond homolytically.
Enthalpy
Homolytic bond cleavage:
Heterolytic bond cleavage:
Enthalpy
Homolytic bond cleavage:
Heterolytic bond cleavage:
Enthalpy
Homolytic bond cleavage:
Enthalpy (#Hor q)
! The kinetic energy exchange between the reaction and its surroundings at
constant pressure!
!The amount of energy necessary to break the bond homolytically.
!Bond dissociation energy (BDE), #H
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Bond Dissociation Energies Bond Dissociation Energies
Bond Dissociation Energies Bond Dissociation Energies
Bond Dissociation Energies Heat of Reaction (#H)
Most reactions involve multiple bonds breaking and forming.
!If during a chemical reaction the reaction temperature decreases, the
reaction causes the surrounding temperature to decreases.
: Exothermic process the system gives energy to the surroundings (#H
is negative).
!If during a chemical reaction the reaction temperatureincreases, the
reaction causes the surrounding temperature to increases.
: Endothermic process the system receives energy to the surroundings
(#H is positive).
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Heat of Reaction (#H)
Most reactions involve multiple bonds breaking and forming.
!If during a chemical reaction the reaction temperature decreases, the
reaction causes the surrounding temperature (e.g. solvent) to decreases.
: Exothermic process the system gives energy to the surroundings (#H
is negative).
!If during a chemical reaction the reaction temperatureincreases, the
reaction causes the surrounding temperature to increases.
: Endothermic process the system receives energy to the surroundings
(#H is positive).
Heat of Reaction (#H)
Most reactions involve multiple bonds breaking and forming.
!If during a chemical reaction the reaction temperature decreases, the
reaction causes the surrounding temperature (e.g. solvent) to decreases.
: Exothermic process the system gives energy to the surroundings (#H
is negative).
!If during a chemical reaction the reaction temperatureincreases, the
reaction causes the surrounding temperature to increases.
: Endothermic process the system receives energy to the surroundings
(#H is positive).
Heat of Reaction (#H)
Most reactions involve multiple bonds breaking and forming.
!If during a chemical reaction the reaction temperature decreases, the
reaction causes the surrounding temperature to decreases.
: Exothermic process the system gives energy to the surroundings (#H
is negative).
!If during a chemical reaction the reaction temperatureincreases, the
reaction causes the surrounding temperature to increases.
: Endothermic process the system receives energy to the surroundings
(#H is positive).
Heat of Reaction (#H)
Most reactions involve multiple bonds breaking and forming.
!If during a chemical reaction the reaction temperature decreases, the
reaction causes the surrounding temperature to decreases.
: Endothermic process the system receives energy to the surroundings
(#H is positive).
!If during a chemical reaction the reaction temperatureincreases, the
reaction causes the surrounding temperature to increases.
: Exothermic process the system gives energy to the surroundings (#H
is negative).
Heat of Reaction (#H)
!Reaction coordinate: the progress of the reaction
Predicting #Hof a Reaction
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Predicting #Hof a Reaction Predicting #Hof a Reaction
Predicting #Hof a Reaction
+ 623 762
Predicting #Hof a Reaction
+ 623 762
!H = 139 kJ/mol
6.2 Entropy
What is the ultimate measure for determining whether or not a reaction can
occur?
Enthalpy and Entropy must both be considered when predicting whether a
reaction will occur.
!Entropy ($S): defined as molecular disorder, randomness, freedom
: particularly statistical thermodynamics (probability)
! Entropy may most accurately be thought of as the number of states that a
molecules energy can be distributed over.
6.2 Entropy
What is the ultimate measure for determining whether or not a reaction can
occur?
Enthalpy and Entropy must both be considered when predicting whether a
reaction will occur.
!Entropy ($S): defined as molecular disorder, randomness, freedom
: particularly statistical thermodynamics (probability)
! Entropy may most accurately be thought of as the number of states that a
molecules energy can be distributed over.
