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 :

Ethylene structure, 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).