Ethylene Synthesis

Ethylene has been synthesised in small quantities from its elements. It has been found in small quantities, rarely exceeding 1 per cent., among the products of the interaction of hydrogen and various kinds of carbon at temperatures between 1800° C. and 2200° C. When, however, the gaseous products are removed quickly from the neighbourhood of the heated carbon, and passed through charcoal cooled in liquid air, the formation of ethylene is more marked. At 1200° C. and 10 to 60 c.m. pressure the rate of formation of ethylene was one-hundredth that of the methane; but at higher temperatures the proportion of ethylene increased, till at 1400° C. the volume of the ethylene was one-tenth that of the methane.

1. Ethylene is generally prepared by the dehydration of ethyl alcohol, the reaction being essentially
   
   C₂H₅OH = C₂H₄ + H₂O.
   
   Zinc chloride, phosphoric anhydride, boric anhydride, and concentrated sulphuric acid have been used as dehydrating agents. In the case of sulphuric acid the half ester, known as ethyl sulphuric acid or sulphovinie acid, is first formed and then decomposes, thus:
   
   C₂H₅OH + HO-SO₂-OH = C₂H₅O-SO₂-OH + H₂O
   C₂H₅O-SO₂-OH = HO-SO₂-OH + C₂H₄.
   
   According to the method of Erlenmeyer and Bunte 4 25 grams of alcohol and 150 grammes of concentrated sulphuric acid are heated in a flask of 2-3 litres capacity to 160°-170° C. and a mixture of 1 part of alcohol and 2 of sulphuric acid is dropped in. Ethylene is evolved, and is washed from alcohol and ether vapour with concentrated sulphuric acid, and from sulphur dioxide with caustic soda. Disadvantages of this process are charring and frothing, and the oxidation of some of the alcohol to carbon dioxide with the simultaneous evolution of sulphur dioxide from the sulphuric acid. Sand is sometimes added to prevent frothing, but, according to J. B. Senderens, it acts also as a catalyst, causing the reaction to proceed at a lower temperature. Ahiminium sulphate, however, is a more effective catalyst. Newth has shown that syrupy phosphoric acid may advantageously be substituted for sulphuric acid, and that a continuous stream of pure ethylene is obtained by dropping alcohol into 50 c.c. of the acid heated until its temperature attains 200°-220° C. A further improvement consists in passing alcohol vapour through the heated phosphoric acid. This may be effected by dropping the alcohol slowly from a tap-funnel down a thistle-funnel closed with a cork. The alcohol then vaporises in the tube of the thistle-funnel within the generating flask, and blows through its finely drawn out point beneath the surface of the phosphoric acid. Or the alcohol may be boiled in a separate flask, and its vapour led into the phosphoric acid.
   
   Other methods of obtaining ethylene are the following:
2. By heating methylene iodide with copper in a sealed tube:
   
   2CH₂I₂ + 4Cu = 2Cu₂I₂ + C₂H₄.
3. By the action of zinc upon ethylene bromide:
   
   C₂H₄Br₂ + Zn = ZnBr₂ + C₂H₄;
   
   and of sodium on ethylene or ethylidene chloride:
   
   + 2Na = + 2NaCl.
   
   + 2Na = + 2NaCl.
4. By the action of alcoholic potash or potassium ethoxide on ethyl bromide:
   
   CH₃-CH₂Br + KOH = C₂H₄ + KBr + H₂O
   CH₃-CH₂Br + KOC₂H₅ = C₂H₄ + KBr + C₂H₅OH.
5. By the addition of hydrogen to acetylene through the action of zinc and ammonia on copper acetylide:
   
   CH≡CH + 2H = CH₂=CH₂
6. By the electrolysis of a concentrated solution of potassium succinate:
   
   COOK-CH₂=CH₂-COOK + 2H₂O = CH₂=CH₂ + 2CO₂ + 2KOH = H₂
7. From alcohol and phosphorus, thus:
   
   3C₂H₅OH + 2P = P(OH)₃ + PH₃ + 3C₂H₄.
8. From carbon monoxide and hydrogen by contact with nickel and palladium:
   
   CO + 2H₂ = H₂O + CH₂; 2CH₂ = C₂H₄.