Chapter 5 • Types of reactions. • Reaction mechanism .
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Transcript Chapter 5 • Types of reactions. • Reaction mechanism .
Chapter 5
• Types of reactions.
• Reaction mechanism
– Radical chlorination - homolytic cleavage, initiation,
propagation, termination, disadvantage
.
– Polar reaction - , reaction enhances polarity, polarizability,
heterolytic cleavage, nucleophile, electrophile.
• Reaction profile - equilibria, kinetics, thermodynamic changes.
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Types of Organic Reactions
• Addition
• Elimination
• Substitution
• Rearrangement
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Addition
H H
H
H
+
H
H
H2
H
H H
H
sp2
sp3
E
R
or
P
RC
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E
P
R
RC
3
Elimination
H H
H
H H
H
OH
sp3
H
H
+
H
O
H
H
sp2
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Substitution
H H
H
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+
Br
H H
+
KOH
H
KBr
OH
5
Rearrangement
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Chapter 5
• Types of reactions.
• Reaction mechanism
– Radical chlorination - homolytic cleavage, initiation,
propagation, termination, disadvantage
.
– Polar reaction - , reaction enhances polarity, polarizability,
heterolytic cleavage, nucleophile, electrophile.
• Reaction profile - equilibria, kinetics, thermodynamic changes.
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Mechanism
A complete step-by-step description
of which bonds break and which
bonds form, in what order, in a
chemical reaction.
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Chapter 5
• Types of reactions.
• Reaction mechanism
– Radical chlorination - homolytic cleavage, initiation,
propagation, termination, disadvantage
.
– Polar reaction - , reaction enhances polarity, polarizability,
heterolytic cleavage, nucleophile, electrophile.
• Reaction profile - equilibria, kinetics, thermodynamic changes.
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Chlorination of Methane
h or heat
CH 4 Cl 2 CH 3Cl HCl
Mechanism: radical reaction.
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Radical reaction - chlorination of methane
• Initiation
• Propagation
• Termination
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Mechanism - Initiation
h or heat
Cl — Cl
2Cl
homolytic
cleavage
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Mechanism - Propagation
Substitution
Cl + H:CH3 HCl + CH3
Addition
CH3 + Cl:Cl CH3Cl + Cl
Chain Reactions
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Mechanism - Termination
Cl + CH3 CH3Cl
Cl + Cl Cl2
CH3 + CH3 CH3CH3
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Chlorination of methane - disadvantage
H
C
H H
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H
Cl2
h or heat
H
C
H H
Cl
Cl2
h or heat
H
C
H Cl
Cl
Cl2
h or heat
15
Chlorination of alkanes
CH3
CH3CH2CH2CHCH2Cl
CH3
CH3CH2CH2CCH3
Cl
CH3
CH3CH2CH2CHCH3
+
CH3
h or
Cl2
heat
CH3CH2CHCHCH3
and others...
Cl
CH3
CH3CHCH2CHCH3
Cl
CH3
CH2CH2CH2CHCH3
Cl
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Chlorination of alkanes
Which of the following compounds will give a single monochloro product?
CH3CH2CH3
C2H6
(CH3)2CCH2CH3
CH3
CH3C
CCH3
Ans.
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Free Radical Chlorination
Write radical reaction mechanism of the chlorination
of methane for the formation of all chlorinated methane
- that is, CH3Cl, CH2Cl2, CHCl3, CCl4.
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Free Radical Chlorination
Considering only monochlorohydrocarbon. Explain
why:
CH3CH2CH2CH2CH2CH3
+
Cl2
CH3CH2CH2CH2CH2CHCl
+
HCl
Low yield
Cl
+
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Cl2
+
HCl
High yield
19
Chapter 5
• Types of reactions.
• Reaction mechanism
– Radical chlorination - homolytic cleavage, initiation,
propagation, termination, disadvantage
.
– Polar reaction - , reaction enhances polarity, polarizability,
heterolytic cleavage, nucleophile, electrophile.
