Chapter 4
The Study of
Chemical Reactions
Jo Blackburn
Richland College,Dallas,TX
Dallas County Community College District
2003,Prentice Hall
Organic Chemistry,5th Edition
L,G,Wade,Jr.
Chapter 4 2
Tools for Study
To determine a reaction’s
mechanism,look at:
Equilibrium constant
Free energy change
Enthalpy
Entropy
Bond dissociation energy
Kinetics
Activation energy =>
Chapter 4 3
Chlorination of Methane
Requires heat or light for initiation.
The most effective wavelength is blue,which
is absorbed by chlorine gas.
Lots of product formed from absorption of
only one photon of light (chain reaction).
=>
C
H
H
H
H + C l 2
h e a t o r l i g h t
C
H
H
H
C l + H C l
Chapter 4 4
Free-Radical Chain Reaction
Initiation generates a reactive intermediate.
Propagation,the intermediate reacts with a
stable molecule to produce another reactive
intermediate (and a product molecule).
Termination,side reactions that destroy the
reactive intermediate,
=>
Chapter 4 5
Initiation Step
A chlorine molecule splits homolytically
into chlorine atoms (free radicals)
=>
C l C l + ph o t o n (h? ) C l + C l
Chapter 4 6
Propagation Step (1)
The chlorine atom collides with a methane
molecule and abstracts (removes) a H,
forming another free radical and one of
the products (HCl).
C
H
H
H
H C l+ C
H
H
H
+ H C l
=>
Chapter 4 7
Propagation Step (2)
The methyl free radical collides with
another chlorine molecule,producing
the other product (methyl chloride) and
regenerating the chlorine radical.
C
H
H
H
+ C l C l C
H
H
H
C l + C l
=>
Chapter 4 8
Overall Reaction
C
H
H
H
H C l+ C
H
H
H
+ H C l
C
H
H
H
+ C l C l C
H
H
H
C l + C l
C
H
H
H
H + C l C l C
H
H
H
C l + H C l=>
C l C l + ph o t o n (h? ) C l + C l
Chapter 4 9
Termination Steps
Collision of any two free radicals
Combination of free radical with
contaminant or collision with wall.
C
H
H
H
C l+ C
H
H
H
C l
Can you suggest others?
=>
Chapter 4 10
Equilibrium constant
Keq = [products]
[reactants]
For chlorination Keq = 1.1 x 1019
Large value indicates reaction,goes to
completion.”
=>
Chapter 4 11
Free Energy Change
DG = free energy of (products - reactants),
amount of energy available to do work.
Negative values indicate spontaneity.
DGo = -RT(lnKeq)
where R = 1.987 cal/K-mol
and T = temperature in kelvins
Since chlorination has a large Keq,the free
energy change is large and negative.
=>
Chapter 4 12
Problem
Given that -X is -OH,the energy difference for
the following reaction is -1.0 kcal/mol.
What percentage of cyclohexanol molecules
will be in the equatorial conformer at
equilibrium at 25° C?
=>
Chapter 4 13
Factors Determining DG?
Free energy change depends on
enthalpy
entropy
DH? = (enthalpy of products) - (enthalpy
of reactants)
DS? = (entropy of products) - (entropy of
reactants)
DG? = DH? - TDS? =>
Chapter 4 14
Enthalpy
DHo = heat released or absorbed during
a chemical reaction at standard
conditions.
Exothermic,(-DH),heat is released.
Endothermic,(+DH),heat is absorbed.
Reactions favor products with lowest
enthalpy (strongest bonds).
=>
Chapter 4 15
Entropy
DSo = change in randomness,disorder,
freedom of movement.
Increasing heat,volume,or number of
particles increases entropy.
Spontaneous reactions maximize
disorder and minimize enthalpy.
In the equation DGo = DHo - TDSo the
entropy value is often small,
=>
Chapter 4 16
Bond Dissociation Energy
Bond breaking requires energy (+BDE)
Bond formation releases energy (-BDE)
Table 4.2 gives BDE for homolytic cleavage
of bonds in a gaseous molecule.
A B A + B
We can use BDE to estimate DH for a reaction.
=>
Chapter 4 17
Which is more likely?
Estimate DH for each step using BDE.
C H
4 H C l
+
+C l C H
3
C H
3 +
C l
2 C H 3 C l + C l
or
C l+C H 4 C H
3
C l + H
H C l 2+ H C l C l+
104 103
58 84
=>
104 84
58 103
Chapter 4 18
Kinetics
Answers question,“How fast?”
