(a) Using bond-dissociation enthalpies from Table 4-2 (page 143), calculate the heat of reaction for each step in the free-radical bromination of methane.

(b) Calculate the overall heat of reaction.

Br2 CH heat or light CH,Br HBr TABLE 4-2 Bond-Dissociation Enthalpies for Homolytic Cleavages A:BA-+-B kcal/mol Bond H-X bonds and X X bonds Bonds to sacondary carbons D-D F-F a-C Br-Br 1-1 H-F H--CI H-Br (CHCH-H (CH CH-F (CH,CH-CI (CH, CH- Br CHHCH-I CHACH OH 38 335 192 151 381 136 431 Bonds to tertiary carbons 381 (CH),CH CHCF (CH)C-C (CH),C-Br (CH),C-I CH,,C-OH 71 119 HO-H 331 213 Medyl bond CH,-H CH3-F 381 91 351 PhCH H (benzylic) C CHH H (allylic) CH CH H (viny) Ph H(aromatic) HC-m C-H (acetylenic) CH, Br 70 CH OH 381 91 113 Bonds to primary carto 410 CH,CH F CH,CH C CH,CH Br CH,CHI CH,CH,-OH CH,CH CH2 H CH,CH CH F CH,CH CH CI CH,CH CH Br CH,CH CH2I CH,CH CH2 OH 107 C-C bonds CH CH CH,CH CH, CH,CH, CH,CH (CH)CH- CH (CH),C CH 285 381 410 91 351 81 381 91