• Reaction profile - equilibria, kinetics, thermodynamic changes.
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Polarity
2.5
3.5
2.5
1.2
2.5
2.5
+
-
-
+
+
-
C—O
C — Mg
H
H
C—I
H
O
C H
H
H
Weakly polar C—O
+
O
C H H
H
Strongly polar C—O
solvent
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A generalized polar reaction
:B-
AB
electrophile
nucleophile
heterolytic cleavage
neutral or
positively
charged
neutral or
negatively
charged
A+
+
O
H+,
CH3X,
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NH3, OH-, Br-
Arrow shows the
flow of electrons.
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Flow of Electrons
H
+
D
D
H
Cl
+H
H
+
+
Cl
+H
H
Cl
H
D
H
+
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Cl
D
D
H
H
+
D
+
Cl
H
Cl
H
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Flow of Electrons
nucleophile
Cl
H
+
D
Cl
H
nucleophile
electrophile
D
H
+H
D
+
H
Cl
H
carbocation
substitution
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Predict Products
O
HO
+
H
C
H H
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C
CH3
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Chapter 5
• Types of reactions.
• Reaction mechanism
– Radical chlorination - homolytic cleavage, initiation,
propagation, termination, disadvantage
.
– Polar reaction - , reaction enhances polarity, polarizability,
heterolytic cleavage, nucleophile, electrophile.
• Reaction profile - equilibria, kinetics, thermodynamic changes.
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Equilibria
A B C D
K eq
[C][ D]
[ A][B]
CH 4 Cl 2 CH 3Cl HCl
K eq
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[CH 3Cl ][ HCl ]
1.11019
[CH 4 ][Cl 2 ]
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Equilibrium
Keq
Thermodynamics
G
G G(products ) G(reac tan ts )
K eq e
G o / RT
@ 25o C, RT 2.48 kJ / mol 0.592kcal / mol
For CH 4 Cl 2 CH 3Cl HCl , K eq 1.11019
G o 108kJ / mol 25.9kcal / mol
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For CH 4 Cl 2 CH 3Cl HCl ,
K eq 1.11019
R=
T=
RT=
8.31E-03 kJ/(mol*K)
298 K
2.48 kJ/mol
Go
Keq
%C_to_P
K eq exp( G o / RT )
100
%C _ to _ P
1 exp(G o / RT)
100
80
60
40
20
0
-20
-40
-60
-80
-100
-120
2.96E-18
9.48E-15
3.04E-11
9.74E-08
3.12E-04
1.00E+00
3.20E+03
1.03E+07
3.29E+10
1.05E+14
3.38E+17
1.08E+21
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2.96E-16
9.48E-13
3.04E-09
9.74E-06
3.12E-02
5.00E+01
1.00E+02
1.00E+02
1.00E+02
1.00E+02
1.00E+02
1.00E+02
% conversion to
products
kJ/mol
120
100
80
60
40
20
0
-150
-100
-50
0
50
100
150
Free Energy(kJ/mol)
29
Thermodynamics
G o H o TS o
Go determines the spontaneity of a reaction.
Ho & So are driving forces of a reaction.
Ho - Exothermic(-) or Endothermic(+).
