(a) When n-heptane burns in a gasoline engine, the combustion process takes place too quickly. The explosive detonation makes a noise called knocking. When 2,2,4-trimethylpentane (isooctane) is burned, combustion takes place in a slower, more controlled manner. Combustion is a free-radical chain reaction, and its rate depends on the reactivity of the free-radical intermediates. Explain why isooctane has less tendency to knock than does n-heptane.

(b) Alkoxy radicals (R-O∙) are generally more stable than alkyl (R∙) radicals. Write an equation showing an alkyl free radical (from burning gasoline) abstracting a hydrogen atom from tert butyl alcohol, (CH₃)₃COH. Explain why tert-butyl alcohol works as an antiknock additive for gasoline.

(c) Use the information in Table 4-2 to explain why toluene (PhCH3) has a very high octane rating of 111. Write an equation to show how toluene reacts with an alkyl free radical to give a relatively stable radical.

Table 4-2

Transcribed Image Text:

A:B → A.+·B Bond-Dissociation Bond-Dissociation Enthalpy Enthalpy Bond kJ/mol kcal/mol Bond kJ/mol kcal/mol H-X bonds and X–X bonds Bonds to secondary carbons 397 (CH3),CH-H (CH3),CH-F H-H 435 104 95 D-D 444 106 444 106 F-F 159 38 (CH3)2CH-CI 335 80 Cl-CI 242 58 (CH3)2CH- Br 285 68 Br -Br 192 46 (CH3)2CH-I (CH3),CH-OH 222 53 I-I 151 36 381 91 H-F 569 136 431 103 Bonds to tertiary carbons 1D-H (CH3),C-H (CH3)3C-F (CH3);C-CI (CH3);C-Br H-Br 368 88 381 91 H-I 297 71 444 106 HO-H 498 119 331 79 НО -ОН 213 51 272 65 Methyl bonds (CH3)3C-I (CH3)3C-OH 209 50 381 91 CH3-H CH3-F 435 104 456 109 Other C-H bonds CH3-CI 351 84 PHCH,–H (benzylic) 356 85 CH3-Br 293 70 CH,— СНCH, — н (allylic) 364 87 CH;-I 234 56 CH,=CH-H (vinyl) 464 111 CH — ОН 381 91 Ph-H (aromatic) 473 113 Bonds to primary carbons НС-С- Н (асetylenic) 523 125 CH;CH2-H 410 98 CH;CH2-F 448 107 C-C bonds CH3CH2-CI 339 81 CH3-CH3 368 88 CH3CH2-Br 285 68 CH;CH,-CH; 356 85 CH;CH2-I CH;CH2-OH 222 53 CH,CH,-CH,CH3 343 82 381 91 (CH3)2CH-CH3 351 84 CH;CH,CH2-H CH;CH,CH2-F 410 98 (CH3);C-CH3 339 81 448 107 CH;CH,CH,-CI CH;CH,CH2- Br CH;CH,CH2-I 339 81 285 68 222 53 CH;CH,CH2-OH 381 91