Organic Chemistry

Section 3.7 Thermodynamics and Kinetics 129

ing the entropy term can be a dangerous practice, however, because many organic

reactions occur with a significant change in entropy or occur at high temperatures and

so have significant terms. It is permissible to use values to approximate

whether a reaction occurs with a favorable equilibrium constant, but if a precise an-

swer is needed, values must be used. When values are used to construct re-

action coordinate diagrams, the y-axis is free energy; when values are used, the

y-axis is potential energy.

Values of can be calculated from bond dissociation energies (Table 3.1). For

example, the for the addition of HBr to ethene is calculated as shown here:

bonds being broken bonds being formed

CC + H H

H

H

H

Br

H

C

C

Br C H

HBr

H

π bond of ethene DH° 0 63 kcal/mol

H

H

H

= DH°=101 kcal/mol

DH°= 0 87 kcal/mol C Br DH°= 69 kcal/mol

=

−

DH°total 150 kcal/mol

∆H° for the reaction=DH° for bonds being broken DH° for bonds being formed

=150 kcal/mol−170 kcal/mol

=−20 kcal/mol

DH°total=170 kcal/mol

¢H°

¢H°

¢H°

¢G° ¢G°

T¢S° ¢H°

Table 3.1 Homolytic Bond Dissociation Energies Y¬Z:Y# + #Z

DH° DH°

Bond Bond

105 439 104 435

101 423 38 159

101 423 58 242

99 414 46 192

97 406 36 150

136 571

88 368 103 432

85 355 87 366

84 351 71 298

80 334

108 451

174 728 84 350

231 966 82 343

81 338

119 497 79 330

104 435 70 294

92 387 69 289

68 285

63 264

57 239

CH 3 CH 2 ¬I 55 230

CH 3 ¬I

(CH 3 ) 3 C¬Br

(CH 3 ) 2 CH¬Br

CH 3 ¬OH CH 3 CH 2 ¬Br

CH 3 O¬H CH 3 ¬Br

HO¬H (CH 3 ) 3 C¬Cl

(CH 3 ) 2 CH¬Cl

HC‚CH CH 3 CH 2 ¬Cl

H 2 C“CH 2 CH 3 ¬Cl

CH 3 ¬F

(CH 3 ) 3 C¬CH 3

(CH 3 ) 2 CH¬CH 3 H¬I

CH 3 CH 2 ¬CH 3 H¬Br

CH 3 ¬CH 3 H¬Cl

H¬F

(CH 3 ) 3 C¬H I¬I

(CH 3 ) 2 CH¬H Br¬Br

CH 3 CH 2 CH 2 ¬H Cl¬Cl

CH 3 CH 2 ¬H F¬F

CH 3 ¬H H¬H

kcal>mol kJ>mol kcal>mol kJ>mol

J. Berkowitz, G.B. Ellison, D. Gutman,J. Phys. Chem.,1994, 98 , 2744.

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