Carbon sp2 hybridization

The vinyl H2C=CH radical can be produced by cleavage of a C-H bond in ethene, and has been studied in the gas phase. The unpaired electron clearly occupies a carbon sp2 hybrid orbital, to lapse into the language of descriptive organic chemistry, but there are regions of space where the /1-spin electrons have... 

Know the meaning of trigonal carbon, sp2 hybridization, restricted rotation, a and n bonds, cis and trans double-bond isomers. 

Two of these carbon sp2 hybrids are used to form CH bonds, so one sp2 and one pure p orbital remain to bind to the neighboring carbons. The sp2 and p orbitals are orthogonal so, to a first approximation, the six MOs of the cyclopropane skeleton can de divided into two groups of linear combinations of the sp2 and p orbitals, respectively (Figure 7.4). 

The second situation referred to above, viz systems containing conjugated double bonds, is perhaps more important to the present discussion. The classical example of such a system is benzene. The molecular orbital treatment regards the six G—G bonds and the six G—H bonds as completely localized molecular orbitals compounded out of carbon sp2 hybrid atomic orbitals and the hydrogen s orbital. So far the treatment is identical with the electron pair theory, discussed in Chapter 4. The G—G bonds will be or bonds formed by the overlap of two sp2 hybrid atomic orbitals, one from each carbon atom and the C—H bonds will also be a bonds formed by the overlap of one sp2 hybrid atomic orbital of carbon with the s atomic orbital of hydrogen. The six carbon 2p atomic orbitals that remain will form completely non-localized molecular orbitals. Thus each 2pt electron will be regarded as existing in the field of six nuclei and will possess a wave function of the form ... 

Overlap of carbon sp2 hybrid orbitals with hydrogen Is orbitals to produce the ethylene a bonded framework. (The axes have been changed for the sake of clarity—i.e. the z axis would be out of the plane of the paper.)... 

Carbon-hydrogen framework in benzene produced by overlap of carbon sp2 hybrid orbitals with hydrogen Is orbitals. All bonds are or bonds. 

Figure 1.13 An sp hybridized carbon. The three equivalent sp2 hybrid orbitals (green) lie in a plane at angles of 120° to one another, and a single unhybridized p orbital (red/blue) is perpendicular to the sp2 plane.

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. 

When two sp2-hybridized carbons approach each other, they form a cr bond by sp2-sp2 head-on overlap. At the same time, the unhybridized p orbitals approach with the correct geometry for sideways overlap, leading to the formation of what is called a pi (ir) bond. The combination of an >p2-sp2 a bond and a 2p-2p 77 bond results iii the sharing of four electrons and the formation of a carbon-carbon double bond (Figure 1.14). Note that the electrons in then-bond occupy the region centered between nuclei, while the electrons in the 77 bond occupy regions on either side of a line drawn between nuclei. 

Like the carbon atoms in ethylene, the carbon atom in formaldehyde is in a double bond and therefore sp2-hybridized. 

Resonance forms differ only in the placement of their tt or nonbonding electrons. Neither the position nor the hybridization of any atom changes from one resonance form to another. In the acetate ion, for example, the carbon atom is sp2-hybridized and the oxygen atoms remain in exactly the same place in both resonance forms. Only the positions of the r electrons in the C=0 bond and the lone-pair electrons on oxygen differ from one form to another. This movement of electrons from one resonance structure to another can be indicated by using curved arrows. A curved arrow always indicates the movement of electrons, not the movement of atoms. An arrow shows that a pair of electrons moves from the atom or bond at the tail of the arrow to the atom or bond at the head of the arrow. 

The dichlorocarbene carbon atom is syj- -hybridized, with a vacant p orbital extending above and below the plane of the three atoms and with an unshared pair of elections occupying the third sp2 lobe. Note that this electronic description of dichlorocarbene is similar to that for a carbocation Section 6.9) with respect to both the sp2 hybridization of carbon and the vacant p orbital. Electrostatic potential maps further show this similarity (Figure 7.6). 

To understand why a racemic product results from the reaction of T120 wjtl 1-butene, think about the reaction mechanism. 1-Butene is first protonaled tc yield an intermediate secondary (2°) carbocation. Since the trivalent carbon i sp2-hybridized and planar, the cation has no chirality centers, has a plane o symmetry, and is achiral. As a result, it can react with H20 equally well fron either the top or the bottom. Reaction from the top leads to (S)-2-butano through transition state 1 (TS 1) in Figure 9.15, and reaction from the bottorr leads to R product through TS 2. The two transition states are mirror images. The] therefore have identical energies, form at identical rates, and are equally likeb to occur. 

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. 

To see why allylic radicals are so stable, look at the orbital picture in Figure 10.3. The radical carbon atom with an unpaired electron can adopt sp2 hybridization, placing the unpaired electron in a p orbital and giving a structure that is electronically symmetrical. The p orbital on the central carbon can therefore overlap equally well with a p orbital on either of the two neighboring carbons. 