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6.2 Entropy
What is the ultimate measure for determining whether or not a reaction can
occur?
Enthalpy and Entropy must both be considered when predicting whether a
reaction will occur.
!Entropy ($S): defined as molecular disorder, randomness, freedom
: particularly statistical thermodynamics (probability)
! Entropy may most accurately be thought of as the number of states that a
molecules energy can be distributed over.
6.2 Entropy
What is the ultimate measure for determining whether or not a reaction can
occur?
Enthalpy and Entropy must both be considered when predicting whether a
reaction will occur.
!Entropy ($S): defined as molecular disorder, randomness, freedom
: particularly statistical thermodynamics (probability)
! Entropy may most accurately be thought of as the number of states that a
molecules energy can be distributed over.
Entropy: Free Expansion of a Gas
a higher state of entropy
!Molecules exhibit vibrational, rotational, and translational motion.
!If the energy of molecules can be distributed in a higher number of vibrational,
rotational, and translational states, the sample will have a greater entropy.
Motion within molecule
Vibrational motion: motion that changes the shape of the molecule stretching, bending, and rotation of bonds
Motion of whole molecule
Rotational motion: whole molecule spins around an axis in three dimensionalspace
Translational motion: whole atom/molecule changes its location in threedimensional space
Entropy: Free Expansion of a Gas
a higher state of entropy
!Molecules exhibit vibrational, rotational, and translational motion.
!If the energy of molecules can be distributed in a higher number of vibrational,
rotational, and translational states, the sample will have a greater entropy.
Entropy: Free Expansion of a Gas
a higher state of entropy
!Molecules exhibit vibrational, rotational, and translational motion.
!If the energy of molecules can be distributed in a higher number of vibrational,
rotational, and translational states, the sample will have a greater entropy.
Entropy
!#$% total entropy change will determine whether a process is spontaneous
(favors the forward direction, an increase):
consider #Ssys:the reaction, #Ssurr:usuallysolvent
The Second Law of Thermodynamics
! For chemical reactions, we must consider the entropy change for both the
system (the reaction) and the surroundings (the solvent usually).
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Gibbs Free Energy
!The spontaneity of a process depends only on #Stot
!#Ssyscan be measured or estimated.
!#$% measurement of #Ssurr?
Gibbs Free Energy
!The spontaneity of a process depends only on #Stot
!#Ssyscan be measured or estimated.
!#$% measurement of #Ssurr?
Gibbs Free Energy
1. The sign of #Ssurr depends on the direction of the heat flow:
At constant temperature, an exothermic process in the system causes heat to flow
into surroundings, increasing the random motions and thus the entropy of
surroundings.
2. The magnitue of #Ssurr depends on the temperature:
The transfer of a given quantity of energy as heat produces a much greater percent
change in the randomness of the surroundings at a low temperature than it does at
a high temperature.
Gibbs Free Energy
!The spontaneity of a process depends only on #Stot
!#Ssyscan be measured or estimated.
!#$% measurement of #Ssurr? see, slide28 for #Gsys
Gibbs Free Energy
!The spontaneity of a process depends only on #Stot
!Multiply both sides by negative temperature (T): The Gibbs Free Energy, #G:
simply #G = T#Stot
Gibbs Free Energy
!Spontaneity of a process:
#Stot:positive
The Gibbs Free Energy,#G: negative
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Gibbs Free Energy
!Spontaneity of a process:
#Stot:positive
The Gibbs Free Energy,#G: negative
Gibbs Free Energy
!Spontaneity of a process:
#Stot:positive
The Gibbs Free Energy,#G: negative
"The second law of thermodynamics
Gibbs Free Energy: Example
1. Predict the sign (+ or ) for #Ssys.
2. In the reaction, two pi bonds are converted into two sigma bonds. Predict the
sign (+ or -) of $Hsys.
