Ethylene hybrid orbitals

Ethylene is planar with bond angles close to 120° (Figure 2 15) therefore some hybridization state other than sp is required The hybridization scheme is determined by the number of atoms to which carbon is directly attached In sp hybridization four atoms are attached to carbon by ct bonds and so four equivalent sp hybrid orbitals are required In ethylene three atoms are attached to each carbon so three equivalent hybrid orbitals... 

Each carbon of ethylene uses two of its sp hybrid orbitals to form ct bonds to two hydrogen atoms as illustrated m the first part of Figure 2 17 The remaining sp orbitals one on each carbon overlap along the mternuclear axis to give a ct bond connecting the two carbons... 

FIGURE 2 17 The carbon-carbon double bond in ethylene has a cr component and a tt compo nent The cr component arises from overlap of sp hybridized orbitals along the internuclear axis The tt component results from a side by side overlap of 2p orbitals... 

The structure of ethylene and the orbital hybridization model for its double bond were presented m Section 2 20 and are briefly reviewed m Figure 5 1 Ethylene is planar each carbon is sp hybridized and the double bond is considered to have a a component and a TT component The ct component arises from overlap of sp hybrid orbitals along a line connecting the two carbons the tt component via a side by side overlap of two p orbitals Regions of high electron density attributed to the tt electrons appear above and below the plane of the molecule and are clearly evident m the electrostatic potential map Most of the reactions of ethylene and other alkenes involve these electrons... 

Unsaturated organic molecules, such as ethylene, can be chemisorbed on transition metal surfaces in two ways, namely in -coordination or di-o coordination. As shown in Fig. 2.24, the n type of bonding of ethylene involves donation of electron density from the doubly occupied n orbital (which is o-symmetric with respect to the normal to the surface) to the metal ds-hybrid orbitals. Electron density is also backdonated from the px and dM metal orbitals into the lowest unoccupied molecular orbital (LUMO) of the ethylene molecule, which is the empty asymmetric 71 orbital. The corresponding overall interaction is relatively weak, thus the sp2 hybridization of the carbon atoms involved in the ethylene double bond is retained. 

The double bond in ethylene contains one a bond and one 7r bond. The a bond forms from the end-on overlap of two hybrid orbitals, and the 7i bond forms from the side-by-side overlap of two atomic p orbitals. Figure 10-21 shows the complete orbital picture of the bonding in ethylene. Ethylene is the simplest of a class of molecules, the alkenes, all of which contain CDC double bonds. The alkenes are the subject of our Box on page 404. 

FIGURE 16.10 The structure and bonding in the anion of Zeise s salt. A cr-bond results from the overlap of a dsp2 hybrid orbital on the metal and the 7T orbital on ethylene. Back donation from a d orbital on the metal to the -k orbital on ethylene gives some 7r bonding (shown in (c)). 

Both carbon atoms in ethylene molecule undergo sp2 hybridization and form three identical sp2 hybrid orbitals. One p orbital remains unhybridized. Two sp2 hybrid orbitals from each carbon atom overlap end to end with the Is orbital of a hydrogen atom and four C — Ho bonds are formed in total. Also, between the two carbon atoms, a C — Co bond is formed as a result of the overlap between two sp2 hybrid orbitals. So, in the C2H4 molecule in total there are five o bonds. Meanwhile, the unhybridized p orbitals of the two carbon atoms overlap side by side and form a rt bond. So between the two carbon atoms in the C2H4 molecule there is one o bond, formed by the overlapping of sp2 hybrid orbitals and one n bond, formed by the side by side overlapping of the unhybridized p orbitals. In total, two bonds are formed, hence a double bond exists between the two carbon atoms. 

Figure 10-4 shows the hybridization that occurs in ethylene, H2C=CH2. Each carbon has sp2 hybridization. On each carbon, two of the hybrid orbitals overlap with an s-orbital on a hydrogen atom to form a carbon-to-hydrogen covalent bond. The third sp2 hybrid orbital overlaps with the sp2 hybrid on the other carbon to form a carbon-to-carbon covalent bond. Note that each carbon has a remaining p-orbital that has not undergone hybridization. These are also overlapping above and below a line joining the carbons. 

The sp hybrid orbitals of carbon were considered as a mix of the 2s orbital with three 2p orbitals. To provide a model for ethylene, we now need to consider hybrid orbitals that are a mix of the 2s orbital with two 2p orbitals, giving three equivalent sp orhitals. In this case, we use just three orbitals to create three new hybrid orbitals. Accordingly, we find that the energy level associated with an sp orbital will be below that of the sp orbital this time, we have mixed just two high-energy p orbitals with the lower energy orbital (Figure 2.13). The... 

