Ethene molecular shape

Ethene, H2C = CH2, serves as the starting material for the synthesis of polyethylene, from which plastic bags and milk jugs are made. Ethyne, H—C=C—H, is used as a fuel for welding torches. The double bond in ethene and the triple bond in ethyne have the same effect on molecular shape as single bonds. Predict the shapes and bond angles of ethene and ethyne. 

Rgure 4.51 Lewis structure and molecular shape of the ethene molecule... 

Figure 6.17 Contour map of p in the interatomic surface associated with the CC bond critical point in ethene. The plane of the plot is perpendicular to the molecular plane. The C and two H nuclei are projected onto the plane of the plot to indicate the orientation of the molecule. We see that electronic charge is preferentially accumulated in the direction perpendicular to the molecular plane, giving an elliptical shape to the electron density in this plane.

Important point Ethene is not actually formed by bringing together two carbon atoms and four hydrogen atoms individual carbon atoms do not hybridize their atomic orbitals and then combine, We are simply trying to rationalize the shapes of molecular orbitals. Hybridization and LCAO are tools to help us accomplish this. 

Successful separation of alkanes and alkenes has been documented when microporous membranes have been used [79,138]. The physiochemical properties, size, and shape of the molecules will play an important role for the separation, hence critical temperatures and gas molecule configurations should be carefully evaluated for the gases in mixture. On the basis of gas properties and process conditions, the separation may be performed according to selective surface flow or molecular sieving (refer to Section 4.2 on transport). The transport may also be enhanced by having a Ag compound in the membrane. The Ag ion will form a reversible complex with the alkene, and facilitated transport results. Selectivities in the range of 200-300 have been reported for separation of ethene-ethane and propene-propane [138]. Successful separation of alkanes and alkenes will be important for the petrochemical industry. Today the surplus hydrocarbons in the purge gas are usually flared. Membranes which should be suitable for this application are the carbon molecular sieves (see Section 4.3.2) and nanostructured materials (Section 4.3.3). 

Make molecular models of ethane and ethene. Notice the tetrahedral shape and 109° bond angles around each carbon of ethane and the trigonal shape and 120° bond angles around each carbon of ethene. Also notice that you can rotate the single bond of ethane but not the double bond of ethene. 

Ethene is chemically more interesting than ethane because of the n system. As you saw in Chapter 5, alkenes can be nucleophiles because the electrons in the n bond are available for donation to an electrophile. But remember that when we combine two atomic orbitals we get two molecular orbitals, from combining the p orbitals either in phase or out of phase. The in-phase combination accounts for the bonding molecular orbital (w), whilst the out-of-phase combination accounts for the antibonding molecular orbital (w ). The shapes of the orbitals as they were introduced in Chapter 4 are shown below, but in this chapter we will also represent them in the form shown in the brown boxes—as the constituent p orbitals. 

Dissociative mechanisms for square-planar substitutions are discussed in a review. A molecular orbital study of insertion of ethene into Pt—H bonds concludes that the reaction can be best described by a series of, preferably, dissociative steps. Rearrangements of three-co-ordinate ML3 T- or Y-shaped i -structures are discussed in this context. Three-co-ordinate intermediates are also suggested in the mechanisms for palladium(ii)-catalysed oxidations of olefins, and for electrophilic cleavage of platinum-carbon ff-bonds by protons. Parallel associative and dissociative processes have been proposed for a substitution reaction of a square-planar rhodium(i) complex in benzene solution. Especially, sterically crowded complexes have been thought to stabilize three-co-ordinate intermediates more easily. Recently determined activation volumes for sterically hindered square-planar complexes both of platinumand palladium are not compatible with dissociative activation, however. 

---