3. Predict the sign (+ or -) for $Ssurr.
4. Predict the sign (+ or -) for $G.
5. How will the spontaneity of the reaction depend on temperature?
Gibbs Free Energy: Example
1. Predict the sign (+ or ) for #Ssys. Negative
2. In the reaction, two pi bonds are converted into two sigma bonds. Predict the
sign (+ or -) of $Hsys.
3. Predict the sign (+ or -) for $Ssurr.
4. Predict the sign (+ or -) for $G.
5. How will the spontaneity of the reaction depend on temperature?
Gibbs Free Energy: Example
1. Predict the sign (+ or ) for #Ssys. Negative
2. In the reaction, two pi bonds are converted into two sigma bonds. Predict the
sign (+ or ) of #Hsys. (overall, three pi bonds to one pi and two sigma bonds.)
(hint: see the next page.)
Enthalpy
Homolytic bond cleavage:
Enthalpy (#Hor q)
! The kinetic energy exchange between the reaction and its surroundings at
constant pressure!
!The amount of energy necessary to break the bond homolytically.
!Bond dissociation energy (BDE), #H
slide 13
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Gibbs Free Energy: Example
1. Predict the sign (+ or ) for #Ssys. Negative
2. In the reaction, two pi bonds are converted into two sigma bonds. Predict the
sign (+ or ) of #Hsys. (overall, three pi bonds to one pi and two sigma bonds.)
Gibbs Free Energy: Example
1. Predict the sign (+ or ) for #Ssys. Negative
2. In the reaction, two pi bonds are converted into two sigma bonds. Predict the
sign (+ or ) of #Hsys. Negative (exothermic)
3. Predict the sign (+ or -) for $Ssurr.
4. Predict the sign (+ or -) for $G.
5. How will the spontaneity of the reaction depend on temperature?
Gibbs Free Energy: Example
1. Predict the sign (+ or ) for #Ssys. Negative
2. In the reaction, two pi bonds are converted into two sigma bonds. Predict the
sign (+ or ) of #Hsys. Negative (exothermic)
3. Predict the sign (+ or ) for #Ssurr.
4. Predict the sign (+ or -) for $G.
5. How will the spontaneity of the reaction depend on temperature?
Gibbs Free Energy: Example
1. Predict the sign (+ or ) for #Ssys. Negative
2. In the reaction, two pi bonds are converted into two sigma bonds. Predict the
sign (+ or ) of #Hsys. Negative (exothermic)
3. Predict the sign (+ or ) for #Ssurr.
4. Predict the sign (+ or -) for $G.
5. How will the spontaneity of the reaction depend on temperature?
Gibbs Free Energy: Example
1. Predict the sign (+ or ) for #Ssys. Negative
2. In the reaction, two pi bonds are converted into two sigma bonds. Predict the
sign (+ or ) of #Hsys. Negative (exothermic)
3. Predict the sign (+ or ) for #Ssurr. Positive
4. Predict the sign (+ or -) for $G.
5. How will the spontaneity of the reaction depend on temperature?
Gibbs Free Energy: Example
1. Predict the sign (+ or ) for #Ssys. Negative
2. In the reaction, two pi bonds are converted into two sigma bonds. Predict the
sign (+ or ) of #Hsys. Negative (exothermic)
3. Predict the sign (+ or ) for #Ssurr. Positive, for #Ssys. Negative.
4. Predict the sign (+ or -) for $G.
5. How will the spontaneity of the reaction depend on temperature?
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Gibbs Free Energy: Example
1. Predict the sign (+ or ) for #Ssys. Negative
2. In the reaction, two pi bonds are converted into two sigma bonds. Predict the
sign (+ or ) of #Hsys. Negative (exothermic)
3. Predict the sign (+ or ) for #Ssurr. Positive, for #Ssys. Negative.
4. Predict the sign (+ or ) for #G.
5. How will the spontaneity of the reaction depend on temperature?
Gibbs Free Energy: Example
1. Predict the sign (+ or ) for #Ssys. Negative
2. In the reaction, two pi bonds are converted into two sigma bonds. Predict the
sign (+ or ) of #Hsys. Negative (exothermic)
3. Predict the sign (+ or ) for #Ssurr. Positive
4. Predict the sign (+ or ) for #G.
5. Depending on temperature!!
Gibbs Free Energy: Exergonic vs. Endergonic
!The spontaneity of a process: negative #G"Exergonic process
: favor the products
!The spontaneity of a process: positive #G"Endergonic process
: favor the reactants
6.4 Equilibria
Consider an exergonic process with a () $G. Will every molecule of A and B be
converted into products?