Rate is proportional to the concentration
of reactants raised to a power.
Rate law is experimentally determined.
=>
Chapter 4 19
Reaction Order
For A + B? C + D,rate = k[A]a[B]b
a is the order with respect to A
a + b is the overall order
Order is the number of molecules of that
reactant which is present in the rate-
determining step of the mechanism.
The value of k depends on temperature as
given by Arrhenius,ln k = -Ea + lnA
RT
=>
Chapter 4 20
Activation Energy
Minimum energy required to reach
the transition state.
At higher temperatures,more molecules
have the required energy.
=>
C
H
H
H
H C l
Chapter 4 21
Reaction-Energy Diagrams
For a one-step reaction:
reactants? transition state? products
A catalyst lowers the energy of the
transition state.
=>
Chapter 4 22
Energy Diagram for a
Two-Step Reaction
Reactants? transition state? intermediate
Intermediate? transition state? product
=>
Chapter 4 23
Rate-Determining Step
Reaction intermediates are stable as long
as they don’t collide with another molecule
or atom,but they are very reactive.
Transition states are at energy maximums.
Intermediates are at energy minimums.
The reaction step with highest Ea will be the
slowest,therefore rate-determining for the
entire reaction,=>
Chapter 4 24
Rate,Ea,and Temperature
X + C H 4 H X + C H 3
X E a Ra t e @ 3 0 0 K Ra t e @ 5 0 0 K
F 1,2 k c a l 1 4 0,0 0 0 3 0 0,0 0 0
Cl 4 k c a l 1300 1 8,0 0 0
Br 1 8 k c a l 9 x 1 0
-8
0,0 1 5
I 3 4 k c a l 2 x 1 0
- 1 9
2 x 1 0
-9
=>
Chapter 4 25
Conclusions
With increasing Ea,rate decreases.
With increasing temperature,rate
increases.
Fluorine reacts explosively.
Chlorine reacts at a moderate rate.
Bromine must be heated to react.
Iodine does not react (detectably),
=>
Chapter 4 26
Chlorination of Propane
There are six 1? H’s and two 2? H’s,We
expect 3:1 product mix,or 75% 1-
chloropropane and 25% 2-chloropropane.
Typical product mix,40% 1-chloropropane
and 60% 2-chloropropane.
Therefore,not all H’s are equally reactive,
=>
1? C
2? CC H 3 C H 2 C H 3 + C l 2 h? C H 2
C l
C H 2 C H 3 + C H 3 C H
C l
C H 3
Chapter 4 27
Reactivity of Hydrogens
To compare hydrogen reactivity,find
amount of product formed per hydrogen,
40% 1-chloropropane from 6 hydrogens
and 60% 2-chloropropane from 2
hydrogens.
40%? 6 = 6.67% per primary H and
60%? 2 = 30% per secondary H
Secondary H’s are 30%? 6.67% = 4.5
times more reactive toward chlorination
than primary H’s,=>
Chapter 4 28
Predict the Product Mix
Given that secondary H’s are 4.5 times as
reactive as primary H’s,predict the
percentage of each monochlorinated
product of n-butane + chlorine,
=>
Chapter 4 29
Free Radical Stabilities
Energy required to break a C-H bond
decreases as substitution on the carbon
increases.
Stability,3? > 2? > 1? > methyl
DH(kcal) 91,95,98,104
=>
Chapter 4 30
Chlorination Energy Diagram
Lower Ea,faster rate,so more stable
intermediate is formed faster.
=>
Chapter 4 31
There are six 1? H’s and two 2?H’s,We
expect 3:1 product mix,or 75% 1-
bromopropane and 25% 2-bromopropane.
Typical product mix,3% 1-bromopropane
and 97% 2-bromopropane !!!
Bromination is more selective than
chlorination,=>
1? C
2? CC H 3 C H 2 C H 3 + C H 2
B r
C H 2 C H 3 +B r 2
h e a t C H
3 C H
B r
C H 3
Bromination of Propane
Chapter 4 32
To compare hydrogen reactivity,find
amount of product formed per hydrogen,
3% 1-bromopropane from 6 hydrogens and
97% 2-bromopropane from 2 hydrogens.
