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4.15 Halogenation of Alkanes

4.15 Halogenation of Alkanes. RH + X 2 Æ RX + HX. Energetics. RH + X 2 Æ RX + HX explosive for F 2 exothermic for Cl 2 and Br 2 endothermic for I 2. 4.16 Chlorination of Methane. Chlorination of Methane. carried out at high temperature (400 °C)

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4.15 Halogenation of Alkanes

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  1. 4.15Halogenation of Alkanes RH + X2Æ RX + HX

  2. Energetics • RH + X2Æ RX + HX • explosive for F2 • exothermic for Cl2 and Br2 • endothermic for I2

  3. 4.16Chlorination of Methane

  4. Chlorination of Methane • carried out at high temperature (400 °C) • CH4 + Cl2 Æ CH3Cl + HCl • CH3Cl + Cl2 Æ CH2Cl2 + HCl • CH2Cl2 + Cl2 Æ CHCl3 + HCl • CHCl3 + Cl2 Æ CCl4 + HCl

  5. 4.17Structure and Stability of Free Radicals

  6. .. .. : O O: . . .. . : : O N Free Radicals • contain unpaired electrons Examples: O2 NO Li 1s2 2s1 .. . : Cl Cl ..

  7. . R R R Alkyl Radicals C • Most free radicals in which carbon bearsthe unpaired electron are too unstable to beisolated. • Alkyl radicals are classified as primary,secondary, or tertiary in the same way thatcarbocations are.

  8. Figure 4.15 Structure of methyl radical. • Methyl radical is planar, which suggests that carbon is sp2 hybridized and that the unpaired electron is in a p orbital.

  9. Alkyl Radicals . R R • The order of stability of free radicals is the same as for carbocations. C R

  10. . . H H3C H H C C H H Ethyl radical(primary) Methyl radical Alkyl Radicals less stablethan

  11. . . H H3C H H C C H H Ethyl radical(primary) Methyl radical . CH3 H3C C H Isopropyl radical(secondary) Alkyl Radicals less stablethan less stablethan

  12. . . H H3C H H C C H H Ethyl radical(primary) Methyl radical . . CH3 CH3 H3C H3C C C H CH3 Isopropyl radical(secondary) tert-Butyl radical(tertiary) Alkyl Radicals less stablethan less stablethan less stablethan

  13. Alkyl Radicals • The order of stability of free radicals can be determined by measuring bond strengths. • By "bond strength" we mean the energy required to break a covalent bond. • A chemical bond can be broken in two different ways—heterolytically or homolytically.

  14. Homolytic • In a homolytic bond cleavage, the two electrons inthe bond are divided equally between the two atoms.One electron goes with one atom, the second with the other atom. • In a heterolytic cleavage, one atom retains bothelectrons. – + Heterolytic

  15. Homolytic • The species formed by a homolytic bond cleavageof a neutral molecule are free radicals. Therefore, measure energy cost of homolytic bond cleavage to gain information about stability of free radicals. • The more stable the free-radical products, the weakerthe bond, and the lower the bond-dissociation energy.

  16. . CH3CH2CH2 + H . CH3CHCH3 + H . . Measures of Free Radical Stability • Bond-dissociation energy measurements tell us that isopropyl radical is 13 kJ/mol more stable than propyl. 410 397 CH3CH2CH3

  17. . (CH3)3C+H . Measures of Free Radical Stability • Bond-dissociation energy measurements tell us that tert-butyl radical is 30 kJ/mol more stable than isobutyl. . (CH3)2CHCH2 + H . 410 380 (CH3)3CH

  18. 4.18Mechanism of Chlorination of Methane

  19. .. .. .. .. . . : : : : : Cl Cl Cl Cl .. .. .. .. Mechanism of Chlorination of Methane Free-radical chain mechanism. • The initiation step "gets the reaction going"by producing free radicals—chlorine atomsfrom chlorine molecules in this case. • Initiation step is followed by propagationsteps. Each propagation step consumes onefree radical but generates another one. Initiation step: +

  20. .. .. . . H3C H : Cl: Cl: .. .. Mechanism of Chlorination of Methane First propagation step: + H3C : H +