No. An equilibrium will eventually
be reached.
A spontaneous process will simply
favor the products meaning there
will be more products than
reactants.
Equilibria
Consider an exergonic process with a () $G. Will every molecule of A and B be
converted into products?
No. An equilibrium will eventually
be reached.
A spontaneous process will simplyfavor the products meaning there
will be more products than
reactants.
Equilibria
Consider an exergonic process with a () $G. Will every molecule of A and B be
converted into products?
No. An equilibrium will eventually
be reached.
A spontaneous process will simplyfavor the products meaning there
will be more products than
reactants
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Equilibria
Why doesnt an exergonic process react 100% to give products? Why will some
reactants remain? (moles of reactants are present.)
In any reaction, collisions are necessary.
!As [A] and [B] decrease, collisions between A and B will occur less often.
!As [C] and [D] increase, collisions between C and D will occur more often.
The more often C and D collide, the more often collisions will occur with
enough free energy for the reverse reaction to take place.
!Recall that EQUILIBRIUM is dynamic and occurs when the forward and
reverse reaction rates are equal.
Equilibria
Why doesnt an exergonic process react 100% to give products? Why will some
reactants remain? (moles of reactants are present.)
In any reaction, collisions are necessary.
!As [A] and [B] decrease, collisions between A and B will occur less often.
!As [C] and [D] increase, collisions between C and D will occur more often.
The more often C and D collide, the more often collisions will occur with
enough free energy for the reverse reaction to take place.
!Recall that EQUILIBRIUM is dynamic and occurs when the forward and
reverse reaction rates are equal.
Equilibria
Why doesnt an exergonic process react 100% to give products? Why will some
reactants remain? (moles of reactants are present.)
In any reaction, collisions are necessary.
!As [A] and [B] decrease, collisions between A and B will occur less often.
!As [C] and [D] increase, collisions between C and D will occur more often.
The more often C and D collide, the more often collisions will occur with
enough free energy for the reverse reaction to take place.
!Recall that EQUILIBRIUM is dynamic and occurs when the forward and
reverse reaction rates are equal.
Equilibria
Why doesnt an exergonic process react 100% to give products? Why will some
reactants remain? (moles of reactants are present.)
In any reaction, collisions are necessary.
!As [A] and [B] decrease, collisions between A and B will occur less often.
!As [C] and [D] increase, collisions between C and D will occur more often.
The more often C and D collide, the more often collisions will occur with
enough free energy for the reverse reaction to take place.
!Recall that EQUILIBRIUM is dynamic and occurs when the forward and
reverse reaction rates are equal.
Equilibria
Why doesnt an exergonic process react 100% to give products? Why will some
reactants remain? (moles of reactants are present.)
In any reaction, collisions are necessary.
!As [A] and [B] decrease, collisions between A and B will occur less often.
!As [C] and [D] increase, collisions between C and D will occur more often.
The more often C and D collide, the more often collisions will occur with
enough free energy for the reverse reaction to take place.
!Recall that EQUILIBRIUM is dynamic and occurs when the forward and
reverse reaction rates are equal.
Equilibria
Why doesnt an exergonic process react 100% to give products? Why will some
reactants remain? (moles of reactants are present.)
In any reaction, collisions are necessary.
!As [A] and [B] decrease, collisions between A and B will occur less often.
!As [C] and [D] increase, collisions between C and D will occur more often.