3%? 6 = 0.5% per primary H and
97%? 2 = 48.5% per secondary H
Secondary H’s are 48.5%? 0.5% = 97
times more reactive toward bromination
than primary H’s,
=>
Reactivity of Hydrogens
Chapter 4 33
Bromination Energy Diagram
Note larger difference in Ea
Why endothermic?
=>
Chapter 4 34
Bromination vs,Chlorination
=>
Chapter 4 35
Endothermic and
Exothermic Diagrams
=>
Chapter 4 36
Hammond Postulate
Related species that are similar in energy are
also similar in structure,The structure of a
transition state resembles the structure of the
closest stable species.
Transition state structure for endothermic
reactions resemble the product.
Transition state structure for exothermic
reactions resemble the reactants.
=>
Chapter 4 37
Radical Inhibitors
Often added to food to retard spoilage.
Without an inhibitor,each initiation step
will cause a chain reaction so that many
molecules will react.
An inhibitor combines with the free
radical to form a stable molecule.
Vitamin E and vitamin C are thought to
protect living cells from free radicals.
=>
Chapter 4 38
Reactive Intermediates
Carbocations (or carbonium ions)
Free radicals
Carbanions
Carbene
=>
Chapter 4 39
Carbocation Structure
Carbon has 6 electrons,
positive charge.
Carbon is sp2 hybridized
with vacant p orbital.
=>
Chapter 4 40
Carbocation Stability
Stabilized by alkyl
substituents 2 ways:
(1) Inductive effect,
donation of electron
density along the
sigma bonds.
(2) Hyperconjugation,
overlap of sigma
bonding orbitals with
empty p orbital.
=>
Chapter 4 41
Free Radicals
Also electron-
deficient
Stabilized by alkyl
substituents
Order of stability:
3? > 2? > 1? >
methyl
=>
Chapter 4 42
Carbanions
Eight electrons on C:
6 bonding + lone pair
Carbon has a negative
charge.
Destabilized by alkyl
substituents.
Methyl >1? > 2? > 3?
=>
Chapter 4 43
Carbenes
Carbon is neutral.
Vacant p orbital,so
can be electrophilic.
Lone pair of
electrons,so can be
nucleophilic,
=>
Chapter 4 44
End of Chapter 4
The Study of
Chemical Reactions
Jo Blackburn
Richland College,Dallas,TX
Dallas County Community College District
2003,Prentice Hall
Organic Chemistry,5th Edition
L,G,Wade,Jr.
Chapter 4 2
Tools for Study
To determine a reaction’s
mechanism,look at:
Equilibrium constant
Free energy change
Enthalpy
Entropy
Bond dissociation energy
Kinetics
Activation energy =>
Chapter 4 3
Chlorination of Methane
Requires heat or light for initiation.
The most effective wavelength is blue,which
is absorbed by chlorine gas.
Lots of product formed from absorption of
only one photon of light (chain reaction).
=>
C
H
H
H
H + C l 2
h e a t o r l i g h t
C
H
H
H
C l + H C l
Chapter 4 4
Free-Radical Chain Reaction
Initiation generates a reactive intermediate.
Propagation,the intermediate reacts with a
stable molecule to produce another reactive
intermediate (and a product molecule).
Termination,side reactions that destroy the
reactive intermediate,
=>
Chapter 4 5
Initiation Step
A chlorine molecule splits homolytically
into chlorine atoms (free radicals)
=>
C l C l + ph o t o n (h? ) C l + C l
Chapter 4 6
Propagation Step (1)
The chlorine atom collides with a methane
molecule and abstracts (removes) a H,
forming another free radical and one of
the products (HCl).
C
H
H
H
H C l+ C
H
H
H
+ H C l
=>
Chapter 4 7
Propagation Step (2)
The methyl free radical collides with
another chlorine molecule,producing
the other product (methyl chloride) and
regenerating the chlorine radical.
C
H
H
H
+ C l C l C
H
H
H
C l + C l
=>
Chapter 4 8
Overall Reaction
C
H
H
H
H C l+ C
H
H
H
+ H C l
C
H
H
H
+ C l C l C
H
H
H
C l + C l
C
H
H
H
H + C l C l C
H
H
H
C l + H C l=>
C l C l + ph o t o n (h? ) C l + C l
Chapter 4 9
Termination Steps
Collision of any two free radicals
Combination of free radical with
contaminant or collision with wall.