  21. .. .. . . H3C H : Cl: Cl: .. .. .. .. .. .. . . : : : : : Cl: Cl Cl Cl H3C H3C .. .. .. .. Mechanism of Chlorination of Methane First propagation step: + H3C : H + Second propagation step: + +

  22. .. .. . . H3C H : Cl: Cl: .. .. .. .. .. .. . . : : : : : Cl: Cl Cl Cl H3C H3C .. .. .. .. .. .. .. .. : : Cl : : : H3C H : Cl: Cl Cl .. .. .. .. Mechanism of Chlorination of Methane First propagation step: + H3C : H + Second propagation step: + + + + H3C : H

  23. .. .. . . H3C H : Cl: Cl: .. .. .. .. .. .. . . : : : : : Cl: Cl Cl Cl H3C H3C .. .. .. .. .. .. .. .. : : Cl : : : H3C H : Cl: Cl Cl .. .. .. .. Almost all of the product is formed by repetitivecycles of the two propagation steps. First propagation step: + H3C : H + Second propagation step: + + + + H3C : H

  24. .. .. . . : : Cl H3C H3C Cl: .. .. Termination Steps • stop chain reaction by consuming free radicals • hardly any product is formed by termination stepbecause concentration of free radicals at anyinstant is extremely low +

  25. 4.19Halogenation of Higher Alkanes

  26. Chlorination of Alkanes • can be used to prepare alkyl chlorides from alkanes in which all of the hydrogens are equivalent to one another 420°C CH3CH3 + Cl2 CH3CH2Cl + HCl (78%) hn + Cl2 + HCl Cl (73%)

  27. Chlorination of Alkanes Major limitation: Chlorination gives every possible monochloride derived from original carbonskeleton. Not much difference in reactivity ofdifferent hydrogens in molecule.

  28. Example • Chlorination of butane gives a mixture of1-chlorobutane and 2-chlorobutane. (28%) CH3CH2CH2CH2Cl Cl2 CH3CH2CH2CH3 hn CH3CHCH2CH3 (72%) Cl

  29. Percentage of product that results from substitution of indicated hydrogen if every collision with chlorine atoms is productive 10% 10% 10% 10% 10% 10% 10% 10% 10% 10% 10%

  30. Percentage of product that actually results from replacement of indicated hydrogen 18% 18% 4.6% 4.6% 4.6% 4.6% 4.6% 4.6% 18% 18% 10%

  31. 4.6 4.6 18 4.6 Relative rates of hydrogen atom abstraction 18% 4.6% • divide by 4.6 = 1 = 3.9 A secondary hydrogen is abstracted 3.9 times faster than a primary hydrogen by a chlorine atom.

  32. CH3 CH3CCH2Cl CH3 H Cl2 CH3CCH3 hn H CH3 CH3CCH3 Cl • Similarly, chlorination of 2-methylbutane gives a mixture of isobutyl chloride and tert-butyl chloride (63%) (37%)

  33. Percentage of product that results from replacement of indicated hydrogen 7.0% 37%

  34. Relative rates of hydrogen atom abstraction • divide by 7 7.0 37 = 5.3 = 1 7 7 A tertiary hydrogen is abstracted 5.3 times faster than a primary hydrogen by a chlorine atom.

  35. Selectivity of free-radical halogenation • R3CH > R2CH2 > RCH3 • chlorination: 5 4 1 • bromination: 1640 82 1 • Chlorination of an alkane gives a mixture of every possible isomer having the same skeletonas the starting alkane. Useful for synthesis only when all hydrogens in a molecule are equivalent. • Bromination is highly regioselective for substitution of tertiary hydrogens. Major synthetic application is in synthesis of tertiary alkyl bromides.

  36. Cl Cl2 hn Synthetic application of chlorination of an alkane • Chlorination is useful for synthesis only when all of the hydrogens in a molecule are equivalent. (64%)

  37. Br H CH3CCH2CH2CH3 CH3CCH2CH2CH3 CH3 CH3 Synthetic application of bromination of an alkane Br2 • Bromination is highly selective for substitution of tertiary hydrogens. • Major synthetic application is in synthesis of tertiary alkyl bromides. hn (76%)

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