The more often C and D collide, the more often collisions will occur with
enough free energy for the reverse reaction to take place.
!Recall that equilibriumis dynamic and occurs when the forward and reverse
reaction rates are equal.
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Equilibria
!Equilibrium is also the state with the
lowest free energy overall.
!Every system seeks to achieve
a minimum of free energy.
K > 1
K = 1
K < 1
Equilibria
!Equilibrium is also the state with the
lowest free energy overall.
!Every system seeks to achieve
a minimum of free energy.
K > 1
K = 1
K < 1
Equilibria
!Keqand $G
R= gas constant (8.314 J/molK)
K > 1
K = 1
K < 1
Equilibria
!Keqand $G
R= gas constant (8.314 J/molK)
Thermodynamics
!Keq, $G, $H, and $S: thermodynamic terms
!Thermodynamics: the study of how energy is distributed under the influence of
entropy.
Thermodynamics
!Keq, $G, $H, and $S: thermodynamic terms
!Thermodynamics: the study of how energy is distributed under the influence of
entropy.
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Thermodynamics
!Keq, $G, $H, and $S: thermodynamic terms
!Thermodynamics: the study of how energy is distributed under the influence of
entropy.
6.5 Kinetics
!Spontaneous process:
thermodynamically favorable, i.e. favoring formation of products
That does not tell us anything about the rate or kinetics for the process.
!Some spontaneous processes: fast (e.g.explosions)
!Some spontaneous processes: slow [e.g.C%(diamond)"C (graphite)]
!The study of reaction rates: kinetics
Kinetics
!Spontaneous process:
thermodynamically favorable, i.e. favoring formation of products
That does not tell us anything about the rate or kinetics for the process.
!Some spontaneous processes: fast (e.g.explosions)
!Some spontaneous processes: slow [e.g.C%(diamond)"C (graphite)]
!The study of reaction rates: kinetics
Kinetics
!Spontaneous process:
thermodynamically favorable, i.e. favoring formation of products
That does not tell us anything about the rate or kinetics for the process.
!Some spontaneous processes: fast (e.g.explosions)
!Some spontaneous processes: slow [e.g.C%(diamond)"C (graphite)]
!The study of reaction rates: kinetics
Kinetics
!Spontaneous process:
thermodynamically favorable, i.e. favoring formation of products
That does not tell us anything about the rate or kinetics for the process.
!Some spontaneous processes: fast (e.g.explosions)
!Some spontaneous processes: slow [e.g.C%(diamond)"C (graphite)]
!The study of reaction rates: kinetics
Kinetics
!Spontaneous process:
thermodynamically favorable, i.e. favoring formation of products
That does not tell us anything about the rate or kinetics for the process.
!Some spontaneous processes: fast (e.g.explosions)
!Some spontaneous processes: slow [e.g.C%(diamond)"C (graphite)]
!The study of reaction rates: kinetics
!Reaction rate: the number of collisions that will result in product production in a
given period of time
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Kinetics
!Spontaneous process:
thermodynamically favorable, i.e. favoring formation of products
That does not tell us anything about the rate or kinetics for the process.
!Some spontaneous processes: fast (e.g.explosions)
!Some spontaneous processes: slow [e.g.C%(diamond)"C (graphite)]
!The study of reaction rates: kinetics
!Reaction rate: the number of collisions that will result in product production in a
given period of time
Rate Equations
!Reaction constant: specific to each reaction
!Concentration: proportional to a frequency of collisions of molecules that lead
to a reaction
Rate Equations
!Reaction constant: specific to each reaction
!Concentration: proportional to a frequency of collisions of molecules that lead
to a reaction
Rate Equations
!The degree to which a change in [reactant] will affect the rate is known as the
order.
!The order is represented by xand yin the rate law equation (experimentally
determined).
Rate Equations
!The degree to which a change in [reactant] will affect the rate is known as the
order.
!The order is represented by xand yin the rate law equation (experimentally
determined).
