CH3NHCHO arises from the reaction of the OH generated in the system with the parent amine to form the CH2NHCH3 alkyl radical, followed by reaction (86). A/,A/,iV',iV'-Tetramethyldiaminomethane, (CH3)2NCH2N(CH3)2, seen in Fig. 6.20C, was shown to be formed in the reaction of dimethylamine with the product HCHO:
Further reaction of this product with 03 was shown to give (CH3)2NCHO, observed as a product in Fig. 6.20A.
It should be noted, however, that in studies of amine photooxidations, it is generally true that a significant fraction of the reacted parent amine remains unaccounted for in the identified products. Clearly, the mechanisms and products are complex and warrant further investigation.
One difficulty in studying the photooxidation of amines is the rapid reaction in the dark with nitrous acid to form nitrosamines (Hanst et al., 1977; Pitts et al., 1978):
As discussed in Chapter 7.B.3, NOz undergoes a surface reaction with water, which is perhaps enhanced at the air-water interface, forming HONO:
Thus, in the course of preparing reactant mixtures for photooxidation studies under typical atmospheric conditions where both N02 and water vapor are present, it is essentially impossible to avoid the production of some HONO, and in the case of studies of amine reactions, some nitrosamines. However, this too is quite relevant, since nitrosamines are carcinogenic in experimental animals. In addition, there are a number of sources that emit nitrosamines directly into the air, including leather tanneries, rocket fuels, tire and amine factories, and tobacco smoke (e.g., see Fine, f980).
The major atmospheric fate of the Af-nitrosamines such as iV-nitrosodimethylamine is photolysis (Tuazon et al., 1984):
The dimethylamino radical then reacts as described earlier.
Hydrazines see widespread use as fuels, for example, in the space shuttle and as a source of emergency power in the F-f6 fighter plane. As a result of the industrial and fuel uses of hydrazines, with their accompanying transport and storage, some emissions to the atmosphere occur and hence there is interest in their atmospheric reactions.
Hydrazines do not photolyze in the actinic UV, but reactions with OH and 03 must be considered. The rate constants for reaction of OH with N2H4 and CH3NHNH2 are (6.1 ± 1.0) X 10"" and (6.5 ± 1.3) X 10"" cm3 molecule"1 s"1, respectively, essentially independent of temperature over the range 298-424 K (Harris et al., 1979). At an OH concentration of f X 106 cm"3, the lifetimes of both N2H4 and CH3NHNH2 will be ~ 4-5 h. Harris and co-workers (1979) estimate that the rate constant for the reaction of OH with 1,1-dimethylhydrazine is ~(5 + 2) X 10"" cm3 molecule"1 s"1, so that its lifetime with respect to OH will be similar, ~ 6 h.
Reaction with 03 is also relatively fast. Tuazon et al. (1981) estimate that the rate constant for the N2H4-03 reaction at 294-297 K is ~1 X 10"16 cm3 molecule"1 s"1. This corresponds to a lifetime of about f h at an 03 concentration of O.f ppm. The rate constants for the 03-CH3NHNH2 and 03-(CH3)2NNH2 reactions were too fast to measure under their experimental conditions; the reactions of ~ 1-3 ppm 03 with 0.4-4 ppm CH3NHNH2 and ~2ppm03with ~ 0.2-2 ppm (CH3)2NNH2 were complete in less than 2-3 min.
From these data, the rate constants must be >10"15 cm3 molecule"1 s-1, and the lifetimes of these two hydrazines must be less than 7 min at 0.1 ppm 03.
The mechanism of the reaction of hydrazines with 03 has been investigated using FTIR (Tuazon et al., 1981; Carter et al., 1981b). In the case of N2H4, the major product was H202, and NzO appeared as a minor product; these are consistent with the following mechanism:
H2NNH2 + 03 -> H2N —NH + OH + 02 (92) Propagation
H2NNH2 + OH ^ H2N —NH + H20 (95) Product Formation
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