Carbon, hybridization schemes

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... 

One more hybridization scheme is important m organic chemistry It is called sp hybridization and applies when carbon is directly bonded to two atoms as m acetylene The structure of acetylene is shown m Figure 2 18 along with its bond distances and bond angles Its most prominent feature is its linear geometry... 

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 ... 

Now consider the alkynes, hydrocarbons with carbon-carbon triple bonds. The Lewis structure of the linear molecule ethyne (acetylene) is H—O C- H. To describe the bonding in a linear molecule, we need a hybridization scheme that produces two equivalent orbitals at 180° from each other this is sp hybridization. Each C atom has one electron in each of its two sp hybrid orbitals and one electron in each of its two perpendicular unhybridized 2p-orbitals (43). The electrons in the sp hybrid orbitals on the two carbon atoms pair and form a carbon—carbon tr-bond. The electrons in the remaining sp hybrid orbitals pair with hydrogen Ls-elec-trons to form two carbon—hydrogen o-bonds. The electrons in the two perpendicular sets of 2/z-orbitals pair with a side-by-side overlap, forming two ir-honds at 90° to each other. As in the N2 molecule, the electron density in the o-bonds forms a cylinder about the C—C bond axis. The resulting bonding pattern is shown in Fig. 3.23. 

Formally, pyrrole can be described as azacyclopentadiene, i.e. cyclopen-tadiene in which a CH2 unit has been replaced by a NH group. However, this translates into a classical structure that does not adequately describe the compound, for pyrrole has aromatic character, even though there are only five atoms in the ring The aromaticity arises because the nitrogen atom contributes two electrons and the four carbons one electron each to form a delocalized sextet of 7t-electrons. In valence bond terms the structure of pyrrole can be represented as a resonance hybrid (Scheme 6.1)... 

Figure 2.47. (Bottom, left) Calculated XAS spectra of adsorbed -octane for different —C bond lengths and — —C angles corresponding to the indicated hybridization schemes. (Right) Calculated XES spectra for change in (top) carbon hybridization, (middle) distance to surface and (bottom) CH distance and orientation. From Ref. [96].

There are two ways in which the state of hybridization at the carbon atoms of [1.1.1]-propellane has been deduced. The results disagree dramatically (Figure 1), and at least one is clearly wrong. The hybridization at the carbon atoms can be derived from (i) the empirical correlations between the s orbital character of a carbon hybrid orbital and the, 3C- H and 13C-13C NMR coupling constants across the bonds it forms, (ii) the analysis of computed wavefunctions, using one of several possible schemes. 

For simplicity, we use the conventional concept of hybridization to describe the bond types, but bonding is frequently more subtle and more extended than implied by this localized description. The parameters of typical hybridization schemes of the carbon atom are listed in Table 14.3.1. 

This alternative hybridization scheme explains how carbon can combine with four atoms in some of its compounds and with three other atoms in other compounds. You may be aware of the conventional way of depicting carbon as being tetravalent in all its compounds it is often stated that carbon always forms four bonds, but that sometimes, as in the case of ethylene, one of these may be a double bond. This concept of the multiple bond preserves the idea of tetravalent carbon while admitting the existence of molecules in which carbon is clearly combined with fewer than four other atoms. 

This arrangement is forbidden by the exclusion principle. An immediate solution is by the assumption that the four bonding electrons between the carbon atoms reside in two different molecular orbitals. There are two possibilities based on sp3 and sp2 hybridization schemes respectively. The first possibility leads to the idea of bent bonds ... 

Probably the most important property of these compounds is the propensity of iV-acyl-imidazoles and -benzimidazoles (as well as other azoles) to become involved in reactions which result in acylation of an attacking nucleophile. The compounds are unlike other tertiary amides in that there is little or no contribution from resonance structures of type (251) to the hybrid (Scheme 142) hence the positive nature of the carbonyl carbon is undiminished. The electron pair on the annular nitrogen is part of the aromatic sextet. The compounds are known as azolides generally, and more specifically as imidazolides . Because the annular nitrogens are not directly adjacent imidazolides are more reactive than the corresponding pyrazolides. 

Other carbon compounds make use of two alternative hybridization schemes. The 5 AO can form hybrids with two of the p AO s to give three sp hybrid orbitals, with one /r-orbital remaining unhybridized. This accounts for the structure of ethylene (elhcne) ... 

Carbon radicals have an unpaired electron in a nonbonding orbital. The possible hybridization schemes are shown below. 

Benzene has a total of 30 valence electrons. To describe the bonding in benzene using hybrid orbitals, we first choose a hybridization scheme consistent with the geometry of the molecule. Because each carbon is surrounded by three atoms at 120° angles, the appropriate hybrid set is sp. Six locahzed C—C tr bonds and six localized C—H <7 bonds are formed from the sf hybrid orbitals, as shown in Figure 9.26(a). Thus, 24 of the valence electrons are used to form the a bonds in the molecule. 

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