$8 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



Preparation of Ethyl Bromide. 



A distilling flask of about i litre capacity is closed by a cork and its neck 

 attached to a condenser. To the end of the condenser is attached an 

 adapter (a bent tube wide at one end and narrow at the other, p. 1 2) which 

 dips under water contained in a conical flask of about 250 c.c. capacity 

 cooled by standing in ice. 54 c.c. (100 gm.) of sulphuric acid are mixed 

 in the flask with 75 c.c. (60 gm.) of absolute alcohol and cooled to the tem- 

 perature of the air. 100 gm. of coarsely powdered potassium bromide 

 are added to the contents of the flask and the mixture is heated on a 

 sand bath or carefully on a gauze. The contents boil and froth up and heavy 

 oily drops of ethyl bromide collect under the water in the receiver. If the 

 frothing is too great the flask is removed from the source of heat for a minute. 

 The heating is continued so long as oily drops distil over. The contents of 

 the receiver are placed in a separating funnel and the lower layer collected. 

 It is purified by returning to the separating funnel and shaking with a dilute 

 solution of sodium carbonate. The ethyl bromide is then shaken with water 

 to remove alkali and it is placed in a clean dry distilling flask and left in con- 

 tact with calcium chloride till it is clear. The flask is furnished with a ther- 

 mometer, attached to a condenser and the ethyl bromide (b.p. 35-40) distilled 

 over from a water- bath. About 75 to 80 gm. should be obtained. 



DIHALOGEN DERIVATIVES. 



Methylene chloride, CH 2 C1 2 , is generally prepared by reducing 

 chloroform in alcoholic solution with zinc and hydrochloric acid. 



Methylene iodide, CH 2 I 2 , is prepared by reducing iodoform with 

 hydriodic acid. 



Methylene bromide, CH 2 Br a , is prepared by treating methylene 

 iodide with bromine: 



CH 2 I 2 + 2Br 2 = CH a Br 2 + 2BrI. 



The numerous isomers of the halogen derivatives of the higher 

 hydrocarbons are prepared by various methods, e.g. : 



(a) by addition of halogen to unsaturated hydrocarbons ; 



(V) by the action of phosphorus pentachloride upon the aldehydes 

 and ketones. 



Preparation of Ethylene Dibromide. 



Ethylene is prepared by dropping a mixture of 30 c.c. of absolute alcohol 

 and 80 c.c. sulphuric acid from a tap funnel upon a mixture of 124 c.c. of 

 alcohol and 1 08 c.c. of concentrated sulphuric acid contained in a 2 litre flask 

 and mixed with sand to prevent frothing, the mixture being gently heated until 

 a steady stream of gas is evolved. The evolved gas is passed through two wash 

 bottles l with safety tubes containing caustic soda solution to remove sulphur 

 dioxide into two ordinary wash bottles containing 50 c.c. bromine and i c.c. 

 of water and 15 c.c. of bromine and i c.c. of water respectively. These two 

 bottles are placed in a basin of water to which ice may be added to prevent 

 the contents becoming warm during the reaction. The outlet tube is 

 connected to a tower containing soda lime so that bromine vapour does not 

 escape into the room. The bromine in the bottles is gradually decolorised 



1 It may be necessary to change these bottles for fresh ones during the preparation. 



