HALOGEN DERIVATIVES OF HYDROCARBONS. 473 



Acetylene, C 2 H 2 , is the first member of a hydrocarbon series of 

 the general composition C n H 2n _ 2 . It has been stated before that 

 acetylene is formed by direct union of the elements when an electric 

 current passes between two carbon poles in an atmosphere of hydro- 

 gen. It is also formed during the incomplete combustion of coal- 

 gas, such as takes place when the flame of a Btinsen burner " strikes 

 back " i. e., burns at the base of the burner. 



The method now extensively used in the manufacture of acetylene 

 for illuminating purposes depends on the decomposition of calcium 

 carbide by water : 



C 2 Ca + H 2 O = CaO + C 2 H 2 . 



Pure acetylene is a gas of agreeable ethereal odor, while the gas as ordi- 

 narily prepared possesses an unpleasant odor, due to impurities. With an 

 ordinary burner acetylene burns with a luminous but sooty flame, while by 

 the use of specially constructed burners flames may be obtained giving a very 

 pure, intensely luminous white light. Like ethene, it combines directly with 

 halogens, and when heated to a sufficiently high temperature it is converted 

 into the polymeric compounds, benzene, C 6 H 6 , and styrene, C 8 H 8 . 



A characteristic property of acetylene is the readiness with which its 

 hydrogen may be replaced by metals ; thus, by treating acetylene with sodium, 

 either monosodium acetylid, C 2 HNa, or disodium acetylid, C 2 Na,, may be ob- 

 tained. Silver acetylid, C 2 Ag 2 , a white crystalline compound, and cuprous 

 acetylid, C 2 Cu 2 , a red powder, may be obtained by passing the gas through 

 ammoniacal solutions of silver and cuprous salts, respectively. When dry, 

 both compounds explode violently when heated, the silver compound even 

 when rubbed with a glass rod. 



Halogen derivatives of hydrocarbons. 



Substitution products. When a mixture of methane and chlorine 

 is exposed to diffused daylight chemical action takes place gradually, 

 resulting in the successive substitution of hydrogen by chlorine, thus : 



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

 CH 3 C1 + 2C1 = CH 2 C1 2 + HC1. 

 CH 2 C1 2 + 2C1 == CHC1 3 + HC1. 

 CHC1 3 + 2C1 = CC1 4 + HC1. 



These reactions between methane and chlorine are more or less 

 characteristic of the general interaction between the halogens and 

 hydrocarbons, most of the latter being very susceptible to the action 

 of halogens. The 4 substitution products formed are designated 

 respectively as monochlor-methane or chlor-methane, dichlor-methane, 

 trichlor-methane, and tetracUor-methane or carbon tetmcMoride. These 

 compounds may also be looked upon as chlorides of the radicals CH 3 ', 



