Bonding and Hybridization

Chapter 3 has discussed a wide range of chemical phenomena involving atoms with partially filled s and p shells. Indeed, the chemistry of these elements is so closely tied to the energies and shapes of s/p orbitals (as well as the hybrids, bonds, and antibonds to which they give rise) that we can virtually speak of s/p-orbital chemistry as a generic type of chemical behavior. 

Fig. 49. Hybrid bond and lone pair orbitals (a) and their expanded forms (b)...

Figure 5.2 Silica and hybrid bonded and end-capped. ( 2006 Waters Corporation. Used with permission.)...

Tabu CLXV, Spatial Arrangement of Hybrid Bonds and Possibility of Multiple Bond Formation... 

The s character of the vinylic C-H bond of cyclopropene is midway between that of sp and sp hybridized bonds and some similarity in behaviour to the acetylenic C—H bond is noted, particularly with regard to the acidity of the proton. Molecular orbital calculations at the ab initio level support an enhanced acidity of C(l)-H over C(3)-H in cyclopropene and correctly predict the preference for a 1-lithio derivative. A bis-lithio derivative is expected to have both lithium atoms bridging the C(l)-C(2) a bond. 

The links in such a chain as that shown in Fig. 9.1 are made of sp3 hybrid bonds and since such a bond is cylindrically symmetrical about its axis, groups of atoms can rotate about any of those bonds. Hence, even though the bond holds its two atoms quite tightly at a fixed distance, and stands quite rigidly at the tetrahedral angle to the other three bonds formed by the same atom, the molecule as a whole has considerable flexibility (Fig. 9.2). 

To summeirize, small ciystallite size, high interlayer spacing, and general structural disorder are the factors that contribute to the unique and stable turbostratic structure of PAN-based carbon fibers (which is likely to include both sp and sp hybrid bonds), and explain their inability to form a graphitic structure even after high-temperature heat-treatment (i.e., 3000°C). 

However, benzene does not behave chemically like an alkene. The entire molecule lies in the same plane, as shown in Figure 2.12. Benzene contains resonance hybrid bonds, and the structure of the benzene ring allows electrons to be spread through delocalized p-orbitals over the whole ring. The structural and skeletal formulas below show benzene as a resonance hybrid, representing the delocalization of electrons. 

In the world of phosphates, orthophosphates and polyphosphates are asymmetric in their properties. Orthophosphates are a single center radical, with sp hybridized bonding, and there are three levels of cationic attraction to the group. This influence is most pronounced with hydrogen ions showing extremely different pKs. 

Therefore, the D feature in Fig. 4.5 corresponds to the O-Cu hybrid bonding and the K feature to the oxygen nonbonding states. The fall of the upper part of d band corresponds to the process of electron transportation from the outer shell of Cu to the deeper empty sp hybrid orbitals of oxygen, or to the even higher empty levels of Cu to form diploes. The rise of the K feature in energy space is independent of the fall of the intensity near the Ep. The exposure dependence of the valence DOS change should provide valuable information about the dynamics of... 

Essential events in surface adsorption include bond contraction and sp-orbit hybridization. Bond and non-bond formation result in all the observations. 

This involves the formation of a carbenium ion which is best described as a hybrid of the two structures shown. This then rearranges by migration of a bond, and in so doing forms a more stable tertiary carbenium ion. Elimination of a proton yields camphene. 

The O atom uses one of its sp or sp hybrids to form the CO a bond and antibond. When sp hybrids are used in conceptualizing the bonding, the other sp hybrid forms a lone pair orbital directed away from the CO bond axis one of the atomic p orbitals is involved in the CO n and 71 orbitals, while the other forms an in-plane non-bonding orbital. Alternatively, when sp hybrids are used, the two sp hybrids that do not interact with the C-atom sp2 orbital form the two non-bonding orbitals. Hence, the final picture of bonding, non-bonding, and antibonding orbitals does not depend on which hybrids one uses as intermediates. 

Atoms with unusual hybridizations can be particularly dihicult to include. Most organic force helds describe atoms with hybridizations whose bond angles are all equivalent (i.e., sp, sp, and sp hybridizations with bond angles of 180, 120, and 109.5°, respectively). In contrast to this, a square planar atom will have some bond angles of 90° and some angles of 180°. In this case, it may be necessary to dehne the bond and angle terms manually, modify the software, or hold the bond angles hxed in the calculation. 

We 11 expand our picture of bonding by introducing two approaches that grew out of the idea that electrons can be described as waves—the valence bond and molecular orbital models In particular one aspect of the valence bond model called orbital hybridization, will be emphasized... 

Bonding m n butane and isobutane continues the theme begun with methane ethane and propane All of the carbon atoms are sp hybridized all of the bonds are ct bonds and the bond angles at carbon are close to tetrahedral This generalization holds for all alkanes regardless of the number of carbons they have... 

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... 

FIGURE 2 20 Bonding in acetylene based on sp hybridization of carbon The carbon-carbon triple bond is viewed as consisting of one cr bond and two tt bonds... 

Section 2 20 Carbon is sp hybridized in ethylene and the double bond has a ct com ponent and a rr component The sp hybridization state is derived by mix mg the 2s and two of the three 2p orbitals Three equivalent sp orbitals result and their axes are coplanar Overlap of an sp orbital of one car bon with an sp orbital of another produces a ct bond between them Each carbon still has one unhybridized p orbital available for bonding and side by side overlap of the p orbitals of adjacent carbons gives a rr bond between them... 

Conformational analysis is far simpler m cyclopropane than m any other cycloalkane Cyclopropane s three carbon atoms are of geometric necessity coplanar and rotation about Its carbon-carbon bonds is impossible You saw m Section 3 4 how angle strain m cyclopropane leads to an abnormally large heat of combustion Let s now look at cyclopropane m more detail to see how our orbital hybridization bonding model may be adapted to molecules of unusual geometry... 

FIGURE 4 9 tert Butyl cation (a) The positively charged carbon is sp hybridized Each methyl group IS attached to the positively charged carbon by a cr bond and these three bonds he in the same plane (b) The sp hybridized car bon has an empty 2p orbital the axis of which is perpen dicular to the plane of the carbon atoms... 

Bonding m alkenes is described according to an sp orbital hybridization model The double bond unites two sp hybridized carbon atoms and is made of a ct component and a rr component The ct bond arises by over lap of an sp hybrid orbital on each carbon The rr bond is weaker than the CT bond and results from a side by side overlap of p orbitals... 

At 146 pm the C 2—C 3 distance m 1 3 butadiene is relatively short for a carbon-carbon single bond This is most reasonably seen as a hybridization effect In ethane both carbons are sp hybridized and are separated by a distance of 153 pm The carbon-carbon single bond m propene unites sp and sp hybridized carbons and is shorter than that of ethane Both C 2 and C 3 are sp hybridized m 1 3 butadiene and a decrease m bond distance between them reflects the tendency of carbon to attract electrons more strongly as its s character increases... 

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