C
H
H
H
C l+ C
H
H
H
C l
Can you suggest others?
=>
Chapter 4 10
Equilibrium constant
Keq = [products]
[reactants]
For chlorination Keq = 1.1 x 1019
Large value indicates reaction,goes to
completion.”
=>
Chapter 4 11
Free Energy Change
DG = free energy of (products - reactants),
amount of energy available to do work.
Negative values indicate spontaneity.
DGo = -RT(lnKeq)
where R = 1.987 cal/K-mol
and T = temperature in kelvins
Since chlorination has a large Keq,the free
energy change is large and negative.
=>
Chapter 4 12
Problem
Given that -X is -OH,the energy difference for
the following reaction is -1.0 kcal/mol.
What percentage of cyclohexanol molecules
will be in the equatorial conformer at
equilibrium at 25° C?
=>
Chapter 4 13
Factors Determining DG?
Free energy change depends on
enthalpy
entropy
DH? = (enthalpy of products) - (enthalpy
of reactants)
DS? = (entropy of products) - (entropy of
reactants)
DG? = DH? - TDS? =>
Chapter 4 14
Enthalpy
DHo = heat released or absorbed during
a chemical reaction at standard
conditions.
Exothermic,(-DH),heat is released.
Endothermic,(+DH),heat is absorbed.
Reactions favor products with lowest
enthalpy (strongest bonds).
=>
Chapter 4 15
Entropy
DSo = change in randomness,disorder,
freedom of movement.
Increasing heat,volume,or number of
particles increases entropy.
Spontaneous reactions maximize
disorder and minimize enthalpy.
In the equation DGo = DHo - TDSo the
entropy value is often small,
=>
Chapter 4 16
Bond Dissociation Energy
Bond breaking requires energy (+BDE)
Bond formation releases energy (-BDE)
Table 4.2 gives BDE for homolytic cleavage
of bonds in a gaseous molecule.
A B A + B
We can use BDE to estimate DH for a reaction.
=>
Chapter 4 17
Which is more likely?
Estimate DH for each step using BDE.
C H
4 H C l
+
+C l C H
3
C H
3 +
C l
2 C H 3 C l + C l
or
C l+C H 4 C H
3
C l + H
H C l 2+ H C l C l+
104 103
58 84
=>
104 84
58 103
Chapter 4 18
Kinetics
Answers question,“How fast?”
Rate is proportional to the concentration
of reactants raised to a power.
Rate law is experimentally determined.
=>
Chapter 4 19
Reaction Order
For A + B? C + D,rate = k[A]a[B]b
a is the order with respect to A
a + b is the overall order
Order is the number of molecules of that
reactant which is present in the rate-
determining step of the mechanism.
The value of k depends on temperature as
given by Arrhenius,ln k = -Ea + lnA
RT
=>
Chapter 4 20
Activation Energy
Minimum energy required to reach
the transition state.
At higher temperatures,more molecules
have the required energy.
=>
C
H
H
H
H C l
Chapter 4 21
Reaction-Energy Diagrams
For a one-step reaction:
reactants? transition state? products
A catalyst lowers the energy of the
transition state.
=>
Chapter 4 22
Energy Diagram for a
Two-Step Reaction
Reactants? transition state? intermediate
Intermediate? transition state? product
=>
Chapter 4 23
Rate-Determining Step
Reaction intermediates are stable as long
as they don’t collide with another molecule
or atom,but they are very reactive.
Transition states are at energy maximums.
Intermediates are at energy minimums.
The reaction step with highest Ea will be the
slowest,therefore rate-determining for the
entire reaction,=>
Chapter 4 24
Rate,Ea,and Temperature
X + C H 4 H X + C H 3
X E a Ra t e @ 3 0 0 K Ra t e @ 5 0 0 K
F 1,2 k c a l 1 4 0,0 0 0 3 0 0,0 0 0
Cl 4 k c a l 1300 1 8,0 0 0
Br 1 8 k c a l 9 x 1 0
-8
0,0 1 5
I 3 4 k c a l 2 x 1 0
- 1 9
2 x 1 0
-9
=>
Chapter 4 25
Conclusions
With increasing Ea,rate decreases.
With increasing temperature,rate
increases.
Fluorine reacts explosively.
Chlorine reacts at a moderate rate.
Bromine must be heated to react.
