1. Chapter 3 Reactions of Alkanes
Bond Dissociation
Bond Strength = Bond Dissociation Energy = DH0 = energy input to break bond
Homolytic Cleavage
Heterolytic Cleavage
DHo = 104 kcal/mol
Bond Dissociation Energies of Various A—B Bonds

2. DHo = 119 kcal/mol
DE much smaller
Polar solvents stabilize ions
DHo depends on how well orbitals overlap: HF > HCl > HBr > HI
Stability of Radicals
Alkyl Radicals: 3o > 2o > 1o > Methyl
Hyperconjugation Explains Radical Stability
Hyperconjugation = delocalization of s-bond electron pair into a partly empty p-orbital
Delocalization = spreading e- over multiple atoms
CH3-H
105
CH3CH2-H
101
(CH3)2CH-H
98.5
(CH3)3C-H
96.5
DHo
Stability

4. 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-
Methyl
Radical

5. Radical Chain Mechanisms
Pyrolysis
Pyrolysis = breaking of CH, C-C bonds with heat
Cracking = breaking into smaller fragments
Control with catalysts
Petroleum into gasoline
Radical Chain Mechanisms
Chlorination of Methane
D = heat, hn = light
Ea is high, so energy is required to start the reaction
Radical
Combinations
DE = -25 kcal/mol

6. Mechanism
Initiation
Propagation 1) 2)
Termination
Termination is rare because radicals are few, small odds of meeting
Radical chains use up “fuel” before termination

9. Reactivities of other Halogens
F2 > Cl2 > Br2 > I2
Propagation 1
F2 DH = -31 kcal/mol
Cl2 DH = +2 kcal/mol
Br2 DH = +18 kcal/mol
I2 DH = +34 kcal/mol
Hammond Postulate
Early TS = little bond breaking = exothermic reaction
Late TS = much bond breaking = endothermic reaction
At TS, Bond Breaking Energy = Bond Forming Energy
Propagation 2
Exothermic for all halides
Total DE for Prop. 1 and Prop. 2
F(-) > Cl(-) > Br(-) > I(+)
Iodine won’t react with methane to give MeI