(Official Hive Translator)
Selective bromination of p-diMeO-benzene with KBr3
(Rated as: excellent)
Selective bromination of aromatic compounds using potassium tribromide.
Ring-brominatable electron-rich aromatic compounds are reacted with potassium tribromide in the presence of water such that a product enriched in monobromo ring substitution is formed. The extent to which multiple ring bromination occurs in the process is effectively suppressed.
The process may be applied to any of a wide variety of electron-rich aromatic compounds so long as the activated benzene ring system has at least two positions in which bromination can occur. A few illustrative compounds of this type include phenol, cresol, the xylenols, resorcinol, hydroquinone, the trihydroxybenzenes, aniline, N-methylaniline, N,N-dimethylaniline, p-phenylenediamine, diphenylamine, anisole, hydroxyanisole, ethoxybenzene, aminophenol, and the like.
The bromination reaction medium may include organic solvents such as methylene dibromide, 1,2-dibromoethane, chlorobenzene, methylene dichloride, carbon tetrachloride, 1,2-chloroethane, hexane, heptane, octane, decane, cyclohexane, toluene, xylene, or the like.
Into a 500 mL round bottom flask were charged 100 grams of 1,4-dimethoxybenzene and 100 grams of methylene dibromide. An endotherm to 12 C occurred when creating this solution. Separately prepared was a solution of 90 grams of KBr in 265 grams of water to which was added 35.5 mL of bromine, and the resulting mixture was stirred to form an aqueous solution of KBr3. A portion of this solution (250 mL) was added slowly to the 1,4-dimethoxybenzene solution. After a slight exotherm from 13 to 22 C. occurred (in 20 minutes) a cooling water bath was applied externally to the flask to keep the temperature at 22-25 C. The total feed time was 70 minutes. The bromine color disappeared within 10 minutes after the addition had been completed. To dissolve some salts that appeared in the reaction mixture, 60 mL of water was added thereto. The phases were separated in a separatory funnel and the aqueous layer was discarded. The organic phase was washed with 30 grams of 10% sodium carbonate solution (pH 7-10) and the phases separated readily. The product solution was stripped at reduced pressure to 65 C. at 0.5 mm Hg pressure, distilled and subjected to analysis.
2-Bromo-1,4-diMeObenzene - 79.8%
p-diMeO-benzene - 10.4%
Dibromo-diMeO-benzene - 8.3%
Monobromo:dibromo ratio - 9.6
Other examples include similar brominaion of methylenedioxybenzene.