WELCOME TO MODERN
ORGANIC CHEMISTRY
Organic Chemistry, 5th Edition
L. G. Wade, Jr.
Chapter 4
The Study of
Chemical Reactions
WHAT IS A REACTION
MECHANISM
A DESCRIPTION OF STRUCTURES AN
ENERGIES OF STARTING MATERIALS AND
PRODUCTS OF A REACTION AS WELL AS
ANY REACTION INTERMEDIATES
IN ADDITION, ALL OF THE TRANSITION
STATES (ENERGY MAXIMA) SEPARATING
THE REACTANTS FROM THE PRODUCTS
(ENERGY MINIMA) MUST BE DETERMINED.
?
PRODUCT DETERMINATION
1. ISOLATE THEM
2. TAKE THEIR SPECTRA
a. IR - FUNCTIONAL GROUPS
b. NMR - environment of
hydrogen and carbon atoms
c. MS - actual MW
d. X-RAY
ORTEP
STRUCTURE OF
TRANSITION STATE?
CAN'T MEASUREMENT DIRECTLY
MEASURE THE RATE LAW TELLS
NUMBER AND KIND OF MOLECULES
INVOLVED IN TS
CERTAIN ENZYMES RECOGNIZE TS THAT IS THEY BOND WITH IT!!
HOW FAR DOES A REACTION
GO?
THERMODYNAMICS
DGo = DHo-TDSo = -2.3xRxTxLogKeq
HOW FAST DOES A REACTION
GO? KINETICS
RATE IS INVERSELY PROPORTIONAL TO
ACTIVATION ENERGY (EA)
Chlorination of Methane
Requires heat or light for initiation.
•The most effective wavelength is blue, which is absorbed by
chlorine gas.
•High quantum yield
Lots of product formed from absorption of only one photon of
light (chain reaction).
Some ethane is formed
THESE FACTORS ARE CHARACHERISTICS OF FREERADICAL REACTIONS
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.
=>
Initiation Step
A chlorine molecule splits homolytically into
chlorine atoms (free radicals)
Cl Cl +
photon ( h)
Cl +
Cl
THE CHLORINE ATOM IS ELECTROPHILIC - LEWIS ACID
SEEKS AN ELECTRON TO REGAIN OCTET OF ELECTRONS
USUALLY ABSTRACTS A HYDROGEN ATOM
=>
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).
H
H C H
H
H
+
Cl
H C
+
H Cl
H
Methyl radicals is also electrophilic -seeks e=>
Methyl radical is a member of the highly reactive
intermediates gang
Propagation Step (2)
The methyl free radical collides with another
chlorine molecule, producing the other
product (methyl chloride) and regenerating
the chlorine radical.
H
H C
H
H
+
Cl
Cl
H C Cl
H
+
Cl
chlorine atom then attacks another methyl=>
step 1 and 2 repeat over and over again
UNTIL -------
Overall Reaction
Cl Cl +
photon ( h)
H
Cl
H
H C H
+
H C
Cl
H
+
H Cl
H
H
H C
Cl +
H
+
Cl
Cl
H C Cl
H
+
Cl
H
H
H C H
H
H
+
Cl
Cl
H C Cl
H
+
H Cl
=>
Termination Steps
• Collision of any two free radicals
• Combination of free radical with
contaminant or collision with wall.
H
H
C
H
+ Cl
H
H
C
Cl
H
Can you suggest others?
H
H C
H
. .
H
C H
H
H
H C
H
=>
.
H
C H
H
Equilibrium constant
• Keq = [products]
[reactants]
• For chlorination Keq = 1.1 x 1019
• Large value indicates reaction “goes to
completion.”
=>
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/Kmol and T = temperature in kelvins
Since chlorination has a large Keq, the free energy chan
Factors Determining DG
• Free energy change depends on
– enthalpy
– entropy
•DH = (enthalpy of products) - (enthalpy of reactant
•DS = (entropy of products) - (entropy of reactants)
DG = DH - TDS
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).
=>
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.
=>
PROBLEMS
Which entropy value shown below is correct?
C2H6 + C2H4
C4H10
POSSIBLE ANSWERS
A. 0
C4H6
B. -35
C6H10
+ C2H4
A. 0
c. +35
B. -18
c. +18
Bond Dissociation Energy
A B
A
+
B
• Bond breaking requires energy (+BDE)
•Bond formation releases energy (-BDE)
Table 4.2 gives BDE for homolytic cleavage of
bonds in a gaseous molecule.