In the Diels-Alder transition state, the two alkene carbons and carbons 1 and 4 of the diene rehybridize from sp2 to sp 5 to form two new single bonds, while carbons 2 and 3 of the diene remain sp2-hybridized to form the new double bond in the cyclohexene product. We ll study this mechanism at greater length in Chapter 30 but will concentrate for the present on learning more about the characteristics and uses of the Diels-Alder reaction. 

Further evidence for the unusual nature of benzene is that all its carbon-carbon bonds have the same length—139 pm—intermediate between typical single (154 pm) and double (134 pm) bonds. In addition, an electrostatic potential map shows that the electron density in all six carbon-carbon bonds is identical. Thus, benzene is a planar molecule with the shape of a regular hexagon. All C-C—C bond angles are 120°, all six carbon atoms are sp2-hybridized. and each carbon has a p orbital perpendicular to the plane of the six-membered ring. 

Pyrrole (two r s, one /) and imidazole are /ive-membered heterocycles, yet both have six tt electrons and are aromatic. In pyrrole, each of the four. sp2-hybridized carbons contributes one tt electron, and the sp2-hybridized nitrogen atom contributes the two from its lone pair, which occupies a p orbital (Figure 15.9). Imidazole, also shown in Figure 15.9, is an analog of pyrrole that has two nitrogen atoms in a five-membered, unsaturated ring. Both nitrogens are sp2-hybridized, but one is in a double bond and contributes only one electron to the aromatic tt system, while the other is not in a double bond and contributes two from its lone pair. 

The electronic structure of benzyne, shown in Figure 16.19, is that of a highly distorted alkyne. Although a typical alkyne triple bond uses sp-hybridized carbon atoms, the benzyne triple bond uses sp2-hybridized carbons. Furthermore, a typical alkyne triple bond has two mutually perpendicular it bonds formed bv p-p overlap, but the benzyne triple bond has one tt bond formed by p-p overlap and one tt bond formed by sp2 sp2 overlap. The latter tt bond is in the plane of the ring and is very weak. 

Figure 16.19 An orbital picture and electrostatic potential map of benzyne. The benzyne carbons are sp2-hybridized, and the "third" bond results from weak overlap of two adjacent sp2 orbitals.

Carboxylic acids are similar in some respects to both ketones and alcohols. Like ketones, the carboxyl carbon is sp2-hybridized, and carboxylic acid groups are therefore planar with C-C=0 and 0=C-0 bond angles of approximately 120° (Table 20.2). 

A conjugated polyene, as we saw in Section 14.1, is one with alternating double and single bonds. According to molecular orbital (MO) theory, the p orbitals on the sp2-hybridized carbons of a conjugated polyene interact to form a set of... 

You may recall that we discussed the bonding in ethene in Chapter 7. The double bond in ethene and other alkenes consists of a sigma bond and a pi bond. The ethene molecule is planar. There is no rotation about the double bond, since that would require breaking the pi bond. The bond angle in ethene is 120°, corresponding to sp2 hybridization about each carbon atom. The geometries of ethene and the next member of the alkene series, QHg, are shown in Figure 22.6. 

The first example of asymmetric catalytic ring-opening of epoxides with sp2-hybridized carbon-centered nucleophiles was reported by Oguni, who demonstrated that phenyllithium and a chiral Schiff base ligand undergo reaction to form a stable system that can be used to catalyze the enantioselective addition of phenyllithium to meso-epoxides (Scheme 7.24) [48]. Oguni proposed that phenyllithium... 

Intramolecular Arylation of Aromatic and Alkene sp2-Hybridized Carbon Atoms 261... 

Klasinc and Schulte-Frohlinde (1968), who also used HMO for the investigation of substituted benzene diazonium ions, were the first to realize that the remarkable distortion of bond lengths and angles of the benzene ring by the diazonio group (Sec. 4.2) has a significant influence on its reactivity. They found that the sp2-hybridization at the carbon atoms of the benzene ring varied from 1.75 for C(l) to 2.16 for C(2). 

C-coupling is of outstanding importance in the azo coupling reaction for the synthesis of azo dyes and pigments. An aromatic or heteroaromatic diazonium ion reacts with the so-called coupling component, which can be an aromatic primary, secondary, or tertiary amine, a phenol, an enol of an open-chain, aromatic, or heteroaromatic carbonyl compound, or an activated methylene compound. These reactions at an sp2-hybridized carbon atom will be discussed in Chapter 12. In the... 

Alkynes can also be arylated by the Meerwein procedure, as shown by Muller in 1949. The reaction of buta-l-en-3-yne (Scheme 10-47) was studied by Kheruze and Petrov (1960). Arylation at an sp2-hybridized carbon is obviously considerably faster than the analogous reaction at an sp-hybridized carbon. A mechanistically interesting case of a Meerwein reaction with phenylacetylene will be discussed later in this section. 

---