Chapter 3 Reactions of Alkanes
I.
Bond Dissociation
A.
Bond Strength = Bond Dissociation Energy = DH0 = energy input to break bond
1) Homolytic Cleavage
H H
2)
2H
DHo = 104 kcal/mol
Heterolytic Cleavage
H
H
H+
+
H-
Bond Dissociation Energies of Various A—B Bonds
3)
H
H
H
OH
H+
OH
+
OH
+ OH-
DHo = 119 kcal/mol
DE much smaller
Polar solvents stabilize ions
4)
B.
DHo depends on how well orbitals overlap: HF > HCl > HBr > HI
Stability of Radicals
1) Alkyl Radicals: 3o > 2o > 1o > Methyl
Stability
CH3-H
105
CH3CH2-H
101
DHo
(CH3)2CH-H 98.5
(CH3)3C-H
2)
96.5
Hyperconjugation Explains Radical Stability
a) Hyperconjugation = delocalization of s-bond electron pair into a
partly empty p-orbital
Delocalization = spreading e- over multiple atoms
b)
c)
d)
Methyl
Radical
Alkyl Radicals are planar (sp2) with single e- in p-orbital
Must have s-bond in proper geometry to help
Hyperconjugation is similar to p-bonding, but has odd # of e-
C.
Pyrolysis
1) Pyrolysis = breaking of CH, C-C bonds with heat
D
CH3 +
Radical
Combinations
+
+
2)
II.
Cracking = breaking into smaller fragments
a) Control with catalysts
b) Petroleum into gasoline
Radical Chain Mechanisms
A.
Chlorination of Methane
CH4 +
1)
2)
Cl2
Dhn
CH3Cl + HCl
DE = -25 kcal/mol
D = heat, hn = light
Ea is high, so energy is required to start the reaction
B.
Mechanism
A. Initiation
B.
Propagation
1)
Cl
2)
C.
Cl
+
Dhn
Cl
HCl +
H-CH3
CH3 + Cl2
Termination
CH3
CH3Cl + Cl
Cl2
2 Cl
Cl
+
2 CH3
1)
2)
2 Cl
CH3
CH3Cl
CH3CH3
Termination is rare because radicals are few, small odds of meeting
Radical chains use up “fuel” before termination
C.
Reactivities of other Halogens
1) F2 > Cl2 > Br2 > I2
2)
Propagation 1
a) F2 DH = -31 kcal/mol
b) Cl2 DH = +2 kcal/mol
c) Br2 DH = +18 kcal/mol
d) I2 DH = +34 kcal/mol
3)
Hammond Postulate
a) Early TS = little bond breaking = exothermic reaction
b) Late TS = much bond breaking = endothermic reaction
c) At TS, Bond Breaking Energy = Bond Forming Energy
4)
Propagation 2
a) Exothermic for all halides
b) Total DE for Prop. 1 and Prop. 2
F(-) > Cl(-) > Br(-) > I(+)
Iodine won’t react with methane to give MeI