Carbon sp3 orbital

At this point in our studies we favor a five-membered cyclic transition state, quite analogous to the four-center transition state (Reaction 5), except that no o- bonds of the attacking electrophile are broken. The fourth bonding center is the carbon sp3 orbital as in structures 25 and 26. 

Jensen and Rickborn (2) discussed the possibility of transition states in which the electrophilic species was partially bound to the o- orbital of carbon—mercury bonds. Thus the fourth center of the four-center bromination mechanism was pictured as the carbon-mercury bond itself, or more specifically, the carbon sp3 orbital (2). 

We saw in Section 1.8 that the carbon-carbon single bond in ethane results from cr (head-on) overlap of carbon sp3 orbitals. If we imagine joining three, four, five, or even more carbon atoms by C-C single bonds, we can generate the large family of molecules called alkanes. 

We know from Sections 1.7 and 1.8 that an sp3 hybridized carbon atom has tetrahedral geometry and that the carbon-carbon bonds in alkanes result from a overlap of carbon sp3 orbitals. Let s now look into the three-dimensional consequences of such bonding. What are the spatial relationships between the hydrogens on one carbon and the hydrogens on a neighboring carbon We ll see in later chapters that an understanding of these spatial relationships is often crucial for understanding chemical behavior. 

What accounts for the stability of conjugated dienes According to valence bond theory (Sections 1.5 and 1.8), the stability is due to orbital hybridization. Typical C—C bonds like those in alkanes result from a overlap of 5p3 orbitals on both carbons. In a conjugated diene, however, the central C—C bond results from conjugated diene results in part from the greater amount of s character in the orbitals forming the C-C bond. 

The beautiful Bohr atomic model is, unfortunately, too simple. The electrons do not follow predetermined orbits. Only population probabilities can be given, which are categorized as shells and orbitals. The orbitals can only accommodate two electrons. Shells and orbitals can also merge ("hybridization"). In the case of carbon, the 2s orbital and the three 2p orbitals adopt a configuration in the shape of a tetrahedron. Each of these sp3 orbitals is occupied by one electron. This gives rise to the sterically directed four-bonding ability of carbon. 

Figure 3.19 Bent-bond representation of the double bond in ethene. The overlap of sp3 orbitals on each carbon atom produces to bend bond (r) orbitals.

The H atom flanked by the two 0=0 groups in (22) exhibits hardly any more acidic character than the analogous one in the corresponding hydrocarbon. The different behaviour of (22) stems from the fact that after proton removal, the carbanion s lone pair would be in an sp3 orbital more or less at right angles to the p orbitals on each of the adjacent carbonyl carbon atoms (cf. p. 259) no sp3/p overlap could thus take place, consequently there would be no stabilisation of the -ve charge through delocalisation, and the (unstabilised) carbanion does not, therefore, form. 

The four sp3 orbitals should be oriented at angles of 109.5° with respect to each other => an sp -hybridized carbon gives a tetrahedral structure for methane. 

The sp3 orbitals of the carbon atoms cannot overlap as effectively as they do in alkane (where perfect end-on overlap is possible). 

In some atoms, the p and s orbitals are mixed together to form several equivalent, hybridized orbitals. The most common example is carbon, where there are four orbitals that are formed by mixing one s orbital with three p orbitals to give four equivalent orbitals designated as sp3 orbitals. 

In this case since carbon has only two unpaired electrons, it seems likely that it will only form only two covalent bonds, but it is known that carbon can form four covalent bonds. To form four bonds, one electron is promoted from the 2s orbital to the 2pz orbital. Then the one 2s orbital and three 2p orbitals mix together to form four new sp3 hybrid orbitals as shown in Figure 5. So in this case of hybridization, three p and one s orbital combine to give four identical sp3 orbitals. 

The carbon atom (6C) has the electron configuration of ls22s22p2. There are 4 valence electrons, of which only two are unpaired in the ground state. During the formation of carbon compounds, one 2s and three 2p orbitals combine to give four identical sp3 orbitals by the promotion of an electron from the 2s orbital to a 2p orbital. These 4 unpaired orbitals then mix to form four identical sp3 hybrid orbitals. 

The cartoon-like drawing of the structure of the parent bicylobutonium ion C4H7+ 36 is adopted from an ingenious forward-looking paper of Olah and coworkers in 1972, 61) long before routine 13C-FT-NMR spectroscopy and routine ab initio quantum chemical calculations were available, which envisaged correctly the stabilization mode of the parent bicyclobutonium ion to arise from the interaction of the backside lobe of the Cy-Hendo sp3 orbital with the empty carbenium carbon p-orbital at Ca. 

There are two possible structures for simple alkyl radicals.176 They might have sp2 bonding, in which case the structure would be planar, with the odd electron in a p orbital, or the bonding might be sp3, which would make the structure pyramidal and place the odd electron in an sp3 orbital. Esr spectra of CH3 and other simple alkyl radicals as well as other evidence indicate that these radicals have planar structures.177 This is in accord with the known loss of optical activity when a free radical is generated at a chiral carbon.178 In addition, electronic spectra of the CH3 and CD3 radicals (generated by flash photolysis) in the gas phase have definitely established that under these conditions the radicals are planar or near-planar.179 Ir spectra of CH3 trapped in solid argon led to a similar conclusion.180... 

The overlap of carbon p orbitals with arsenic d orbitals is less effective than with the d orbitals of phosphorus, and so the covalent canonical structure is expected to make less of a contribution to the hybrid structure. This has been confirmed in an X-ray study of 2-acetyl-3,4,5-triphenylcyclopentadienetriphenylarsorane.6 Yamamoto and Schmidbaur7 found (13CNMR) that the bonding in arsenic ylides was probably sp3 (cf. phosphorus, which changes from sp3—>sp2), resulting in arsenic pseudotetrahedral geometry (cf. phosphorus ylides, which are planar). 

All four sp3 orbitals are of equal energy. Therefore, according to Hund s rule (Section 1.1) the four valence electrons of carbon are distributed equally among them, making four half-filled orbitals available for bonding. 

The axes of the sp3 orbitals point toward the corners of a tetrahedron. Therefore, sp3 hybridization of carbon is consistent with the tetrahedral structure of methane. Each C—H bond is a a bond in which a half-filled Is orbital of hydrogen overlaps with a half-filled sp3 orbital of carbon along a line drawn between them. 

FIGURE 2.8 sp3 Hybridization (a) Electron configuration of carbon in its most stable state, (b) Mixing the s orbital with the three p orbitals generates four sp3 hybrid orbitals. The four sp3 hybrid orbitals are of equal energy therefore, the four valence electrons are distributed evenly among them. The axes of the four sp3 orbitals are directed toward the corners of a tetrahedron. 

FIGURE 2.10 The C—C a bond in ethane, pictured as an overlap of a half-filled sp3 orbital of one carbon with a half-filled sp3 hybrid orbital of the other. 

FIGURE 4.19 Bonding w in methyl radical, (a) If the structure of the CH3 radical is planar, then carbon is sp2-hybridized with an unpaired electron in 2p orbital. (ft) If CH3 is pyramidal, then carbon is sp3-hybridized with an electron in sp3 orbital. Model (a) is more consistent with experimental observations. 

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