Rate Equations
!The degree to which a change in [reactant] will affect the rate is known as the
order.
!The order is represented by xand yin the rate law equation (experimentally
determined).
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Factors Affecting the Reaction Rate
0. The concentrations of the reactants
1. Energy of activation
2. The temperature
3. Geometry and sterics
Factors Affecting the Reaction Rate
0. The concentrations of the reactants
1. Energy of activation
2. The temperature
3. Geometry and sterics
related to the rate constant (k)
Factors Affecting the Reaction Rate (Rate Constant, k)
1. Energy of activation
: energy barrier between the reactants and the products the minimum
amount of energy required for a reaction to occur between two reactants that
collide
Factors Affecting the Reaction Rate (Rate Constant, k)
1. Energy of activation
!A certain threshold kinetic energyof molecules (Ea)
Factors Affecting the Reaction Rate (Rate Constant, k)
1. Energy of activation
!A certain threshold kinetic energyof molecules (Ea)
!Distribution of kinetic energy
Factors Affecting the Reaction Rate (Rate Constant, k)
1. Energy of activation
!A certain threshold kinetic energyof molecules (Ea)
!Distribution of kinetic energy
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Factors Affecting the Reaction Rate (Rate Constant, k)
1. Energy of activation
!A certain threshold kinetic energyof molecules (Ea)
!Distribution of kinetic energy
!Catalysts (and enzymes)
catalyst: a compound that
can speed up the rate of a
reaction without itself being
consumed by the reaction
alternating a reaction pathway
not changing $Greactants,
$gproducts and the position of
equilibrium
Factors Affecting the Reaction Rate (Rate Constant, k)
1. Energy of activation
!A certain threshold kinetic energyof molecules (Ea)
!Distribution of kinetic energy
!Catalysts (and enzymes)
catalyst: a compound that
can speed up the rate of a
reaction without itself being
consumed by the reaction
alternating a reaction pathway
not changing $Greactants,
$gproducts and the position of
equilibrium
Factors Affecting the Reaction Rate (Rate Constant, k)
1. Energy of activation
!A certain threshold kinetic energyof molecules (Ea)
!Distribution of kinetic energy
!Catalysts (and enzymes)
catalyst: a compound that
can speed up the rate of a
reaction without itself being
consumed by the reaction
alternating a reaction pathway
not changing $Greactants,
$Gproducts and the position of
equilibrium
Factors Affecting the Reaction Rate (Rate Constant, k)
2. Temperature
: a measure of a systems averagekinetic energy
Factors Affecting the Reaction Rate (Rate Constant, k)
3. Geometric and Steric
: the geometry of the reactants and the orientation of their collision can have
an impact on the frequency of collisions that lead to a reaction.
Factors Affecting the Reaction Rate (Rate Constant, k)
3. Geometric and Steric
: the geometry of the reactants and the orientation of their collision can have
an impact on the frequency of collisions that lead to a reaction.
We are going to discuss this factor in chapter 7.
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6.6 Energy Diagrams: Kinetics vs.Thermodynamics Kinetics vs.Thermodynamics
!C + D is kineticallyand thermodynamicallyfavorable!
Kinetics vs.Thermodynamics
!C + D is thermodynamicallyfavorable!
!E + F is kineticallyfavorable!
Kinetics vs.Thermodynamics
!C + D is thermodynamicallyfavorable!
!E + F is kineticallyfavorable!
At high temperature, C + D vs. E + F ??
Kinetics vs.Thermodynamics
!C + D is thermodynamicallyfavorable!
!E + F is kineticallyfavorable!
At high temperature, C + D vs. E + F ??
Transition States vs. Intermediates
!Transition states: at all localenergy maxima
!Intermediates: at all localenergy minima
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Transition States vs. Intermediates
!Transition states: at all local energy maxima
transiently exist and cannot be isolated!
bonds are in the process of being
broken and/or formed simultaneously.
