Orbital hybridization ethylene

We conclude this introduction to hydrocarbons by describing the orbital hybridization model of bonding m ethylene and acetylene parents of the alkene and alkyne families respectively... 

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... 

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)). 

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). 

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. 

Parallel p orbitals in ethylene. The pi bond in ethylene is formed by overlap of the unhybridized p orbitals on the sp2 hybrid carbon atoms. This overlap requires the two ends of the molecule to be coplanar. 

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... 

One of the celebrated successes of orbital hybridization is the elucidation of multiple bonding, which stems from the Lewis formulation of a chemical bond as a shared electron pair. In a compound such as ethylene the saturation of the carbon valence shells can only be achieved by the sharing of two electron pairs between the two carbon atoms. 

Problem 12.17 The overlap integrals for Slater sp hybrid orbitals in ethylene are ... 

The structure of ethylene and the orbital hybridization model for the double bond were presented in Section 1.17. To review. Figure 5.1 depicts the planar structure of ethylene, its bond distances, and its bond angles. Each of the carbon atoms is xp -hybridized, and the double bond possesses a o component and a tt component. The o component results when an sp orbital of one carbon, oriented so that its axis lies along the intemuclear axis, overlaps with a similarly disposed sp orbital of the other carbon. Each sp orbital contains one electron, and the resulting a bond contains two of the four electrons of the double bond. The tt bond contributes the other two electrons and is formed by a side-by-side overlap of singly occupied p orbitals of the two carbons. 

FIGURE 17.1 Similarities between the orbital hybridization models of bonding in (a) ethylene and ( >) formaldehyde. Both molecules have the same number of electrons, and carbon Is sp -hybrldlzed in both. In formaldehyde, one of the carbons Is replaced by an sp -hybrldlzed oxygen (shown in red). Oxygen has two unshared electron pairs each pair occupies an sp -hybrldlzed orbital. Like the carbon-carbon double bond of ethylene, the carbon-oxygen double bond of formaldehyde Is composed of a two-electron cr component and a two-electron tr component. 

This represents rotational stability about the ethylenic bond produced by s/ 2-orbital-hybridized carbon atoms. Since free rotation about the carbon-to-carbon axis is not possible, two stable forms of the molecule can now exist. The six components of both forms are in the same plane. The orientations of substituents X and Y relative to each other, however,... 

The pi molecular orbitals of ethylene. The pi bonding orbital of ethylene is formed by constructive overlap of unhybridized p orbitals on the xp"-hybrid carbon atoms. Destructive overlap of these two orbitals forms the antibonding pi orbital Combination of twop orbitals must give exaaly two molecular orbitals. 

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