Page 1042 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

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+ O 2 OOH
O O
CHAPTER 11
Free Radical Reactions
This reaction is the cause of a widely recognized laboratory hazard. The peroxides
formed from several commonly used ethers such as diethyl ether and tetrahydrofuran
are explosive. Appreciable amounts of such peroxides can build up in ether samples
that have been exposed to air. Since the hydroperoxides are less volatile than the ethers,
they are concentrated by evaporation or distillation and the concentrated peroxide
solutions may explode. For this reason, storage of ethers that have been exposed to
oxygen is extremely hazardous.

11.4. Free Radical Addition Reactions

11.4.1. Addition of Hydrogen Halides

As with halogen substitution, thermochemical relationships impose limits on
free radical chain addition reactions of the hydrogen halides. These relationships are
summarized in Figure 11.13. There are significant endothermic steps for HF and HI
and a slightly endothermic step for HCl. Radical chain additions of hydrogen fluoride
and hydrogen iodide to alkenes are not observed. In the case of hydrogen iodide, the
addition of an iodine atom to an alkene is an endothermic process and is too slow
to permit a chain reaction, even though the hydrogen abstraction step is favorable. In
the case of hydrogen fluoride, the abstraction of hydrogen from hydrogen fluoride is
energetically prohibitive. Only in the case of HBr is the thermochemistry consistent
with a radical chain process. The HBr addition has one thermal neutral step and an

X· H – X
RCH=CH 2 RCHCH X RCH CH X + X·
.
2
2
2
+36 F
+14.2 I
+ 5 Cl
0 0 Br

–11 Br
–14.2 Cl

–15.2 I

–43.1 F
Fig. 11.13. Thermochemistry for the steps in radical chain
additions of the hydrogen halides to alkenes. The C=C and
C-H bond energies were taken as 70 and 98 kcal/mol, respec-
tively, and primary C-X bond energies were used.