Preparation of cyanogen bromide
To the two-neck two-liter flask which is cooled with ice-salt bath slowly 1000 g (320 ml) bromine and 150 ml of water is added. To this bromine and water mixture with vigorous stirring at a rate of 1 drop per second solution containing 420 g of sodium cyanide (sodium cyanide us used in 30% excess than the theoretical) in 850 ml of water is slowly added. The temperature of the reaction mixture should be below 20 °C. Local excess of cyanide in the reaction flask should be avoided as this could lead to the formation of side products such as (CN)x. The last 150 ml solution of sodium cyanide is half diluted with additional water. During addition of sodium cyanide vigorous stirring should be applied. The reaction is complete and addition of sodium cyanide should be stopped when the color turns to brow (brownish) and no longer disappears with addition of sodium cyanide. Cyanogen bromide is slowly removed from the reaction flask by gently warming it. Cyanogen bromide is dried by passing vapors trough calcium chloride during distillation process and collected in 750-1000 ml flask which is immersed in salt-ice cooled bath.
The product is a snow-white crystals (yield of cyanogen bromide is 590 g, or 90% of theory based on the bromine); in this form it can be stored for a long time. Cyanogen bromide with brown color, can not be stored. When storing cyanogen bromide the bottle is necessary to close with cork stopper and seal it well with paraffin.
A process could be scaled up according to following procedure (Org. Syn. Coll. 2. 150, 1948). 500 g of bromine in 50 ml of water was mixed with a solution containing 170 g sodium cyanide (0.125 times more than necessary by the equation) in 1200 ml of water (2.5 times greater than in the method above). The yield of cyanogen bromide is 73-85%.
Cyanogen bromide crystallizes in the form of colorless needles. Melting point 52° C; boiling point 61.4° C. Cyanogen bromide is soluble in ether, alcohol and water. Extremely toxic so all synthesis should be performed in well ventilated hood! Furthermore it is recommended to wear a mask.
Руководство по неорганическому синтезу. Брауэра Г. – Ч. 3 – 691-692, 1985 (Guide to preparative inorganic chemistry, H. Brauer, Vol. 3, 691-692, 1985)
Bromide, Cyanogen, Cyanogen Bromide
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