HALOGEN DERIVATIVES OF THE HYDROCARBONS. 



The only chemical property of the saturated hydrocarbons is that 

 they are attacked by the halogens yielding halogen substitution deriva- 

 tives, one atom of hydrogen being progressively replaced by an atom 

 of halogen ; thus, from methane by the action of chlorine, we can 

 obtain 



CH 3 C1 CH a Cl 2 CHC1 S CC1 4 . 



A mixture of the compounds results and the reaction is slow, so that, 

 in practice, these compounds are not prepared from the hydrocarbon, 

 but from other compounds. 



The unsaturated hydrocarbons differ from the saturated hydrocar- 

 bons in their behaviour to the halogens. They react by addition, thus, 

 e.g. ethylene combines with two atoms of bromine, forming the saturated 

 compound, ethylene dibromide : 



CH 2 CH 2 Br 



CH 2 CH 2 Br. 



Dihalogen compounds of this type are generally prepared by this 

 reaction. 



MONOHALOGEN DERIVATIVES. ALKYL HALIDES. 



Preparation. 



The alkyl halides are prepared from the corresponding alcohol 

 by the action of the halogen acid, or by the action of the phosphorus 

 halide : 



CH 8 OH + HBr = CH 3 Br + H 2 O 

 3CH 3 OH + PL, = CHJ + H 3 PO. 

 CH 3 OH + PC1 8 = CH 3 C1 + POC1 3 + HC1. 



Preparation of Methyl Iodide. 



1 8 gm. of methyl alcohol and 5 gm. of red phosphorus are placed in a 

 small flask (250 c.c.) and a reflux condenser is attached to it. 50 gm. of 

 iodine are slowly added by detaching the flask from the condenser and rapidly 

 refixing. Heat is evolved in the reaction and loss of alcohol and iodide is 

 prevented by the condenser. The apparatus and mixture is allowed to stand 

 for 12 to 24 hours so that the reaction completes itself. The contents of the 

 flask are distilled from a water-bath. The distillate is shaken in a separating 

 funnel with dilute caustic soda to remove iodine and hydriodic acid, and if 

 sufficient has been used the lower layer of methyl iodide will be colourless. 

 The lower layer of methyl iodide is separated, allowed to stand with a little 

 calcium chloride till it is clear and distilled from a water-bath (b.p. 44. 

 About 45 gm. or 75 per cent, of the theoretical yield should be obtained. 



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