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Chapter 3 Reactions of Alkanes

Chapter 3 Reactions of Alkanes. Bond Dissociation Bond Strength = Bond Dissociation Energy = DH 0 = energy input to break bond Homolytic Cleavage Heterolytic Cleavage. DH o = 104 kcal/mol. Bond Dissociation Energies of Various A—B Bonds. DH o = 119 kcal/mol

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Chapter 3 Reactions of Alkanes

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  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 DHo Stability

  3. 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

  4. 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

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

  6. Reactivities of other Halogens • F2 > Cl2 > Br2 > I2 • Propagation 1 • F2DH = -31 kcal/mol • Cl2DH = +2 kcal/mol • Br2DH = +18 kcal/mol • I2DH = +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

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