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Solutions for The Study of Chemical Reactions
Br BrC
H
H
H
C
H
H
H
H Br
Br Br
HBr + • CH3Br • + CH4
(b) The step leading to the highest energy transition state is rate-limiting. In this mechanism, the first 
propagation step is rate-limiting:
δ •δ •
‡
(3)
(2)
‡
δ • δ •
(1) ("δ •" means partial radical character on the atom)δ •δ •(c)
 + 67 kJ/mole + − 101 kJ/mole = − 34 kJ/mole
( + 16 kcal/mole + − 24 kcal/mole = − 8 kcal/mole)
∆H° for the reaction is the sum of the ∆H° values of the individual propagation steps (refer to the 
solution to 10 (a) and (b)):
(d)
‡
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hν
+I •
+ I •+ • CH3
+ • CH3
2 I •(1) 
(2) 
(3)
initiation
propagation
(a)
I CH3I I
H IH CH3
I I
step (1) break I–I
step (2) break H–CH3
 make H–I 
 step (2)
∆H° = + 151 kJ/mole ( + 36 kcal/mole)
∆H° = + 435 kJ/mole ( + 104 kcal/mole)
∆H° = – 297 kJ/mole ( – 71 kcal/mole)
∆H° = + 138 kJ/mole ( + 33 kcal/mole)
∆H° = + 151 kJ/mole ( + 36 kcal/mole)
∆H° = – 234 kJ/mole ( – 56 kcal/mole)
∆H° = − 83 kJ/mole ( − 20 kcal/mole)
step (3) break I–I
 make I–CH3 
 step (3)
16 continued
(b) ∆H° for the reaction is the sum of the ∆H° values of the individual propagation steps:
+ 138 kJ/mole + − 83 kJ/mole = + 55 kJ/mole
(+ 33 kcal/mole + − 20 kcal/mole = + 13 kcal/mole)
(c) Iodination of methane is unfavorable for both kinetic and thermodynamic reasons. Kinetically, the rate 
of the first propagation step must be very slow because it is very endothermic; the activation energy must 
be at least + 138 kJ/mole. Thermodynamically, the overall reaction is endothermic, so an equilibrium 
would favor reactants, not products; there is no energy decrease to drive the reaction to products.
18 Propane has six primary hydrogens and two secondary hydrogens, a ratio of 3 : 1. If primary and 
secondary hydrogens were replaced by chlorine at equal rates, the chloropropane isomers would reflect the 
same 3 : 1 ratio, that is, 75% 1-chloropropane and 25% 2-chloropropane.
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