Iodine does not react (detectably),
=>
Chapter 4 26
Chlorination of Propane
There are six 1? H’s and two 2? H’s,We
expect 3:1 product mix,or 75% 1-
chloropropane and 25% 2-chloropropane.
Typical product mix,40% 1-chloropropane
and 60% 2-chloropropane.
Therefore,not all H’s are equally reactive,
=>
1? C
2? CC H 3 C H 2 C H 3 + C l 2 h? C H 2
C l
C H 2 C H 3 + C H 3 C H
C l
C H 3
Chapter 4 27
Reactivity of Hydrogens
To compare hydrogen reactivity,find
amount of product formed per hydrogen,
40% 1-chloropropane from 6 hydrogens
and 60% 2-chloropropane from 2
hydrogens.
40%? 6 = 6.67% per primary H and
60%? 2 = 30% per secondary H
Secondary H’s are 30%? 6.67% = 4.5
times more reactive toward chlorination
than primary H’s,=>
Chapter 4 28
Predict the Product Mix
Given that secondary H’s are 4.5 times as
reactive as primary H’s,predict the
percentage of each monochlorinated
product of n-butane + chlorine,
=>
Chapter 4 29
Free Radical Stabilities
Energy required to break a C-H bond
decreases as substitution on the carbon
increases.
Stability,3? > 2? > 1? > methyl
DH(kcal) 91,95,98,104
=>
Chapter 4 30
Chlorination Energy Diagram
Lower Ea,faster rate,so more stable
intermediate is formed faster.
=>
Chapter 4 31
There are six 1? H’s and two 2?H’s,We
expect 3:1 product mix,or 75% 1-
bromopropane and 25% 2-bromopropane.
Typical product mix,3% 1-bromopropane
and 97% 2-bromopropane !!!
Bromination is more selective than
chlorination,=>
1? C
2? CC H 3 C H 2 C H 3 + C H 2
B r
C H 2 C H 3 +B r 2
h e a t C H
3 C H
B r
C H 3
Bromination of Propane
Chapter 4 32
To compare hydrogen reactivity,find
amount of product formed per hydrogen,
3% 1-bromopropane from 6 hydrogens and
97% 2-bromopropane from 2 hydrogens.
3%? 6 = 0.5% per primary H and
97%? 2 = 48.5% per secondary H
Secondary H’s are 48.5%? 0.5% = 97
times more reactive toward bromination
than primary H’s,
=>
Reactivity of Hydrogens
Chapter 4 33
Bromination Energy Diagram
Note larger difference in Ea
Why endothermic?
=>
Chapter 4 34
Bromination vs,Chlorination
=>
Chapter 4 35
Endothermic and
Exothermic Diagrams
=>
Chapter 4 36
Hammond Postulate
Related species that are similar in energy are
also similar in structure,The structure of a
transition state resembles the structure of the
closest stable species.
Transition state structure for endothermic
reactions resemble the product.
Transition state structure for exothermic
reactions resemble the reactants.
=>
Chapter 4 37
Radical Inhibitors
Often added to food to retard spoilage.
Without an inhibitor,each initiation step
will cause a chain reaction so that many
molecules will react.
An inhibitor combines with the free
radical to form a stable molecule.
Vitamin E and vitamin C are thought to
protect living cells from free radicals.
=>
Chapter 4 38
Reactive Intermediates
Carbocations (or carbonium ions)
Free radicals
Carbanions
Carbene
=>
Chapter 4 39
Carbocation Structure
Carbon has 6 electrons,
positive charge.
Carbon is sp2 hybridized
with vacant p orbital.
=>
Chapter 4 40
Carbocation Stability
Stabilized by alkyl
substituents 2 ways:
(1) Inductive effect,
donation of electron
density along the
sigma bonds.
(2) Hyperconjugation,
overlap of sigma
bonding orbitals with
empty p orbital.
=>
Chapter 4 41
Free Radicals
Also electron-
deficient
Stabilized by alkyl
substituents
Order of stability:
3? > 2? > 1? >
methyl
=>
Chapter 4 42
Carbanions
Eight electrons on C:
6 bonding + lone pair
Carbon has a negative
charge.
Destabilized by alkyl
substituents.
Methyl >1? > 2? > 3?
=>
Chapter 4 43
Carbenes
Carbon is neutral.
Vacant p orbital,so
can be electrophilic.
Lone pair of
electrons,so can be
nucleophilic,
=>
Chapter 4 44
End of Chapter 4
