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1. The mercuric oxide used was Mallinckrodt or Baker powdered red mercuric oxide, analytical reagent grade. Old mercuric oxide gives variable results and may lower the yield. The 1,1,2,2-tetrachloroethane used was a technical grade and was distilled to make sure no low-boiling impurities were present. Rea-gent-grade solvent has been used without distillation. The vapors of this chlorinated hydrocarbon are toxic, and its distillation as well as the reaction should be carried out in a hood. Suitable cyclopropanecarboxylic acid2 is obtainable from Aldrich Chemical Company.
2. The reaction starts spontaneously and is mildly exothermic. Moderating the temperature by use of a water bath diminishes the amount of bromine and product carried off by the carbon dioxide evolved. The reaction can be followed by use of a tetra-cbloroethane bubbler, and at the end of the reaction the solvent in the bubbler can be used to wash the mercuric bromide. The checkers followed the reaction with a wet test meter presaturated with carbon dioxide; 52-60% of the theoretical amount of carbon dioxide was evolved.
3. The checkers used a sintered glass pressure filter (Corning Glass Works, Cat. No. 34020) rather than a suction filter in order -to minimize evaporation losses. An ordinary water aspirator can cause the mixture to boil at room temperature. The flask and filter can be cleaned readily with a little acetone, which dissolves mercuric bromide rapidly.
4. Once the boiling point starts to rise, it goes up quite rapidly. The fractions collected between 75° and 90° contain a little product and can be reworked if a second distillation is carried out.
3. Methods of Preparation
Bromocyclopropane has been prepared by the Hunsdiecker reaction by adding silver cyclopropanecarboxylate to bromine in dichlorodifluoromethane at —29° (53% yield) or in tetrachloro-ethane at —20° to —25° (15-20% yield).3 Decomposition of the peroxide of cyclopropanecarboxylic acid in the presence of carbon tetrabromide gave bromocyclopropane in 43% yield.4 An attempt to prepare the bromide via the von Braun reaction was unsuccessful.3
4. Merits of the Preparation
The present procedure is substantially simpler and quicker than the best previous procedure,3 which requires 4 days instead of 4 hours. It is also safer, for no explosions have been encountered with the present procedure, even on a 1.2-mole scale,6 whereas care must be taken to prevent explosion of the intermediate hypobromite when the Hunsdiecker method is used,® and one detonation has been reported.6 In comparison with the peroxide method,4 it is simpler and gives better yields.
The present procedure seems to be a general one for producing alkyl halides from acids. To aid in isolating higher-boning or solid products, solvents such as carbon tetrachloride and cyclo-hexane can be used.7 In preparing a solid, the mercuric halide can be removed by extraction with 5% potassium iodide.
1 Department of Chemistry, University of Colorado, Boulder, Colorado.
* С. M. McCloskey and G. H. Coleman, Org. Syntheses, Coll. Vol. S, 221 (1955).
* J. D. Roberts and V. C. Chambers, J. Am. Chem-. Soc., 73, 3176, 5030 (1951).
4 E. Renk, P. R. Shafer, W. H. Graham, R. H. Mazur, and J. D. Roberts, /.
Am. Chem. Soc., 83, 1987 (1961).
* A. A. Holzschuh, Dow Chemical Co., private communication.
' J. W. Rowe, Master’s Thesis, University of Colorado, 1952.
’ S. J. Cristol and W. C. Firth, Jr., J. Org. Chem., 26, 280 (1961).
ORGANIC SYNTHESES, VOL. 43
Submitted by K. G. Rutherfoed and W. Redmond.1 Checked by M. Pauxshock and В. C. McKusick.
A. o-Bromobenzenediazonium hexafluorophosphate. A solution of 95 ml. of 12N hydrochloric acid in 650 ml. of water is added with stirring to 60 g. of o-bromoaniline (0.35 mole; Note 1) in a
2-1. three-necked flask equipped with stirrer and thermometer. Solution is effected by heating the mixture on a steam bath (Note 2). A solution of 29 g. (0.42 mole) of sodium nitrite in 75 ml. of water is added with stirring while the mixture is maintained at —5° to —10° by means of a bath of ice and salt or of dry ice and acetone. At the end of the addition there is an excess of nitrous acid, which can be detected with starch iodide paper. Seventy-four milliliters (134 g., 0.60 mole) of 65% hexafluorophosphoric acid (Note 3) is added in one portion, with vigorous stirring, to the cold solution of the diazonium salt. Cooling and slow stirring are continued for an additional 30 minutes, and the precipitated diazonium hexafluorophosphate is then collected on a Buchner funnel. The diazonium salt is washed on the funnel with 300 ml. of cold water and with a solution of 80 ml. of methanol in 320 ml.
of ether (Note 4). The salt is partly dried by drawing air through the funnel for 2 hours. It is then transferred to a pile of several filter papers, powdered with a spatula, and dried at about 25°/1 mm. for at least 12 hours. The dried o-bromobenzenediazonium hexafluorophosphate is cream-colored; weight 108-111 g. (94— 97%); m.p. 151-156° (dec.) (Note 5).