Carbon sp3 hybridization

I jfpr example, descriptive organic chemistry emphasizes the carbon sp3 hybrid imic orbitals at the expense of the more usual 2s, 2px, 2py and 2p, ones. 

FIGURE 7.7 The bonding in methane. Each of the four C-H bonds results from head-on (singly occupied carbon sp3 hybrid orbital with a singly occupied... 

Tetrahedral displacement of carbon sp3 hybrid orbitals which overlap withhydrogen Is orbitals. 

The structure of ethane. The carbon-carbon bond is formed by a- overlap of two carbon sp3 hybrid orbitals. (For clarity, the smaller lobes of the sp3 hybrid orbitals are not shown.)... 

The concept of hybridization explains how carbon forms four equivalent tetrahedral bonds but not why it does so. The shape of the hybrid orbital suggests the answer. When an 5 orbital hybridizes rvith three p orbitals, the resultant sp3 hybrid orbitals are unsyimmetrical about the nucleus. One of the two... 

Although sp3 hybridization is the most common electronic state of carbon, it s not the only possibility. Look at ethylene, C2H4, for example. It was recognized more than 100 years ago that ethylene carbons can be tetravalent only if they share four electrons and are linked by a double bond. Furthermore, ethylene is planar (flat) and has bond angles of approximately 120° rather than 109.5°. 

When we discussed sp3 hybrid orbitals in Section 1.6, we said that the four valence-shell atomic orbitals of carbon combine to form four equivalent sp3 hybrids. Imagine instead that the 2s orbital combines with only two of the three available 2p orbitals. Three sp2 hybrid orbitals result, and one 2p orbital remains unchanged- The three sp2 orbitals lie in a plane at angles of 120° to one another, with the remaining p orbital perpendicular to the sp2 plane, as shown in Figure 1.13. 

Like the carbon atom in methane and the nitrogen atom in methylamine, the oxygen atom in methanol (methyl alcohol) and many other organic molecules can also be described as sp3-hybridized. The C-O-H bond angle in methanol is 108.5°, very close to the 109.5° tetrahedral angle. Two of the four sp3 hybrid... 

Sulfur is most commonly encountered in biological molecules either in compounds called thiols, which have a sulfur atom bonded to one hydrogen and one carbon, or in sulfides, which have a sulfur atom bonded to two carbons. Produced by some bacteria, methanethiol (CH3SH) is the simplest example of a thiol, and dimethyl sulfide [(ChP S l is the simplest example of a sulfide. Both can be described by approximate sp3 hybridization around sulfur, although both have significant deviation from the 109.5° tetrahedral angle. 

In addition to compounds with planar, sp2-hybridized carbons, compounds with tetrahedral, sp3-hybridized atoms can also be prochiral. An vp3-hybridizec) atom is said to be a prochirality center if, by changing one of its attached groups, it becomes a chirality center. The —GH2OH carbon atom of ethanol, for instance, is a prochirality center because changing one of its attached -H atoms converts it into a chirality center. 

Alkyl halides contain a halogen bonded to a saturated, sp3-hybridized carbon atom. The C-X bond is polar, and alkyl halides can therefore behave as electrophiles. 

Another trend is that sp3-hybridized carbons generally absorb from 0 to 90 8, while sp2 carbons absorb from 110 to 220 8. Carbonyl carbons (C=0) are... 

Table 13.3 shows the correlation of H chemical shift with electronic environment in more detail. In general, protons bonded to saturated, sp3-hybridized carbons absorb at higher fields, whereas protons bonded to s/ 2-hybridized carbons absorb at lower fields. Protons on carbons that are bonded to electronegative atoms, such as N, O, or halogen, also absorb at lower fields. 

Like a carbonyl group, a nitrile group is strongly polarized and has an electrophilic carbon atom. Nitriles therefore react with nucleophiles to yield 5p2-hybridized imine anions in a reaction analogous to the formation of an sp3-hybridized alkoxide ion by nucleophilic addition to a carbonyl group. 

Earlier in this section we made reference to the azo coupling of triphenylphos-phonium cyclopentadienylide (12.109). Makhailov et al. (1984) found that penta-methylcyclopentadiene (12.121) reacts with mono-, di-, and trinitrobenzenediazo-nium salts to give a mixture of 12.122 and 12.123, formed by arylazo substitutions at the sp3-hybridized carbon of the five-membered ring and at the methyl group attached to the same carbon atom, respectively. This is definitely not a classical azo coupling ... 

On the other hand, the rate constant k does not depend on the changing steric influence of substitutents in the 8-position, but correlates surprisingly well with the Hammett-Brown constant cr. This result indicates that the formation of an sp3-hybridized carbon atom (at the 1-position of the o-complex) leads to a compound without significant steric interaction of the electrophile with substituents in the 8-position. The o-complex cannot be planar and is asymmetric. The preferred conformation of a o-complex of this type is illustrated in Figure 12-6. The pseudoax-ial position of the electrophile E reduces the steric interaction between this group and the peri substituent R. 

The 13C NMR data for representative three-membered sulfones and sulfoxides are given in Table 5. The chemical shifts of the sp3-hybridized a-carbon in the parent thiirane70 and the five-membered ring86 sulfide, sulfoxide and sulfone are 18.1, 31.7, 54.3 and 51.1, respectively, whereas those of cyclopropenone, diphenylcyclopropenone and dimethylcyclopropenone are 169.087, 148.788 and 157.9, respectively. 

In diamond, each carbon atom is sp3 hybridized and linked tetrahedrally to its four neighbors, with all electrons in C C cr-bonds (Fig. 14.30). Diamond is a rigid, transparent, electrically insulating solid. It is the hardest substance known and the best conductor ol heat, being about five times better than copper. These last two properties make it an ideal abrasive, because it can scratch all other substances, yet the heat generated by friction is quickly conducted away. 

In diamond, carbon is sp hybridized and forms a tetrahedral, three-dimensional network structure, which is extremely rigid. Graphite carbon is sp2 hybridized and planar. Its application as a lubricant results from the fact that the two-dimensional sheets can slide across one another, thereby reducing friction. In graphite, the unhybridized p-electrons are free to move from one carbon atom to another, which results in its high electrical conductivity. In diamond, all electrons are localized in sp3 hybridized C—C cr-bonds, so diamond is a poor conductor of electricity. 

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