L in Scheme 11.3) departs. Nucleophilic addition to the intermediate benzyne (step D) is readily explained by perturbative MO arguments. The extra and orbitals of benzyne are compared to those of ethylene in Figure 11.7. The aromatic n system is not involved in the special properties of benzyne. The third benzyne n bond is due to the overlap in fashion of the two sp2 hybrid orbitals which lie in the nodal plane of the intact 6 electron system. Two factors contribute to a very low LUMO for benzyne. First, the sp2 hybrid orbitals are lower in energy than the 2p orbitals from which the ethylene orbitals are constructed. Second, the intrinsic interaction between the two sp2 orbitals is less than the normal / cc since the orbitals have less p character and are tipped away from each other. The low LUMO of benzyne makes the molecule a strong Lewis acid, susceptible to attack by bases, and a reactive dienophile in Diels-Alder reactions, as we shall see later. 

The carbon-oxygen double bond in aldehydes and ketones is similar and can be described in either of these two ways. If we adopt the iocalised-orbital description, formaldehyde will have two directed lone pairs in place of two of the C-H bonds in ethylene. In this case the axes of these hybrid orbitals will be in the molecular plane (unlike the oxygen lone pairs in water). Either the components of the double bond or the lone pairs can be transformed back into symmetry forms. The alternative description of the lone pairs would he one er-type along the 0-0 direction and one jr-type with axis perpendicular to the 0-0 bond hut in the molecular plane. It is the latter orbital which has the highest energy, so that an electron is removed from it in. ionisation or excitation to the lowest excited state. 

Boron trifluoride and ethylene are but two of the many instances where the directional properties of covalent bonds are better described in terms of overlap of hybrid orbitals than in terms of simple atomic orbitals. 

The side-on bonding of H2 to the metal involves the transfer of a bonding electrons of H2 to a vacant metal d orbital (or hybrid orbital), together with the transfer of electrons from a tilled metal d orbital into the empty a orbital of H2, as shown in Fig. 11.5.2. This synergistic (mutually assisting) bonding mode is similar to that of CO and ethylene with metal atoms. The n back donation from... 

Rule 3 If two or three pairs of electrons form a multiple bond between two atoms, the first bond is a sigma bond formed by a hybrid orbital. The second bond is a pi bond, consisting of two lobes above and below the sigma bond, formed by two unhybridized p orbitals (see the structure of ethylene in Figure 2-17). The third bond of a triple bond is another pi bond, perpendicular to the first pi bond (shown in Figure 2-18). 

Each carbon atom has an octet, and there is a double bond between the carbon atoms. Each carbon is bonded to three other atoms (three sigma bonds), and there are no lone pairs. The carbon atoms are sp2 hybridized, and the bond angles are trigonal about 120°. The double bond is composed of a sigma bond formed by overlap of two sp2 hybridized orbitals, plus a pi bond formed by overlap of the unhybridized p orbitals remaining on the carbon atoms. Because the pi bond requires parallel alignment of its two p orbitals, the ethylene molecule must be planar (Figure 2-17). 

The double bond between carbon and oxygen looks just like the double bond in ethylene. There is a sigma bond formed by overlap of sp2 hybrid orbitals and a pi bond formed by overlap of the unhybridizedp orbitals on carbon and oxygen (Figure 2-19). 

Each of the carbon-hydrogen sigma bonds is formed by overlap of an sp2 hybrid orbital on carbon with the Is orbital of a hydrogen atom. The C—H bond length in ethylene (1.08 A) is slightly shorter than the C—H bond in ethane (1.09 A) because the sp2 orbital in ethylene has more s character (one-third, v) than an sp3 orbital (one-fourth, v). The s orbital is closer to the nucleus than the p orbital, contributing to shorter bonds. 

The triple bond is relatively short because of the attractive overlap of three bonding pairs of electrons and the high s character of the sp hybrid orbitals. The sp hybrid orbitals are about one-half s character (as opposed to one-third s character of sp2 hybrids and one-fourth of sp3 hybrids), using more of the closer, tightly held s orbital. The sp hybrid orbitals also account for the slightly shorter C — H bonds in acetylene compared with ethylene. 

The chemical bonding of re coordinated ethylene is very similar to the Chatt-Dewar-Duncanson picture of CO coordination (Fig. 4.6). The donating orbital is the doubly occupied n orbital that is a-symmetric with respect to the normal to the surface. When adsorbed atop it interacts with the highly occupied dz2 surface atomic orbital and the partially filled s and pz orbitals. The ethylene LUMO is the empty asymmetric n orbital, which interacts with the surface dxz and px orbitals. The corresponding overall interaction is relatively weak and no hybridization on ethylene is assumed to occur. The orbital interaction diagram of the occupied ethylene n orbital with surface atom dz2 orbital is analogous to that sketched for the CO 5a orbital in Fig. 4.4. When this dz2 becomes nearly completely occupied, as occurs, for instance, for Pd or Pt, the ethylene-rc surface atom dz2 interaction... 

These three views of the ethylene molecule emphasize different aspects of the disposition of shared electron pairs in the various bonding orbitals of ethene (ethylene), (a) The backbone structure consisting of sigma (a) bonds formed from the three sp2-hybridized orbitals on each carbon, (b) The % (pi) bonding system formed by overlap of the unhybridized pz orbital on each carbon. The pi orbital has two regions of electron density extending above and below the plane of the molecule, (c) A cutaway view of the combined sigma and pi system. 

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