FIGURE 13.21 Summary of major reaction paths in the tropospheric chemistry of HFC-134a.


With no abstractable hydrogen atoms on the alkoxy radical, reaction with 02 cannot occur and it decomposes by scission of the C-C bond (Edney and Driscoll, 1992; Tuazon and Atkinson, 1993b; Sehested et al., 1993):

The CF3 radical then reacts as described earlier, forming COF2, FNO, and CF3OH.

Figure 13.22 summarizes the chemistry of HCFC-125.

c. Chemistry of HCFC-123 (CHCl2CF3)

The initial OH chemistry of HCFC-123 (CHC12CF3) produces a fully halogenated alkoxy radical where further hydrogen abstraction by 02 is not an option:

CF3CCI2 + 02 ™ CF3CC1200, (18) CF3CC1200 + NO -> CF3CC120 + N02. (19)

However, recall that the C-Cl bond is much weaker than a C-F bond. As a result, one additional potential route of decomposition in this case is elimination of a chlorine atom:

In fact, this predominates over the competing C-C bond scission,

with yields of CF3C(0)C1 approaching 100% (Edney et al., 1991; Tuazon and Atkinson, 1993b; Hayman et al.,

1994). Similar chlorine atom elimination has been observed for other chlorine-containing alkoxy radicals (e.g., see Wu and Carr, 1992; and Bhatnagar and Carr,

1995). The CF3C(0)C1 hydrolyzes in clouds and fogs to form CF3COOH, trifluoroacetic acid.

It should be noted that, although it is not important in this particular case, there is an additional path available for some alkoxy radicals formed in the oxidation of certain halogenated organics, that is, the intramolecular elimination of HC1 from a-monochloro-alkoxy radicals. For example, the alkoxy radical CH2C1CHC10 both reacts with 02 and eliminates HC1, cf3chf2 + oh o2

FIGURE 13.22 Summary of major tropospheric reactions of HCFC-125.

cf3cf2oo + h2o

FIGURE 13.22 Summary of major tropospheric reactions of HCFC-125.

with the latter favored at 1 atm in air (Wallington et al„ 1996b).

Figure 13.23 summarizes the chemistry of HCFC-123.

d. Chemistry of HCFC- 141b (CH3CFCl2)

Hydrogen atom abstraction from HCFC-141b (CH3CFC12) produces the alkoxy radical OCH2CFCl2:

CH2CFC12 + 02 ^ OOCH2CFCl2, (23) OOCH2CFCl2 + NO ^ N02 + OCH2CFCl2. (24) In this case, reaction of the alkoxy radical with 02, OCH2CFCl2 + 02 -» H02 + HC(0)CFC12, (25)

cf3chci2 + oh

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