Zinc dimethyl, decomposition

On thermal decomposition of alstonine at 300-330°, the bases produced distilled at l20-170°/0-15 mm., and on fractionation as picrates gave three products Base D, C4,HigN2, picrate, m.p. 254-6°. Base E, C18H20N2, or CjgHgaNj, picrate m.p. 193-5-195°, not identical with Sharp s alstyrine, and base F, Cj3Hi2N2, m.p. 79-81°, pierate, m.p. 261-262-5°, hydrochloride, m.p. about 275° (dec.), methiodide, m.p. 283-4° (dec.). Base F has an ultraviolet absorption spectrum very similar to that of 2-ethyl- -carboline, but it is not that substance nor is it 1 2-dimethyl-/3-carboline, 2 3-dimethyl-j3-carboline, 1-ethyl- -carboline or 3-ethyl- -wocarboline. Base F was also produced when alstonine was distilled with zinc dust. 

The pyrolysis of dimethyl zinc has been studied in a toluene carrier flow system46, in sealed ampoules using both Zn(CH3)2 and Zn(CD3)247 and in an argon carrier system coupled to a mass spectrometer48. The decomposition in the toluene carrier system may be discussed in terms of the mechanism... 

Reaction (5) was not included in the mechanism originally suggested by Price and Trotman-Dickenson46. A more recent study of the thermal decomposition of dimethyl zinc in a benzene carrier system54 gave C6H5ZnCH3 as a major product. The activation energy of reaction (10) has been shown to be approx-... 

This reaction was therefore negligible when C6D12/Zn(CH3)2 = 13. The rates of reactions (1) and (14) are strongly dependent on the nature of the surface. In an ampoule coated with tar from decomposition of a large quantity of dimethyl zinc, the rate of both reactions is only J-i the rate observed in unconditioned vessels (tested at 348 °C with fraction Zn(CH3)2 reacted in conditioned vessel = 0.035). It has also been shown that in 90 min at 290 °C, the overall decomposition of dimethyl zinc in the absence of cyclohexane is 94 % complete if a zinc oxide surface is used, but only 4.5 % complete in a conditioned vessel. Decompositions carried out in conditioned vessels were assumed to be homogeneous. 

It is therefore possible that the initial fate of ZnCH3 in the static system work is dimerization. Under the conditions used diffusion to the surface can compete successfully with reaction (2) so formation of an intermediate dimer could be either a homogeneous or a heterogeneous process. By elimination, the two hydrogen atoms required to convert the dimer to 2 Zn+2 CH4 must come from dimethyl zinc itself and must leave a product that does not undergo subsequent decomposition. It is possible that this occurs via a cyclic intermediate, with adsorbed dimethyl zinc leaving surface-adsorbed radicals which may undergo polymerization, viz. 

If all the hydrogens in the adsorbed dimethyl zinc are abstracted, then Lambert s value for kt is increased by a neglible amount at all % decompositions used, and the combined calculation with the estimated infinite pressure data of Price and Trotman-Dickenson gives... 

Fig. 2. Arrhenius plots for the decomposition of dimethyl zinc. 1, Price and Trotman-Dickenson solid line, 16 torr toluene dotted line, 5 torr toluene 2, Lambert, approx. 760 torr (cyclohexane +7 % alkyl) 3, composite curve obtained from rate coefficients of Price and Trotman-Dickenson corrected to infinite pressure and from Lambert s values. The composite curve has been displaced upwards by 2.0 log units. Open circles Topor, 22-24 torr (argon+2.4 torr dimethyl zinc). 

Methanol can be converted to hydrocarbons over acidic catalysts. However, with the exception of some zeolites, most catalysts deactivate rapidly. The first observation of hydrocarbon formation from methanol in molten ZnCl2 was reported in 1880, when decomposition of methanol was described to yield hexamethylbenzene and methane.414 Significant amounts of light hydrocarbons, mostly isobutane, were formed when methanol or dimethyl ether reacted over ZnCl2 under superatmo-spheric pressure.415 More recently, bulk zinc bromide and zinc iodide were found to convert methanol to gasoline range (C4-C13) fraction (mainly 2,2,3-trimethyl-butane) at 200°C with excellent yield (>99%).416... 

Dimethylbiphenyl can be prepared from w-bromotoluene8 or m-iodotoluene9 and sodium by the action of copper powder on w-iodotoluene 10 by treating 4,4 -dihydroxy-3,3/-dimethyl-biphenyl with zinc dust 11 by heating 4,4,-dichloro-3,3,-dimethyl-biphenyl with hydriodic acid and phosphorus 11 by treating o-tolidine with nitrous acid in alcoholic solution 12 by the decomposition of tetrazotized o-tolidine with methanol or ethanol... 

A study in Japan looked at the migration of dimethylamine (DMA) into water and hydrochloric acid from 25 rubber articles (including stoppers, chopping boards, spatulas and teats). After one hour of refluxing, the water extracts contained 3 to 1280 mg of DMA per kg of rubber. The study also showed that the thiuram accelerators that were present (TMTD and TMTM) were almost totally decomposed to DMA (a nitrosatable substance). However, in the case of dimethyl dithiocarbamate salts (sodium, zinc, copper and lead examples were included), the decomposition to DMA depended on the solvent used and the salt compound. 

Mixtures of 10-15 percent with air may be ignited with difficulty. Moderately explosive when exposed to sparks or flame. Forms explosive mixtures with air within narrow limits at atmospheric pressure, with wider limits at higher pressure. The explosive sensidvity of mixtures with air may be increased by the presence of aluminum, magnesium, zinc, or their alloys. Incompatible with metals, dimethyl sulfoxide, ethylene oxide. To fight fire, use foam, water, CO2, dry chemical. When heated to decomposition it emits toxic fumes of Br". See also BROMIDES. 

SAFETY PROFILE Poison by inhalation and ingestion. A corrosive eye, skin, and mucous membrane irritant. Potentially explosive reaction with water evolves hydrogen chloride and phosphine, which then ignites. Explosive reaction with 2,6-dimethylpyridine N-oxide, dimethyl sulfoxide, ferrocene-1,1 -dicarboxylic acid, pyridine N-oxide (above 60°C), sodium -L heat. Violent reaction or ignition with BI3, carbon disulfide, 2,5-dimethyl pyrrole + dimethyl formamide, organic matter, zinc powder. Reacts with water or steam to produce heat and toxic and corrosive fumes. Incompatible with carbon disulfide, N,N-dimethyl-formamide, 2,5-dimethylpyrrole, 2,6-dimethylpyridine N-oxide, dimethylsulfoxide, ferrocene-1,1-dicarboxylic acid, water, zinc. When heated to decomposition it emits highly toxic fumes of Cl" and POx. 

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