Halogenated hydrocarbons hydrolysis

P.M. Jeffers and N.L. Wolfe, Hydrolysis of methyl bromide, ethyl bromide, chloropicrin, 1,4-dichloro-2-butene, and other halogenated hydrocarbons, in Fumigants Environmental Fate, Exposure, and Analysis, ed. J.N. Seiber, J.A. Knuteson, J.E. Woodrow, N.L. Wolfe, M.V. Yates, and S.R. Yates, ACS Symposium Series No. 652, American Chemical Society, Washington, DC, pp. 32-41 (1997). 

DBCP. The predictions suggest that DBCP is volatile and diffuses rapidly into the atmosphere and that it is also readily leached into the soil profile. In the model soil, its volatilization half-life was only 1.2 days when it was assumed to be evenly distributed into the top 10 cm of soil. However, DBCP could be leached as much as 50 cm deep by only 25 cm of water, and at this depth diffusion to the surface would be slow. From the literature study of transformation processes, we found no clear evidence for rapid oxidation or hydrolysis. Photolysis would not occur below the soil surface. No useable data for estimating biodegradation rates were found although Castro and Belser (28) showed that biodegradation did occur. The rate was assumed to be slow because all halogenated hydrocarbons degrade slowly. DBCP was therefore assumed to be persistent. 

Let us now look at some examples to illustrate what we have discussed so far to get a feeling of how structural moieties influence the mechanisms, and to see some rates of nucleophilic substitution reactions of halogenated hydrocarbons in the environment. Table 13.6 summarizes the (neutral) hydrolysis half-lives of various mono-halogenated compounds at 25°C. We can see that, as anticipated, for a given type of compound, the carbon-bromine and carbon-iodine bonds hydrolyze fastest, about 1-2 orders of magnitude faster than the carbon-chlorine bond. Furthermore, we note that for the compounds of interest to us, SN1 or SN2 hydrolysis of carbon-fluorine bonds is likely to be too slow to be of great environmental significance. 

New experimental data on the hydrolysis of dilute solutions of carbon tetrachloride indicate clearly that the reaction is first order in CCI4.157 This is contrary to die classical work of Fells and Moelwyn-Hughes, whose data have been reanalysed and shown to be consistent with first-order kinetics. The same research group has carried out extensive work on the kinetics of hydrolysis (neutral or alkaline) of many halogenated hydrocarbons in relation to studies in environmental and toxicological chemistry.158... 

The apparent rate of hydrolysis and the relative abundance of reaction products is often a function of pH because alternative reaction pathways are preferred at different pH. In the case of halogenated hydrocarbons, base-catalyzed hydrolysis will result in elimination reactions while neutral hydrolysis will take place via nucleophilic displacement reactions. An example of the pH dependence of hydrolysis is illustrated by the base-catalyzed hydrolysis of the structurally similar insecticides DDT and methoxy-chlor. Under a common range of natural pH (5 to 8) the hydrolysis rate of methoxychlor is invariant while the hydrolysis of DDT is about 15-fold faster at pH 8 compared to pH 5. Only at higher pH (>8) does the hydrolysis rate of methoxychlor increase. In addition the major product of DDT hydrolysis throughout this pH range is the same (DDE), while the methoxychlor hydrolysis product shifts from the alcohol at pH 5-8 (nucleophilic substitution) to the dehydrochlorinated DMDE at pH > 8 (elimination). This illustrates the necessity to conduct detailed mechanistic experiments as a function of pH for hydrolytic reactions. 

Colorless gas. Pungent, suffocating odor. Corrosive to skin. Avoid inhalation Forms dense white fumes in moist air, bp -127.1". bp -100.4°. d4 (-100.4 liq) 1,57. d(gas at STP) 3.07666 gil. Soly in water (0 ) 332 g/100 g some hydrolysis occurs to form fluoboric and boric acids. Soly in anhydrous H2SO, 1.94 g/100 g acid. Forms solid complex with nitric acid (HNOj.2BFj), Sol in most saturated and halogenated hydrocarbons and in aromatic compds. Polymerizes unsaturated molecules. Easily forms coordination complexes with molecules having at Least one unshared pair of electrons. Reacts with incandescence when heated with alkali metals or alkaline earth metals except magnesium. 

Halogenated hydrocarbons may be destroyed by incineration, hydrolysis with caustic alkalies, and biodegradation. Ultrahigh destruction of volatile organic compounds to sub-ppt level in fumes or air stream has been reported using catalytically stabilized thermal incineration process (Pfefferle 1992). Methylene... 

Classes of organic pollutants that hydrolyze via nucleophilic substitution reactions include the halogenated hydrocarbons, epoxides, and phosphorus esters. Further discussion of the factors affecting the reactivity of nucleophilic substitution reactions will be made as the hydrolysis mechanisms of these chemicals are examined in greater detail. 