Transition States vs. Intermediates
!Transition states: at all local energy maxima
transiently exist and cannot be isolated!
bonds are in the process of being
broken and/or formed simultaneously.
Transition States vs. Intermediates
!Intermediates: at all localenergy minima
intermediates generally exist long enough to be observed.
bonds are notin the process of breaking or forming.
The Hammond Postulate
!Two points on an energy diagram that are close in energy should be similar in
structure.
The Hammond Postulate
!Two points on an energy diagram that are close in energy should be similar in
structure.
6.7 Nucleophiles and Electrophiles
Three major reaction categories:
1. Ionic (polar) reactions
2. Radical reactions (chapter 11)
3. Pericyclic reactions (chapter 17)
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Nucleophiles and Electrophiles
1. Ionic (polar) reactions
the participation of ions as reactants, intermediates, or products
the force of attraction between opposite charges
Nucleophiles and Electrophiles
1. Ionic (polar) reactions
the participation of ions as reactants, intermediates, or products
the force of attraction between opposite charges
!Nucleophile (nucleophilic reagent): a reagent that forms a bond to its reaction
partner (the electrophile) by donating both bonding electrons. (nucleus lover)
!Nucleophilic center: an electron-rich atom that is capable of donating a pair of
electron
Nucleophiles and Electrophiles
electrophile nucleophile
an electrophilecenter
Nucleophiles and Electrophiles
!Nucleophile (nucleophilic reagent): a reagent that forms a bond to its reaction
partner (the electrophile) by donating both bonding electrons. (nucleus lover)
!Nucleophilic center: an electron-rich atom that is capable of donating a pair of
electron
electrophile nucleophile
an electrophilecenter
a nucleophilecenter
Nucleophiles and Electrophiles
!Nucleophile (nucleophilic reagent): a reagent that forms a bond to its reaction
partner (the electrophile) by donating both bonding electrons. (nucleus lover)
!Nucleophilic center: an electron-rich atom that is capable of donating a pair of
electron
!Polarizability: the ease of distortion of the electron cloud of a molecular entity
by an electric field (such as that due to the proximity of a charged reagent)
affecting the strength of nucleophilicity. ROH vs.RSH
Nucleophiles and Electrophiles
!Nucleophile (nucleophilic reagent): a reagent that forms a bond to its reaction
partner (the electrophile) by donating both bonding electrons. (nucleus lover)
!Nucleophilic center: an electron-rich atom that is capable of donating a pair of
electron
!Polarizability: the ease of distortion of the electron cloud of a molecular entity
by an electric field (such as that due to the proximity of a charged reagent)
affecting the strength of nucleophilicity. ROH vs.RSH
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Nucleophiles and Electrophiles
!Electrophile (electrophilic reagent): a reagent that forms a bond to its reaction
partner (the nucleophile) by accepting both bonding electrons from its reaction
partner. (electron lover)
!Electrophilic center: an electron-deficient atom that is capable of accepting a
pair of electrons
electrophile nucleophile
a nucleophilecenter
Nucleophiles and Electrophiles
!Electrophile (electrophilic reagent): a reagent that forms a bond to its reaction
partner (the nucleophile) by accepting both bonding electrons from its reaction
partner. (electron lover)
!Electrophilic center: an electron-deficient atom that is capable of accepting a
pair of electrons
electrophile nucleophile
an electrophilecenter
a nucleophilecenter
Nucleophiles and Electrophiles
!Electrophile (electrophilic reagent): a reagent that forms a bond to its reaction
partner (the nucleophile) by accepting both bonding electrons from its reaction
partner. (electron lover)
!Electrophilic center: an electron-deficient atom that is capable of accepting a
pair of electrons
carbocation
Nucleophiles and Electrophiles
Nucleophiles and Electrophiles 6.8 Mechanism and Arrow Pushing
!Acid-base reaction