So - Randomness or Freedom of Motion
For most (not all) organic reactions, Ho > So
Thus, Go Ho
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Enthalpy Change in Chlorination of Methane
CH4 + Cl2 CH3Cl + HCl
Table 5.3
page 172
Bonds Broken(+)
Bonds Formed(-)
CH3—H +438
Cl—Cl +243
CH3—Cl -351
H—Cl -432
+681
-783
Ho (or Go) = +681 + (-783) = -102 kJ/mol
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Enthalpy Change in Chlorination of Methane
CH4 + Cl2
E
Ho (or Go) = -102 kJ/mol
CH3Cl + HCl
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Reaction Coordinate
32
Enthalpy Change in Bromination of Methane
CH4 + Br2 CH3Br + HBr
Table 5.3
page 172
Bonds Broken(+)
Bonds Formed(-)
CH3—H +438
+193
CH3—Br -293
H—Br -366
+631
-659
Br—Br
H (or Go) = +631 + (-659) = -28 kJ/mol
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Enthalpy Change in Bromination of Methane
CH4 + Br2
E
H (or Go) = -28 kJ/mol
CH3Br + HBr
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Reaction Coordinate
34
Chlorination and Bromination of Methane
CH4 + Cl2
CH4 + Br2
-28 kJ/mol
E
-102 kJ/mol
CH3Br + HBr
CH3Cl + HCl
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Reaction Coordinate
35
H of Chlorination of Alkane
CH3
CH3
CH3CH2CHCH3
+
Cl2
h or heat
CH3CH2CHCH2Cl
1o
Ho
1o
+
HCl
-107
+
HCl
-107
+
HCl
-127
+
HCl
-129
CH3
ClCH2CH2CHCH3
2o
CH3
CH3CHCHCH3
Cl
CH3
CH3CHCCH3
Cl
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3o
36
Kinetics
A+BC+D
Rate k[A]a [B]b
k: rate constant
a: order with respect to A.
b: order with respect to B.
a+b: overall order of the reaction.
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Rate Law
CH3Br
+
OH-
CH3OH
+
Br
rate k[CH3 ][OH ]
(CH3)3CBr +
OH-
(CH3)3OH
+
Br
rate k[(CH 3 ) 3 Br]
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Rate Law Determination
• Reactions that appear to be similar do not
necessarily have the same rate law (or rate
equation). That is, in general, rate law can
not be predicted by the stoichiometry of the
reaction.
• Rate law can only be determined
experimentally.
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Example
Given:
CH3Cl + -CN CH3C N + ClWhen [CH3Cl] is doubled, the observed rate doubled.
When [-CN] tripled, the observed rate tripled.
Questions:
What is the order with respect to CH3Cl?
What is the order with respect to -CN?
What is the rate law of this reaction?
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Example
Given:
H
H
H
+
C C
H
H2
H
H H C
C
H
H H
When [H2] is fixed and [ethylene] is doubled, no change in observed rate.
When [ethylene] is fixed and [H2] tripled, no change in observed rate.
Questions:
What is the order with respect to ethylene?
What is the order with respect to H2?
What is the rate law of this reaction?
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Temperature Dependence of Rate
A+BC+D
Rate = k[A]a[B]b
k = exp(-G‡/RT)
G‡: activation energy
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Activation Energy
Page 175
“…many organic reactions have activation energies
in the range 40-150 kJ/mol … activation energies
less than 80 kJ/mol take place at or below room
temperature, … ”
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Graphic Description of Reaction Profile
Single Step Reaction
transition state
[ CH3----H----Cl ]‡
E
G‡
H
•CH3 + HCl
CH4 + Cl•
Reaction Coordinate
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Multistep Reactions
Propagation of Methane:
Ho or Go
Cl + CH4 HCl + CH3
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CH3 + Cl2 CH3Cl + Cl
G‡
kJ/mol
kJ/mol
4
17
-109
4
45
Graphic Description of Reaction Profile
Multistep Reaction
E
Reaction Coordinate
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H of Chlorination of Alkane
CH3
CH3
CH3CH2CHCH3
+
Cl2
h or heat
CH3CH2CHCH2Cl
1o
2o
E
1o
1o
+
HCl
-107
+
HCl
-107
+
HCl
-127
+
HCl
-129
CH3
ClCH2CH2CHCH3
Reactants
Ho
1o
CH3
CH3CHCHCH3
Cl
2o
3o
RC
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CH3
CH3CHCCH3
Cl
3o
47
Chapter 5
• Types of reactions.
• Reaction mechanism
– Radical chlorination - homolytic cleavage, initiation,
propagation, termination, disadvantage
.
– Polar reaction - , reaction enhances polarity, polarizability,
heterolytic cleavage, nucleophile, electrophile.
• Reaction profile - equilibria, kinetics, thermodynamic changes.
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