56 Biological Chemistry. 



products when treated with certain of the halogens. 

 When a paraffin is allowed to stand in the presence of 

 chlorine or bromine in sunlight, reaction takes place : very 

 readily in the case of chlorine, but less so in the case of 

 bromine. A hydrogen atom in the carbon compound is 

 " substituted " by one atom of halogen and removed in 

 combination with another atom. 



Thus CH 4 + C1 2 = CH 3 C1 + HC1 



or CH 4 + Br 2 = CH 3 Br + HBr 



In this way one or more atoms of the hydrogen of the 

 hydrocarbon can be " substituted " by a halogen. From 

 methane itself, the following chlorine derivatives are 

 obtainable by the gradual substitution of the hydrogen 

 atoms CH 3 C1, or methyl chloride or monochlormethane ; 

 CH 2 C1 2 , methylene dichloride or dichlormethane ; CHC1 3 , 

 chloroform or trichlormethane ; and CC1 4 , carbon tetra- 

 chloride or tetrachlormethane. In view of the fact that 

 mixtures are obtained when a hydrocarbon is treated with 

 a halogen, the method of preparation by direct substitution 

 is not the most convenient one for obtaining the halogen 

 derivatives of the hydrocarbons. Other methods will be 

 discussed later. It is important to remember, however, 

 that halogens act upon hydrocarbons by substitution, and 

 that the hydrogen compound of the halogen is formed at 

 the same time. Iodine does not act readily in the same 

 way as chlorine or bromine, and the iodine substitution 

 products are generally prepared by other methods. 



THE OLEFINES OR ETHYLENE HYDROCARBONS. 

 In addition to compounds containing only hydrogen 

 and carbon of the general formula C n Ho n+2 , a series of the 

 formula C n H 2n is also known. The typical member of 

 this group of substances is the gas which is formed when 

 alcohol C 2 H e O is treated under certain conditions with a 



