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Radical Chain Reactions

Radical Chain Reactions. Substitution Theory. Substitution Theory. 1838 - chlorination of acetic acid. C 4 H 4 O 2 + Cl 6 = C 4 HCl 3 O 2 + H 3 Cl 3. C = 6, O = 16. C 2 H 4 O 2 + 3Cl 2 = C 2 HCl 3 O 2 + 3HCl. J. B. Dumas (1800 -1884). CH 4 + Cl 2 CH 3 Cl + HCl.

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Radical Chain Reactions

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  1. Radical Chain Reactions

  2. Substitution Theory Substitution Theory 1838 - chlorination of acetic acid C4H4O2 + Cl6 = C4HCl3O2 + H3Cl3 C = 6, O = 16 C2H4O2 + 3Cl2 = C2HCl3O2 + 3HCl J. B. Dumas (1800 -1884)

  3. CH4 + Cl2 CH3Cl + HCl . CH3 H Cl Ho = +1 kcal/mol . Ea = +4 kcal/mol CH3 Cl Cl Ea = +2 kcal/mol . light P1 Initiation: Cl2 CH4 + Cl. CH3. + HCl 2Cl Propagation Step 1 (P1) DHo = 104 - 103 = 1 kcal/mol Propagation Step 2 (P2) DHo = 58 - 84 = -26 kcal/mol Ho = -25 kcal/mol Ho = -26 kcal/mol . CH3 + Cl2 P2 CH3Cl + Cl. Free Radical Chain Reaction of Methane with Chlorine

  4. +104 kcal/mol -103 kcal/mol +104 kcal/mol -84 kcal/mol CH4 + Cl. CH3. + HCl CH4 + Cl. H. + CH3Cl +58 kcal/mol -84 kcal/mol +58 kcal/mol -103 kcal/mol CH3. + Cl2 CH3Cl + Cl. H. + Cl2 HCl+ H. Ho = +20 kcal/mol Ho = -45 kcal/mol Ho = +1 kcal/mol Ho = -25 kcal/mol Ho = -25 kcal/mol Ho = -26 kcal/mol Free Radical Chain Reaction of Methane with Chlorine: An Alternative Mechanism? CH4 + Cl2 CH3Cl + HCl

  5. C2H6 + Cl2 C2H5Cl + HCl +98 kcal/mol -103 kcal/mol C2H6 + Cl. C2H5.+ HCl Ho = -5 kcal/mol +58 kcal/mol -81 kcal/mol C2H5. + Cl2 C2H5Cl + Cl. Ho = -26 kcal/mol Ho = -28 kcal/mol Free Radical Chain Reaction of Ethane with Chlorine

  6. Free Radical Chain Reaction of Propane with Chlorine Reactivity of 1o vs. 2o C-H Bonds Ho = -5 kcal/mol C3H8 + Cl2 C3H7Cl + HCl Eact = ~ 1 kcal/mol 1-Chloropropane 40% 2-Chloropropane 60% Propagation Step 2 Ho = -8 kcal/mol +58 kcal/mol -81 kcal/mol 1-C3H7.+ Cl2 1-C3H7Cl + Cl. Propagation Step 1 Ho = -22 kcal/mol +58 kcal/mol -80 kcal/mol 2-C3H7.+ Cl2 2-C3H7Cl+ Cl. +98 kcal/mol -103 kcal/mol C3H8 + Cl. 1-C3H7.+ HCl Ho = -23 kcal/mol +95 kcal/mol -103 kcal/mol C3H8 + Cl. 2-C3H7.+ HCl Ho = -28 kcal/mol Ho = -30 kcal/mol

  7. Free Radical Chain Reaction of Propane with Chlorine Reactivity of 1o vs. 2o C-H Bonds secondary C-H primary C-H C3H8 + Cl2 C3H7Cl + HCl 1-Chloropropane 40% 2-Chloropropane 60% Primary C-H bonds are less reactive (BDE = 98 kcal/mol) than secondary C-H bonds (BDE = 95 kcal/mol), but there are more primary C-H bonds than secondary C-H bonds.

  8. Free Radical Chain Reaction of Isobutane with Chlorine Reactivity of 1o vs. 3o C-H Bonds tertiary C-H primary C-H 1-Chloro-2-methylpropane 62% 2-Chloro-2-methylpropane 38% Type C-H # Yield (%) %/# Relative Reactivity 1o 9 62 6.88 1 3o 1 38 38 5.5 C4H10 + Cl2 C4H9Cl + HCl Primary C-H bonds have the numbers but not the reactivity! (Tertiary C-H bond: 91 kcal/mol)

  9. Free Radical Chain Reaction of 2-Methylbutane with Chlorine Predicting Product Ratios C5H12 + Cl2 C5H11Cl + HCl 1-Chloro-2-methylbutane 1-Chloro-3-methylbutane 2-Chloro-3-methylbutane 2-Chloro-2-methylbutane Primary 1 Primary 2 Type # Relative Reactivity # x R.R. fraction % Primary 1 6 1 6 6/23.5 25.5 Secondary Primary 2 3 1 3 3/23.5 12.8 Tertiary Secondary 2 4.5 9 9/23.5 38.3 Tertiary 1 5.5 5.5 5.5/23.5 23.4

  10. Why do radical halogenations stop at the monochloro compound? They don’t! The reaction of molar quantities of methane and chlorine yields a distribution of chlorinated methanes. Readily separated by distillation.

  11. . . CH4 + X CH3 + HX . . CH3 + X2 CH3X + X +33 kcal/mol Endothermic! Iodination proceeds in the reverse direction. iodination Energy (kcal/mol) +13 kcal/mol +16 kcal/mol bromination +1 kcal/mol -8 kcal/mol chlorination -26 kcal/mol Reaction Coordinate What about bromination and iodination? At 27oC, the chlorination of methane is ~1011 times faster than the bromination and, bromination is ~1010 times faster than iodination under the same conditions!

  12. CH4 + X2 CH3X + HX X Ea (kcal/mol) BDE HX [CH3X] oC 1o 2o 3o 1 F 1.2 136 [115] 27 1.2 1.4 1 1 Cl 4 103 [84] 27 3.9 4.5 5.1 5.5 1 1 Br 18 88 [70] 127 82 97 1600 - Activation Energy of Fluorine, Chlorine and Bromine Atoms with Methane And the Relative Reactivity of the Halogen Atoms with C-H Bonds Reaction Selectivities: Anson, Fredricks, Tedder (1958);Wade’s Text

  13. 2-Methylbutane Cyclohexene Toluene Typical C-H Bond Dissociation Energies

  14. 1st Propagation Step Generation of Br2 in low concentration N-Bromosuccinimide (more dense than CCl4) Succinimide (less dense than CCl4) Allylic Bromination 2nd Propagation Step Allylic Bromination rate = kallylic[alkene][Br2] Addition of Br2 to the double bond rate = kaddn[alkene][Br2]2

  15. The End F. E Ziegler 2009

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