We can use BDE to estimate DH for a reaction.
=>
Determination of delta H for
overall reaction
H
H C H
H
104 kcal
+ Cl-Cl
58 kcal
H
H C Cl
H
+ H-Cl
-103 kcal
-84 kcal
-187kcal
162 kcal
-25 kcal
EXOTHERMIC
INDIVIDUAL STEPS
RDS
H
H C H + . Cl
H
104 kcal
H
+ Cl-Cl
H C
H
58 kcal
.
H
H C
H
.
+
H-Cl
-103 kcal
H
H C Cl + Cl
H
-84 kcal
DH
+1
.
THIS IS THE OFFICIAL MECHANISM
-26
-25
Which is more likely?
Estimate DH for each step using BDE.
CH4 + Cl
104
CH3 + Cl2
58
CH4 + Cl
104
H
+ Cl2
58
CH3 +
or
HCl
103
CH3Cl + Cl
84
CH3Cl + H
84
HCl + Cl
103
+1 kcal
+20 kcal
=>
Kinetics
• Answers question, “How fast?”
• Rate is proportional to the concentration of
reactants raised to a power.
• Rate law is experimentally determined.
=>
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
=>
Activation Energy
• Minimum energy required to reach
H
the transition state.
H C
H
Cl
H
• At higher temperatures, more molecules
have the required energy.
=>
Reaction-Energy Diagrams
• For a one-step reaction:
reactants  transition state  products
• A catalyst lowers the energy of the transition
state.
=>
Energy Diagram for a
Two-Step Reaction
• Reactants  transition state  intermediate
• Intermediate  transition state  product
=>
Rate-Determining Step
•Reaction intermediates are stable as long as they
don’t collide with another molecule or atom,
but they are very reactive.
Trapping agents are used to determine intermediate
Transition states are at energy maxima.
The reaction step with highest Ea will be the slowest,
therefore rate-determining for the entire reaction.
Rate, Ea, and Temperature
X
X
F
Cl
Br
I
+ CH4
Ea
1.2 kcal
4 kcal
18 kcal
34 kcal
HX
Rate @ 300K
140,000
1300
9 x 10-8
-19
2 x 10
+
CH3
Rate @ 500K
300,000
18,000
0.015
-9
2 x 10
=>
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).
=>
Chlorination of Propane
1 C
CH3
CH2
2 C
CH3
+ Cl2
h
Cl
CH2 CH2
Cl
CH3 + CH3
CH CH3
• There are six 1 H’s and two 2 H’s. We expect 3:1
product mix, or 75% 1-chloropropane and 25% 2chloropropane.
• Typical product mix: 45% 1-chloropropane and 55% 2chloropropane.
• Therefore, not all H’s are equally reactive.
=>
Reactivity of Hydrogens
• To compare hydrogen reactivity, find amount
of product formed per hydrogen: 45% 1chloropropane from 6 hydrogens and 55% 2chloropropane from 2 hydrogens.
• 45%  6 = 7.5% per primary H and
55%  2 = 27.5% per secondary H
• Secondary H’s are 27.5%  7.5% = 3.7 times
more reactive toward chlorination than
primary H’s.
=>
Chlorination of Isobutane
methylpropane
H3C
H3C
H
H3C
Cl2 / light
CH3
+
CH3
H3C
Cl
CH3
3o H
36%
reactivity of 3o = 36/1 = 36% per 1 H
o
reactivity of 1o = 64/9 = 7.1 per 1 H
RelativeReactivity = 3o
>
2o
> 1o
H3C
H
CH2Cl
64%
Rel reactive of 3o/ 1o = 36/7.1 = 5.1
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
=>
Chlorination Energy Diagram
Lower Ea, faster rate, so more stable
intermediate is formed faster.
=>
Bromination of Propane
1 C
Br
Br
CH3
CH2
2 C
CH3
+ Br2
heat
CH2
CH2
CH3 + CH3
CH CH3
• 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.
=>
Reactivity of Hydrogens
• To compare hydrogen reactivity, find amount of
product formed per hydrogen: 3% 1bromopropane from 6 hydrogens and 97% 2bromopropane 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.
=>
Bromination Energy Diagram
• Note larger difference in Ea
• Why endothermic?
=>
Bromination vs. Chlorination
=>
Endothermic and
Exothermic Diagrams
=>
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.
=>
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.
=>