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

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

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

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

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

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