56 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



(b) ACETYLENES. 



Preparation. 



Acetylene is formed by the incomplete combustion of other hydro- 

 carbons, but is most usually prepared by the action of water upon cal- 

 cium carbide : 



CaC 2 + H 2 = C 2 H 2 + CaO. 



The hydrocarbons of this group are prepared by the action of alco- 

 holic potash upon halogen compounds in the same way as ethylene : 



CH 2 Br CHBr 



+ KOH = || + KBr + H 2 O 



CH 2 Br CH 2 



Vinyl bromide. 

 CHBr CH 



|| + KOH = I)) + KBr + H 2 O. 



CH 2 CH 



Properties. 



The lower members are gases, the higher members are liquids. 

 Acetylene is soluble in water (i : i) and other organic liquids. Acetone 

 dissolves thirty-one times its own volume of the gas at N.T.P. Acety- 

 lene burns with a smoky, intensely hot flame which is very luminous ; 

 it is consequently employed for illuminating purposes, the burners, 

 generally of clay, being designed so that complete cpmbustion is effected. 



Addition Reactions. 



Acetylene and its homologues behave like the defines, but react 

 with two molecules : 



(1) Hydrogen. 



C a H 2 + H 2 = C 2 H 4 (ethylene) 

 C 2 H 4 + H a = C 2 H 6 (ethane). 



(2) Halogen Acid. 



CH 2 

 C 2 H, + HC1 = || (vinyl chloride) 



CHC1 



CH 2 CH. 



|| ' +HC1= | (ethylidene chloride). 



CHC1 CHC1 2 



(3) Halogens. 



CH CHBr 



(I) + Br 2 = || (acetylene dibromide) 



CH CHBr 



CHBr CHBr 2 



|| + Br a = I (ethane tetrabromide). 



CHBr CHBr 2 



Acetylene and the other members of the series form characteristic 

 compounds with copper, silver and other heavy metals. 



Cuprous acetylide, C 2 Cu 2 ,and silver acetylide, C 2 Ag 2 , are precipitated 

 as amorphous compounds when acetylene is passed through ammoni- 

 acal solutions of cuprous chloride or silver nitrate. In the dry state 

 these compounds are very explosive ; they are decomposed on treat- 

 ment with hydrochloric acid or potassium cyanide yielding acetylene. 

 Acetylene may be separated from other hydrocarbons by this property. 



