I/P-hybridized carbon

AUcenes are relatively nonpolar. Alkyl snbstitnents donate electrons to an i p -hybridized carbon to which they are attached slightly better than hydrogen does. 

A considerable body of data is available on the acidity of substituted benzoic acids. Benzoic acid itself is a somewhat stronger acid than acetic acid. Its carboxyl group is attached to an i p -hybridized carbon and ionizes to a greater extent than one that is attached to an -hybridized carbon. Remember, carbon becomes more electron-withdrawing as its s character increases. 

As we ve seen on a number of occasions, bonds to i p -hybridized carbon are shorter than those to sp -hybridized carbon, and the case of phenols is no exception. The carbon-oxygen bond distance in phenol is slightly less than that in methanol. 

The broad, intense absorption at 3300 cm is attributable to a hydroxyl group. Although both phenol and benzyl alcohol are possibilities, the peaks at 2800-2900 cm reveal the presence of hydrogen bonded to i p -hybridized carbon. All carbons are xp -hybridized in phenol. The infrared spectrum is that of benzyl alcohol. 

The carbon that bears the functional group is 5p -hybridized in alcohols and alkyl halides. Figure 4.1 illustrates bonding in methanol. The bond angles at carbon are approximately tetrahedral, as is the C—O—H angle. A similar orbital hybridization model applies to alkyl halides, with the halogen coimected to i-p -hybridized carbon by a ct bond. Carbon-halogen bond distances in alkyl halides increase in the order C—F (140 pm) < C—Cl (179 pm) < C—Br (197 pm) < C—1 (216 pm). 

An alkyl radical is neutral and has one more electron than the corresponding carbo-cation. Thus, bonding in methyl radical may be approximated by simply adding an electron to the vacant 2p orbital of i p -hybridized carbon in methyl cation (Figure 4.17a). Alternatively, we could assume that carbon is i p -hybridized and place the unpaired electron in an sp orbital (Figure A.llb). 

The reactions tolerate many functional groups, including OH, C=0, and NO2 elsewhere in the molecule and as shown in the Heck example, the stereochemistry of the double bond is retained when cross-coupling involves an i p -hybridized carbon. 

Acetylene is linear and alkynes have a linear geometry of their X—C=C—Y units. The carbon-carbon triple bond in alkynes is composed of a CT and two tt components. The triply bonded carbons are sp-hybridized. The a component of the triple bond contains two electrons in an orbital generated by the overlap of i p-hybridized orbitals on adjacent carbons. Each to these carbons also has two 2p orbitals, which overlap in pairs so as to give two tt orbitals, each of which contains two electrons. 

Carbon and silicon are both i p -hybridized. The C—Si bond involves overlap of a half-filled sp orbital of carbon with a half-filled sp hybrid orbital of silicon. The C—H and Si—H bonds involve hydrogen li orbitals and sp hybrid orbitals of C and Si, respectively. The principal quantum number of the valence orbitals of silicon is 3. 

Decomposition of organonickei compounds bearing two p -hybridized carbon moieties is known to cause a ligand coupling reaction. The arylalkenyl-nickel compound (CXL), on thermolysis or treatment with bromine, affords the ligand coupling product (Miller et al., 1968) (see also Section I,A). 

Although Sn2 reactions are stereospecific and proceed with inversion of configuration at carbon, the situation is not as clear-cut for S l. When the leaving group departs from a chirality center of an optically active halide, the positively charged carbon that results is i-p -hybridized and cannot be a chirality center. The three bonds to that carbon define a plane of symmetry. 

Section 10.7 Conjugated dienes aie stabilized by election delocalization to the extent of 12-16 kJ/mol (3 kcal/mol). Overlap of the p orbitals of four adjacent 5/i -hybridized carbons in a conjugated diene gives an extended tt system through which the electrons are delocalized. 

