Ethylene

Ethylene

Ethylene may be prepared by most of the general methods of preparation, but the most convenient laboratory method is to heat ethanol with excess of concentrated sulphuric acid. It is obtained in huge quantities as a by-product in the cracking crude oil and of ethane and propane.

Ethylene is a colourless gas, b.p. -105^oC, sparingly soluble in water. It burns with a smoky luminous flame. When ethylene is heated with chlorine at 350-450^oC, vinyl chloride is obtained :

CH₂=CH₂ + Cl₂  →  CH₂=CHCl + HCl

Ethylene may be oxidized to thhylene oxide and polymerises under high pressure and high temperature to form polyethylene or polythene :

nCH₂=CH₂ →  ­—(—CH₂—CH_2—)_n—

 This polymerisationis catalysed by traces of oxygen (which produces the free radicals). Polythene is very resistant to acids, bases, and most of the usual organic solvents.

Polythene is also manufactured by the Ziegler process. Ethylene is passed, under pressure, into a hydrocarbon solvent containing a suspension of, e.g., Et₃Al + TiCl₄ as catalyst at about 70^oC.

 Ethylene is used for ripening fruit. Unripe fruit may be transported easily without damage, and ripens on exposure to ethylene gas for a few days, the product being apparently indistinguishable from the natural ripened fruit. It appears that all fruits give off ethylene (Burg et al., 1962). Ethylene is also used as an anesthetic, in the manufacture of mustard gas and plastics (polythene, polystyrene), and in the preparation of various solvents such as glycol, dioxin, cellosolves, etc.