Methylchloroformate

In 1976 the United States banned the use of CFCs as aerosol propellants. No further steps were taken until 1987 when the United States and some 50 other countries adopted the Montreal Protocol, specifing a 50% reduction of fully halogenated CFCs by 1999. In 1990, an agreement was reached among 93 nations to accelerate the discontinuation of CFCs and completely eliminate production by the year 2000. The 1990 Clean Air Act Amendments contain a phaseout schedule for CFCs, halons, carbon tetrachloride, and methylchloroform. Such steps should stop the iacrease of CFCs ia the atmosphere but, because of the long lifetimes, CFCs will remain ia the atmosphere for centuries. 

Confirmation of the destmetion of ozone by chlorine and bromine from halofluorocarbons has led to international efforts to reduce emissions of ozone-destroying CPCs and Halons into the atmosphere. The 1987 Montreal Protocol on Substances That Deplete the Ozone Layer (150) (and its 1990 and 1992 revisions) calls for an end to the production of Halons in 1994 and CPCs, carbon tetrachloride, and methylchloroform byjanuary 1, 1996. In 1993, worldwide production of CPCs was reduced to 50% of 1986 levels of 1.13 x 10 and decreases in growth rates of CPC-11 and CPC-12 have been observed (151). 

A mixture of 25 ml of 50% sodium hydroxide in 5 ml of water is added to a mixture of 15 g of cyanamide, 2.2 g of methylchloroformate, 3.5 ml of water and 3 ml of acetone over 45 minutes below 10°C, pH raised to 6.5. A moiar equivaient soiution of the diamine in 100 ml of ethanol isadded. The mixture is heated until the easily volatile solventsare expelled, to about 85°C, then maintained at this temperature with some water added for one-half hour. The product, methyl 5-propylthio-2-benzimidazolecarbamate, is separated, washed to give a colorless crystalline solid, MP 208°-210°C. 

Schwetz BA, Leong KJ, Gehring PJ. 1975. The effect of maternally inhaled trichloroethylene, perchloroethylene, methylchloroform, and methylene chloride on embryonal and fetal development in mice and rats. Toxicol Appl Pharmacol 32 84-96. 

Krol, M., P. J. van Leeuwen, and J. Lelieveld, Global OH Trend Inferred from Methylchloroform Measurements, . /. Geophys. Res., 103, f0697-I07ff (f998). 

While there are a variety of other chlorinated organics such as methylchloroform (CH3CC13) that are emitted, these have relatively short tropospheric lifetimes because they have an abstractable hydrogen atom (e.g., see WMO, 1995). For example, while the stratospheric lifetime of methylchloroform is estimated to be 34 7 years (Volk et al., 1997), its overall atmospheric lifetime is only 5-6 years, primarily due to the removal by OH in the troposphere (toii 6.6 years), with a much smaller contribution from uptake by the ocean (roi i an 85 years) (WMO, 1995). 

However, about this time, a variety of research indicated that even with full implementation of the Montreal Protocol, the atmospheric abundance of chlorine could reach as much as 6-9 ppb between the years 2050 and 2075. This delay is due to the relatively long time between emission of these compounds into the troposphere and when they reach the stratosphere and photolyze to produce an active chlorine atom. Figure 13.1, for example, compares the estimated equivalent effective stratospheric chlorine from 1960 until the year 2100 with no controls and a 3% increase per year in CFC and methylchloroform emissions to those with the controls agreed to in the Montreal Protocol. Equivalent effective stratospheric chlorine loading depends on emissions as well as removal processes, which determine what fraction of the CFCs emitted at the earth s... 

Despite the growth of HCFCs, the National Oceanic Atmospheric Administration (NOAA) researchers data show that ozone-depleting chemicals in the atmosphere have declined overall about 3% since 1994. But that decrease is due almost entirely to a dramatic drop in the amount of methylchloroform, which was used as a cleaning solvent. Natural oxidation processes in the atmosphere remove methylchloroform so efficiently that its atmospheric concentration has been falling rapidly since production stopped. 

Protection of the secondary alcohol as the corresponding methoxy methyl (MOM) ether, followed by removal of the Boc group with ZnBr2 in dichloromethane and acylation of the incipient secondary amine with bromoacetyl bromide in the presence of K2CO3 afforded the bromoacetamide 114 in 86% yield from 113. Treatment of 114 with methanolic ammonia afforded the corresponding glycinamide which was directly subjected to cyclization in the presence of NaH in toluene/HMPA to afford the bicyclic compound 115 in 79% overall yield from 114. Next, a one-pot double carbomethoxylation reaction was performed by the sequencial addition of n-BuLi in THF followed by addition of methylchloroformate, that carbomethoxylated the amide nitrogen atom. Subsequent addition of four equiv of methyl chloroformate followed by the addition of 5 equiv of LiN(TMS)2 afforded 116 as a mixture of diastereomers in 93% yield that were taken on directly without separation. 

Astrand, I., Kilbom, A., Wahlberg, I. Ovrum, R (1973) Methylchloroform exposure I. Concentration in alveolar air and blood at rest and during exercise. Work Environ. Health, 10, 69-81... 

Carlson, GP. (1981) Effect of alterations in drug metabolism on epinephrine-induced cardiac arrhythmias in rabbits exposed to methylchloroform. Toxicol. Lett., 9, 307-313... 

Eben, A. Kimmerle, G. (1974) Metabolism, excretion and toxicology of methylchloroform in acute and subacute exposed rats. Arch. Toxicol., 31. 233-242... 

Reitz, R.H., McDougal. J.N.. Himmelstein, M.W., Nolan, R.J. Schumann, A.M. (1988) Physiologically based phannacokinetic modeling with methylchloroform implications for interspecies, high dose/low dose, and dose route extrapolations. Toxicol, appl. Pharmacol., 95, 185-199... 

Another prominent example of a gas-phase-controlled exchange flux is the evaporation of pure organic liquids. To illustrate this case, we again use the example of methylchloroform (MCF). In the following the subscript i stands for MCF. 

Another modification of the process can be used to meet the growing demand for 1,1,1-trichloroethane (methylchloroform). In this version, the chlorination of dichloroethane can be directed toward maximum production of 1,1,2-trichloroethane (9). This product when dehydrochlorinated yields vinylidene chloride, a widely used monomer. Hydrochlorination of vinylidene chloride yields 1,1,1-trichloroethane, a solvent of increasing importance. 

Prinn, R., Cunnold, D., Rasmussen, R., Simmonds, R, Alyea, F., Crawford, A., Fraser, P., Rosen, R. (1987) Atmospheric trends in methylchloroform and the global average for the hydroxyl radical. Science 238, 945-950. 

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