The valence atomic orbitals which are available to form the orbitals of a CC single bond, directed along the x axis, are the 2s and 2px atomic orbitals on each carbon atom. Their admixture—in proportions which depend on the number of neighbors at each carbon and on the subsequent hybridization—creates two (s, p ) hybrids on each atom. One of these hybrids points away from the other atom and can be used for bonding to additional atoms. The pair of hybrids which point at each other overlap and interact in the conventional fashion [we symbolize the non-interacting orbitals by an interruption of the bond axis (Fig. 1)]. The two bond orbitals which are formed in this manner both have [Pg.3]

Methylene is the simplest example of a carbene, a molecule containing a carbon formally bearing only six valence electrons. Of these, four electrons are involved in the C-H bonds. The orbital occupation of the last two electrons defines the specific electronic state of methylene. If we assume a bent structure, we can use the simple model of an sp -hybridized carbon. The four bonding electrons occupy two of these sp hybrids. This leaves the third sp hybrid (3ai) and the p-orbital (l i) available for the last two electrons (see Figure 5.1). Placing one electron in each of these orbitals with their spins aligned creates a triplet state. The electronic configuration of this triplet state is... 

A 1,2,4-triphosphapentadienide anion from 2,4,6-trimethylbenzoyl chloride [36] A subsequent nucleophilic attack of the negatively polarized sp -hybridized carbon of 3-phenyl-1,3-bis(trimethylsilyl)-1,2-diphosphapropenide at phosphorus, i.e., the positively polarized atom in the P group of phenylmethylidynephosphane, and an attendant 1,3-shift of the trimethylsilyl group fi-om carbon in position 3 to carbon in position 5 should result in the formation of the open chained 3,5-diphenyl-l,5-bis(trimethylsilyl)-l,2,4-triphosphapentadienide anion (Eq. 14). 

The electronic structure of benzyne, shown in Fipire 16.2C. is that sf a highly distorted alkyno. Although a typical alkyne triple bond uses sp hybridized carbon atoms, the bt rtiByne triple bond uses sp hyhi idized tar-honSs Furthermore, a typical alKyne triple bnnd has two mutually perpendicular n bonds formed by p-p overlap, but the benzyne triple bond Ims one w bond foTinwl by p-p uv( >rUp and o fte ir bond I ormed by. p -4ip overlap The latter w bond is in the plane of the- ring and U very weak. 

As mentioned in the introduction, the wave function of CO may be approximated as a superposition of three resonance structures (see Fig. la). In the example of Cr(CO)6, we discussed the bonding on the basis of structure I. The only orbitals which seem to incorporate structures II and III are the delocalized double bond resonance structures may be important in describing the bonding in such molecules. In our recent study of the PtCO molecule we have found that besides the Pt-CO dative bond VBO structure, the Pt=CO structures shown schematically in Fig. 5a contribute quite significantly. The SOPP orbitals for one of the double bond resonance structures are shown in Fig. 5. The bonds between the d,s,p-hybrids on Pt and the hybrids on the carbon atom are seen to be bent. 

I he clichlorocarbene carbon atom is 5/-> -liybridizecl, with a vacant p orbital extending above and below the plane of the three atoms and with an unshared jiair of elections occupying the third sp- lobe. Note that this electronic description of didilorocciibene is similar to that for a carhocation (Section 6.9) with respect to both the s/j- hybridization of carbon and the vacant p orbital. Electrostatic potential maps fuither show this similarity (l-igure 7.6). 

FIGURE 6 Membership functions of different a-environments for multiplicity 2 and sp hybridization state. Reprinted from Analitica Chim. Acta, 298, I. P. Bangov, I. Laude, D. Cabrol-Bass, Combinatorial pmblems in the treatment of fuzzy C NMR spectra in the process of computer aided structure elucidation Estimation of the carbon atom hybridization and a-environ-ment states, pp. 33-52, (1994) with kind permission of Elsevier Science-NL, Sara Burgerhart-straat 25, 1055 KV Amsterdam, The Netherlands. 

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