54 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



B. UNSATU RATED. 



In addition to the series of saturated hydrocarbons there are other 

 series which contain less hydrogen in the molecule and are represented 

 by the general formulae C n H Zn (olefines) and C W H 2 ^_ 2 (acetylenes). 

 The two compounds ethylene or ethene, C 2 H 4 , and acetylene or ethine, 

 C 2 H 2 , are the first and typical examples. They are represented by 



the constitutional formulae : 



CH 2 CH 



I! HI 



CH 2 CH 



Ethylene. Acetylene. 



The four valencies of the carbon atoms are not completely satisfied by 

 hydrogen atoms and they are therefore termed the unsaturated hydro- 

 carbons. The unsaturated hydrocarbons are given the suffix ene and 

 ine respectively. The higher members are derived from the corre- 

 sponding saturated hydrocarbons by the loss of two or four hydrogen 

 atoms and the insertion of a double or triple bond. Isomers exist 

 amongst the higher members, and further, compounds are known which 

 contain two or more double bonds in their molecule, e.g. 



CH 



>C CH=CH 2 

 CH/ 



Isoprene. 



It should be noted that the double and the triple bonds do not indi- 

 cate greater, but on the contrary lesser, stability. 



(a) OLEFINES. 



Preparation. 



1. The olefines are most usually prepared by abstracting the ele- 

 ments of water from alcohols by means of dehydrating agents zinc 

 chloride, sulphuric acid, phosphoric acid : 



C 2 H 6 OH = CH 2 =CH 2 + H 2 0. 



The preparation of ethylene by this method is described on p. 58. 



2. Olefines are prepared by the action of alcoholic potash upon 



the alkyl halide (p. 57). 



C 2 H 5 I + KOH = CH 2 =CH 2 + H 2 O + KI. 

 Ethylene may be prepared as follows : 



50 c.c. of a 20 per cent, solution of caustic potash in alcohol is placed 

 in a 250 c.c. distilling flask in the neck of which a tap funnel is fastened with 

 a cork. The distilling flask is fixed on a stand at an angle so that its neck 

 may be attached to an inverted condenser (or its neck bent at an angle). A 

 glass tube suitably bent leads from the condenser to a water trough. The 

 potash solution is warmed and about 15 c.c. of ethyl iodide are slowly dropped 

 in. Ethylene is evolved and potassium iodide is precipitated. When all the 

 air has been displaced from the apparatus the gas may be collected in a gas 

 cylinder over water. 



. Ether is formed in the reaction according to the equation : 

 KOC 2 H + C 2 H 5 I = KI + C 2 H 6 . O . C 3 H 8 . 



