Ethene orbital hybridization

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. 

We can account for the structure of ethyne on the basis of orbital hybridization as we did for ethane and ethene. In our model for ethane (Section 1.12B) we saw that the carbon orbitals are sp hybridized, and in our model for ethene (Section 1.13) we saw that they are sp hybridized. In our model for ethyne we shall see that the carbon atoms are sp hybridized. 

Hybridizing two p orbitals with one t orbital yields three s- orbitals. Atoms that are sp hybridized point the axes of their three orbitals toward the corners of an equilateral triangle. The carbon atoms of ethene are hybridized and trigonal planar. 

Figure 7.36 The MO diagram for the D21, molecule ethene, using hybrid orbitals.

The element before carbon in Period 2, boron, has one electron less than carbon, and forms many covalent compounds of type BX3 where X is a monovalent atom or group. In these, the boron uses three sp hybrid orbitals to form three trigonal planar bonds, like carbon in ethene, but the unhybridised 2p orbital is vacant, i.e. it contains no electrons. In the nitrogen atom (one more electron than carbon) one orbital must contain two electrons—the lone pair hence sp hybridisation will give four tetrahedral orbitals, one containing this lone pair. Oxygen similarly hybridised will have two orbitals occupied by lone pairs, and fluorine, three. Hence the hydrides of the elements from carbon to fluorine have the structures... 

The vinyl H2C=CH radical can be produced by cleavage of a C-H bond in ethene, and has been studied in the gas phase. The unpaired electron clearly occupies a carbon sp hybrid orbital, to lapse into the language of descriptive organic chemistry, but there are regions of space where the, 6-spin electrons have... 

Formation of a <7-bond by donation from the 7r-orbital of ethene into a vacant metal dsp2 hybrid orbital... 

We use different hybridization schemes to describe other arrangements of electron pairs (Fig. 3.16). For example, to explain a trigonal planar electron arrangement, like that in BF, and each carbon atom in ethene, we mix one s-orbital with two /7-orbitals and so produce three sp2 hybrid orbitals ... 

Similar, but different, redeployment is envisaged when a carbon atom combines with three other atoms, e.g. in ethene (ethylene) (p. 8) three sp2 hybrid atomic orbitals disposed at 120° to each other in the same plane (plane trigonal hybridisation) are then employed. Finally, when carbon combines with two other atoms, e.g. in ethyne (acetylene) (p. 9) two sp1 hybrid atomic orbitals disposed at 180° to each other (idigonal hybridisation) are employed. In each case the s orbital is always involved as it is the one of lowest energy level. 

Both carbon atoms in ethene undergo sp2 hybridization. The C-H bonds involve overlap of sp1 carbon orbitals with Is orbitals of the H atoms. The carbon-carbon double bond involves the overlap of sp2 orbitals from each carbon to give the o bond and the side-on overlap of a p orbital from each carbon atom to give the n bond. 

All alkenes contain a carbon-to-carbon double bond. We can use ethene as a typical example to explain the bonding in alkenes. On each carbon atom of the double bond, the 2s orbital mixes with two of the 2p orbitals to form three degenerate sp hybrid orbitals. The remaining 2p orbital is left unhybridised. 

Each carbon atom in ethene uses its three sp hybrid orbitals to form a bonds with two hydrogen atoms and with the other carbon atom. The unhybridised 2p orbitals left on the carbon atoms overlap side-on to form a k bond. The formation of the bonds in ethene is illustrated in the following diagram. 

As can be seen from Table 2, the amount of s-character of the CH hybrid orbital of 1 is indeed increased relative to that of the CH hybrid orbitals of ethene, ethane and methane while at the same time the s-character of the CC hybrid orbitals is decreased. This seems to confirm predictions based on model calculations with orthogonal hybrid orbitals. However, closer inspection of the data in Table 2 reveals that s-character of the CH hybrid... 

In the valence-bond approach, the 7r bond of ethene is considered to be a hybrid of all reasonable electronic configurations of two indistinguishable paired electrons distributed between two p orbitals. Each of the configurations that can be written, 4a, 4b, 4c, and 4d, have identical locations of the atomic nuclei in space ... 

The four valence-bond structures or configurations, 4a-d, are combined mathematically to give four hybrid states, and of these, the lowest-energy one corresponds approximately to the normal state of the molecule. The calculation shows that the structures 4a and 4b, which have one electron in each p orbital, are the major contributors to the hybrid of ethene. The valence-bond structures, 4c and 4d, are ionic structures, which correspond to the conventional formulas, 4e and 4f ... 

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