Ethylene structure

Ethylene structure

The molecular formula of ethylene is C₂H₄. Two carbon atoms have the power to combine with six univalent atoms or groups, as in ethane, neopentane, etc. There are only four univalent hydrogen atoms present in ethylene: therefore ethylene is said to be unsaturated, and should be capable of adding on two univalent atoms or groups. Thus the structure of ethylene must be such as to be capable of undergoing addition reactions. Assuming carbon to be quardrivalent and hydrogen univalent, three structures are possible for ethylene :

Two isomeric compounds of molecular formula C₂H₄Cl₂ are possible : CH₃CHCl₂ and CH₂ClCH₂Cl. Both isomers are known, one (ethylene dichloride) being formed by the direct combination between ethylene and chlorine, and the other (ethylidene dichloride) by the action of phosphorus pentachloride on acetaldehyde. The structure of ethylidene dichloride is CH₃CHCl₂, hence the structure of ethylene dichloride is CH₂ClCH₂Cl. If (I) were the structure of ethylene, then the addition of chlorine should give ethylidene dichloride, and not ethylene dichloride. We may, therefore, reject structure (I). Furthermore, since (I) is unsymmetrical it would have a fairly large dipole moment; actually ethylene has a zero dipole moment.

Structure (II) represents ethylene as possessing a double bond. Such  a bond would prevent free rotation and would therefore explain geometrical isomerism. Hence (II) was acceptable to the classical chemists. The modern theory of a double bond is that it consists of one 𝜎– and one 𝜋–bond or two ‘bent’ bonds. Structure (III) represents ethylene as a free diradical, but since ethylene does not exhibit the usual properties of a diradical, we must reject (III).