The decomposition of ethyl bromide

The thermostat is an iron box about thirty centimeters square filled with molten lead. An iron blade keeps the lead stirred and a platinum resistance thermometer functions both as a thermometer and as a thermoregulator. The Wheatstone bridge, which contains the resistance thermometer as one arm, is adjusted for a given temperature, and the beam of light from the galvanometer strikes a zero reading on the scale. At this position is introduced a photoelectric cell connected with a radio tube and amplifier, which in turn actuates an electro-magnetic relay. When the temperature reaches the predetermined value, the beam of 

The diaphragm changes its zero point with temperature, but it is calibrated at the high temperature. A new diaphragm behaves erratically, and must be seasoned for an hour or two before making actual determinations. Tests were made with air to determine the extent of any time-lag. When the flask is plunged into the lead bath from room temperature the diaphragm settles down to a uniform value in about a minute and a half and thermal equilibrium with the gas is reached in about five minutes. 

Analysis of the products showed that ethylene and hydrobromic acid are practically the only gases remaining after the reaction is completed. The flask was opened under a starch iodide solution, but no test for free bromine could be obtained. In none of the work did the brown color of bromine develop even at high tem- 

It is evident that two molecules are produced from one and that the reaction can be followed quantitatively by the increase in pressure. There is a complication, however, in that hydrobromic acid would be expected to add to ethylene and reverse the reaction. 

The method of calculating the specific reaction rates k from these pressure-time curves calls for some consideration. If there were no reverse reaction and no complications the final pressure should be just twice the initial pressure. Actually it is always less and the difference between twice the initial pressure, 2pif and the final pressure, pj, gives a measure of the extent of the reverse reaction, provided that the reaction mixture has not stayed in the reaction chamber long enough (several hours) for the polymerization of ethylene and the formation of a film. 

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Under appropriate conditions the decomposition of ethyl bromide may be explained by the following mechanism. 

The two parallel thermal decompositions of bromopropane-1,1,2,2-d4, together with the decomposition of ethyl bromide, have been investigated by using the very low-pressure pyrolysis (VLPP) method107 (equations 20 and 21). 

Fig. 17.—Pressure-time curves for the decomposition of ethyl bromide.

Different Methods of Calculating k for the Decomposition of Ethyl Bromide ifeXlO4 sec-1 at 305°... 

This extended discussion of the decomposition of ethyl bromide has shown the difficulty of definitely assigning a single mechanism to a reaction. Perhaps still further experiments will enable... 

In studying the connections between chemical kinetics and infrared vibration frequencies the ethyl halides were chosen because the kinetics of the decomposition of ethyl bromide had been the object of such an extended investigation.7 In attempting to... 

The decomposition of ethyl bromide in the gas phase is a first-order reaction. The data are ... 

Lin et al. (162, 193) also obtained a similar spectrum by decomposition of ethyl bromide on Cu(100), but with poorer resolution in the 900-cur1 region. This spectrum does, however, show a much lower wavenumber vCCu mode at 370 cm-1 than that for vCPt at 484 cm 1. This difference between the two metals is similar to that found from the spectra of the adsorbed methyl species. 

Details of the preparation of methyl (-)-cis-chrysanthemate from (+)-car-3-ene have appeared (Vol. 5, p. 15 is misleading).159 Both ( )-cis- and (+)-trans-chrysanthemic acids are again reported from (+)-car-3-ene via ozonolysis 160 this work is very similar to that reported (Vol. 5, p. 15) by Sukh Dev and illustrates the lamentable delay from receipt to publication in some journals. The decomposition of ethyl diazoacetate in 2,5-dimethylhexa-2,4-diene in the presence of the chiral copper complex (72) leads to cis- and frvms-chrysanthemic acids in 60—70% optical yield the degree of asymmetric induction is dependent upon the steric bulk of R1 and R2 in (72).161 cis-Chrysanthemic acid has also been prepared from 3,3-dimethylcyclopropene, isoprene, and 2-methylpropenylmagnesium bromide followed by treatment with carbon dioxide.162... 

