Page 1036 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

1020 Hydrogen atom abstraction step Halogen atom abstraction step
R CH + X· R C· + H X R 3 C· + X X R 3 CX + X·
3
3
CHAPTER 11
+34.0 M
Free Radical Reactions
+29.5 E
X = I
+27.1 P
+24.7 i B
+18.0 M
+13.5 E
X = Br
+11.1 P
+9.7 i B
+3.0 M
0
–2.5 E
X = Cl
–4.9 P
–7.3 i B –18.3 iB
–19.8 E
X = I
–20.1 P
–21.1 M
–25.0 M,E,iB
X = Br
–26.5 P
–26.7 M
–27.1 iB X = Cl
–27.2 E
–27.6 P
29.0 M
33.5 E X = F
35.9 P
38.3 iB

–70.0 M
–75.1 E X = F
–77.4 iB,P
Fig. 11.12. Thermochemical relationships for steps in radical chain halogenation M = methane, E = ethane,
P = propane, and iB = isobutane.

for NBS bromination. 133 Br is maintained at a low concentration during the course of
2
the reaction by formation from NBS and HBr.
. .
Br + H R Br H + R
O O

Br H + N Br N H + Br 2
O O
Br 2 + R . RBr + Br .

133
R. E. Pearson and J. C. Martin, J. Am. Chem. Soc., 85, 354, 3142 (1963); G. A. Russell, C. DeBoer, and
K. M. Desmond, J. Am. Chem. Soc., 85, 365 (1963); J. H. Incremona and J. C. Martin, J. Am. Chem.
Soc., 92, 627 (1970); J. C. Day, M. J. Lindstrom, and P. S. Skell, J. Am. Chem. Soc., 96, 5616 (1974).