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Four Patterns of Ionic Mechanism
1. Nucleophilic attack
2. Loss of a leaving group
3. Proton transfers (acid/base)
4. Rearrangements
Four Patterns of Ionic Mechanism
1. Nucleophilic attack
a lone pair:
a nucleophilic center
empty porbital:
an electrophilic center
Four Patterns of Ionic Mechanism
1. Nucleophilic attack
a lone pair:
a nucleophilic center
empty porbital:
an electrophilic center
Four Patterns of Ionic Mechanism
1. Nucleophilic attack
a lone pair:
a nucleophilic center
inductive effect (carbon
with a &+ charge):
an electrophilic center
Four Patterns of Ionic Mechanism
1. Nucleophilic attack
a lone pair:
a nucleophilic center
inductive effect (carbon
with a &+ charge):
an electrophilic center
1. Nucleophilic Attack
A. Stepwise view: Resonance contribution
B. One-step electron flow
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2. Loss of Leaving Group 2. Loss of Leaving Group
2. Loss of Leaving Group
or
!Acid-base reaction (protonated)
!Acid-base reaction (deprotonated)
3. Proton Transfer
or
3. Proton Transfer
A. One-step electron flow
B. Stepwise view: Resonance contribution
3. Proton Transfer
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4. Rearrangement
Carbocations can be stabilized by neighboring groups through slight orbital
overlap called hyperconjugation.
4. Rearrangement
Carbocations can be stabilized by neighboring groups (e.g. alkyl group) through
slight orbital overlap called hyperconjugation.
4. Rearrangement
Carbocations can be stabilized by neighboring groups (e.g. alkyl group) through
slight orbital overlap called hyperconjugation.
4. Rearrangement
Carbocation stability: depending upon # of alkyl groups attached directly to the
carbocation
4. Rearrangement
Carbocation rearrangement
1. Hydride (H) Shift
2. Methyl Shift
4. Rearrangement
Carbocation rearrangement
1. Hydride (H) Shift
2. Methyl Shift
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4. Rearrangement
Carbocation rearrangement
1. Hydride (H) Shift
2. Methyl Shift
4. Rearrangement
What is the driving force of the Carbocation rearrangement?
1. Hydride (H) Shift
2. Methyl Shift
4. Rearrangement
What is the driving force of the Carbocation rearrangement? Stability of C+
1. Hydride (H) Shift
2. Methyl Shift
4. Rearrangement
Carbocation rearrangement?
4. Rearrangement
Carbocation rearrangement?
4. Rearrangement
Carbocation rearrangement?
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4. Rearrangement
Carbocation rearrangements generally do not occur when the carbocation is
already tertiary unless a rearrangement will produce a resonance-stabilized
carbocation;
4. Rearrangement
Carbocation rearrangements generally do not occur when the carbocation is
already tertiary unless a rearrangement will produce a resonance-stabilized
carbocation;
4. Rearrangement
Carbocation rearrangements generally do not occur when the carbocation is
already tertiary unless a rearrangement will produce a resonance-stabilized
carbocation;
6.9 Combining the Patterns of Arrow Pushing'
Combining the Patterns of Arrow Pushing'
!Two arrow-pushing patterns simultaneously
!Concerted process: Two or more primitive changes are said to be concerted
(or to constitute a concerted process) if they occur within the same elementary
reaction.
1. The arrow starts on a pair of electrons (a bonded pair or a lone pair)
2. The arrow ends on a nucleus (as the formation of a lone pair) .
6.10 Drawing Curved Arrows
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1. The arrow starts on a pair of electrons (a bonded pair or a lone pair)
2. The arrow ends on a nucleus (as the formation of a lone pair) .
6.10 Drawing Curved Arrows
3. Avoid breaking the octet rule. Never give C, N, O, or F more than 8 valence
electrons.
4. Draw arrows that follow the four key patterns.
6.10 Drawing Curved Arrows
3. Avoid breaking the octet rule. Never give C, N, O, or F more than 8 valence
electrons.
4. Draw arrows that follow the four key patterns.
6.10 Drawing Curved Arrows
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