This should be a reasonably facile step, and the replacement of an j-radical by a bromine atom in this way may account in part for the catalytic action of HBr in these systems. Incorporating these steps, Maccoll has proposed the following as a possible chain mechanism of the thermal decomposition of ethyl bromide... 

The sequence of steps of the catalyzed decomposition of ethyl bromide is written as... 

Other hydrides used for the conversion of esters to alcohols are magnesium aluminum hydride in tetrahydrofuran [89, 577] and magnesium bromohydride prepared by decomposition of ethylmagnesium bromide at 235° for 2.5 hours at 0.5mm [7055]. They do not offer special advantages (the latter giving only 35% yield of benzyl alcohols from ethyl benzoate). 

Under suitable conditions, certain chemical reactions will give rise to nacreous sulphur the most satisfactory result is obtained by allowing slow inter-diffusion of solutions of sodium thiosulphate and potassium hydrogen sulphate to occur.7 Another method involves the gradual decomposition of sulphur chloride or bromide by the vapour of water or methyl alcohol at the ordinary temperature.8 The decomposition of calcium polysulphidcs by hydrochloric acid,9 and of hydrogen persulphide by the addition of alcohol, ether, ethyl acetate or other organic solvents, also yields sulphur of the desired modification. 

Fig. 19.—Effect of decreased pressure on the specific decomposition rate of ethyl bromide.

Again the decomposition of the ethyl bromide might take place by the rupture into free radicals and the recombination to give the final products ethylene and hydrobromic acid... 

A given alcohol is treated with 25 percent excess of aqueous (48 per cent) hydrobromic acid together with sulfuric acid. The mixture is refluxed in order to convert the alcohol as completely as possible into the corresponding bromide, and the latter is then removed from the reaction mixture by distillation. Slight variations from this procedure depend upon the physical and chemical properties of the alcohol used, or of the bromide formed in the reaction. For example, in the preparations of ethyl and allyl bromides, the reaction mixture is not refluxed because of the volatility of the former compound, and because of the chemical reactivity of the latter in the preparation of wo amyl bromide, too large a proportion of sulfuric acid may produce appreciable decomposition while halides of high molecular weight, because of their low volatility, are separated from the reaction mixture mechanically, instead of by distillation. 

Apart from the molecular and the radical-chain mechanisms for the decomposition of halogenated hydrocarbons, the study of allyl bromide pyrolysis " revealed a third possibility, namely a radical non-chain mechanism, a type originally proposed by Daniels and Veltman for ethyl bromide pyrolysis. Its application to allyl bromide pyrolysis is shown in the following scheme as proposed by Maccoll ... 

The decomposition of acetaldehyde, sensitized by biacetyl, was studied at 499 °C by Rice and Walters , and between 410 and 490 °C by Boyer et al. They found the initial rate to be proportional to the square root of the biacetyl concentration and to the first power of the aldehyde concentration. The chains are initiated by the radicals originating from the decomposition of the biacetyl molecule. The decomposition of acetaldehyde can be induced also by di-r-butyl peroxide (at 150-210 °C, about 10-50 molecules decompose per peroxide molecule added), as well as by ethylene oxide (around 450 °C each added ethylene oxide molecule brings about the decomposition of up to 300 acetaldehyde molecules). For the influence of added diethylether, vinyl ethyl ether, ethyl bromide, and ethyl iodide etc., see Steacie °. 

Ethylphenyl Carbinol. Employ the same procedure as in section (A). Use ethyl bromide 4.4 g magnesium 1.0 g ether 10 ml and benzaldehyde 4.25 ml. The ether extract, after the decomposition of the addition compound, is shaken with 5 ml of saturated sodium bisulfite solution to remove the unreacted benzaldehyde, with which it forms a crystalline compound. The upper layer is... 

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