In nonpolar aprotic solvents, such as the aliphatic and aromatic hydrocarbons and symmetrical halogenated hydrocarbons, or in polar solvents, such as aldehydes, ethers, and asymmetric halogenated hydrocarbons, the cathodic process can be the reduction of oxygen, which is quite soluble in hydrocarbons, or the reduction of protons allowed by the presence of halogen acid resulting from the hydrolysis of halogenated hydrocarbons. The polar behavior of the solvent favors the solvating effect of metal ions, the solubilization of the corrosion products of the corrosion process. 

Conductivity is extremely sensitive to the presence of ions. Hence, conductivity measurements are particularly suitable for the indication of all decomposition processes (hydrolysis, oxidation) accompanied by the liberation of hydrogen ions. If the high mobility of the hydrogen ion is also taken into account, the special sensitivity of this method in these processes is readily understood. In addition, it may be utilized to follow the production of other ionic formations, e.g., to indicate the hydrolytic decomposition of halogenated hydrocarbons. 

The reaction mechanism described in Scheme 17.2 depends on the atomic H/Cl ratio of the tested molecules. Thus, totally chlorinated hydrocarbons such as carbon tetrachloride or hexachloroethane are essentially destroyed by hydrolysis in the presence of water, and the underchlorinated hydrocarbons undergo competitive hydrolysis and oxidation reactions [52]. Following the same mechanism, less halogenated hydrocarbons may also lead to alkenes (Scheme 17.3). 

ETHYLENE DICHLORIDE. CH CI CHjCI. Limited laboratory tests indicate that 3003 alloy was resistant to dry ethylene dichloride vapor at the boiling point. The presence of water causes increased corrosion because of hydrochloric acid formed by hydrolysis. CAUTION-. See "Halogenated Hydrocarbons." See also Ref (1) p. 132, (2) p. 268. 

Halogenated hydrocarbons may decompose by hydrolysis if water is present or by other processes to yield mineral acids such as hydrochloric acid. These acids corrode aluminum alloys because they destix the protective surface oxide film naturally present that provides inherent resistance to corrodon. Corrosion of aluminum alloys by these acids may also promote reactions of the hydrocarbons themselves because aluminum halides formed by corrosion are catalysts for some of these reactions (e.g. AlClj for a Friedel-Crafts reaction). In some instances, aluminum alkyls may be produced. Because of the rapid rate of evolution of heat, corrosion of aluminum and reaction of a halogenated hydrocarbon, once initiated, may tend to become autocacalytic. 

Phosphorus—Carbon Bond. The P—C bond is 0.184—0.194-nm long and has an energy of ca 272 kj/mol (65 kcal/mol). It is one of the more stable bonds formed by phosphoms, resistant to both hydrolysis and oxidation (7,8). Unlike the phosphoms—halogen or phosphoms—oxygen bonds, the P—C linkage is inert to exchange. A phosphoms atom connected to carbon behaves similarly to another carbon atom in a hydrocarbon chain. 

Hydrolysis of acetals yields aldehydes, which are intermediates in the biochemical /3-oxidation of hydrocarbon chains. Acid catalyzed hydrolysis of unsubstituted acetals is generally facile and occurs at a reasonable rate at pH 4-5 at room temperature. Electron-withdrawing substituents, such as hydroxyl, ether oxygen, and halogens, reduce the hydrolysis rate, however [50]. Anionic acetal surfactants are more labile than cationic [40], a fact that can be ascribed to the locally high oxonium ion activity around such micelles. The same effect can also be seen for surfactants forming vesicular aggregates. 

Some preliminary examples of hydrolysis reactions illustrate the very wide range of reactivity of organic compounds. For example, triesters of phosphoric acid hydrolyze in nearneutral solution at ambient temperatures with half-lives ranging from several days to several years (Wolfe, 1980), whereas the halogenated alkanes pentachloroethane, carbon tetrachloride, and hexachloroethane have "environmental" (pH = 7 25°C) half-lives of about 2 hr, 50 yr, and 1000 millennia, respectively (Mabey and Mill, 1978 Jeffers et al., 1989). On the other hand, pure hydrocarbons from methane through the PAHs are not hydrolyzed under any circumstances that are environmentally relevant. 

The direct oxidation of the hydrocarbon to alcohol is difiicult because the oxidation proceeds to the acid stage, thus giving rise to mixtures. In only a very few cases is it possible to develop a practical method. Aside from the hydrolysis of the halogen compoxmds, the reduction of aldehydes or acids can, according to the above diagram, be used for the preparation of alcohols. Reductions of this type are possible under certain conditions for the.se the student should refer to the text. 

The members of two classes of common pollutant chemicals are likely to undergo hydrolysis. One class includes the alkyl halides, which are straight-chain or branched hydrocarbons in which one or more hydrogen atoms have been replaced by a chlorine, fluorine, bromine, or iodine atom. Using X to represent a halogen atom and R to represent the hydrocarbon group, the overall hydrolysis reaction can be written... 

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