Hybridization in methane

These bond angles are consistent with the hybridization of the atomic orbitals of the carbon atom involved in the bonds sp hybridization in methane, sp hybridization in ethene (see... 

Section 2.6 Bonding in methane is most often described by an orbital hybridization model, which is a modified for m of valence bond theory. Four equivalent sp hybrid orbitals of carbon are generated by mixing the 2s, 2p 2py, and 2p orbitals. Overlap of each half-filled sp hybrid orbital with a half-filled hydrogen I5 orbital gives a a bond. 

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... 

We are now ready to account for the bonding in methane. In the promoted, hybridized atom each of the electrons in the four sp3 hybrid orbitals can pair with an electron in a hydrogen ls-orbital. Their overlapping orbitals form four o-bonds that point toward the corners of a tetrahedron (Fig. 3.14). The valence-bond description is now consistent with experimental data on molecular geometry. 

FIGURE 3.14 Each C H bond in methane is formed by the pairing of an electron in a hydrogen U-orbital and an electron in one of the four sp hybrid orbitals of carbon. Therefore, valence-bond theory predicts four equivalent cr-bonds in a tetrahedral arrangement, which is consistent with experimental results. 

The reaction enthalpy and thus the RSE will be negative for all radicals, which are more stable than the methyl radical. Equation 1 describes nothing else but the difference in the bond dissociation energies (BDE) of CH3 - H and R - H, but avoids most of the technical complications involved in the determination of absolute BDEs. It can thus be expected that even moderately accurate theoretical methods give reasonable RSE values, while this is not so for the prediction of absolute BDEs. In principle, the isodesmic reaction described in Eq. 1 lends itself to all types of carbon-centered radicals. However, the error compensation responsible for the success of isodesmic equations becomes less effective with increasingly different electronic characteristics of the C - H bond in methane and the R - H bond. As a consequence the stability of a-radicals located at sp2 hybridized carbon atoms may best be described relative to the vinyl radical 3 and ethylene 4 ... 

There is also a third type of reactive species that we shall discuss in detail in Chapter 9, namely radicals. Briefly, radicals are uncharged entities that carry an unpaired electron. A methyl radical CH3 results from the fission of a C-H bond in methane so that each atom retains one of the electrons. In the methyl radical, carbon is sp hybridized and forms three CT C-H bonds, whilst a single unpaired electron is held in a 2/ orbital oriented at right angles to the plane containing the ct bonds. The unpaired electron is always shown as a dot. The simplest of the radical species is the other fission product, a hydrogen atom. 

Alkanes have only -hybridized carbons. The conformation of alkanes is discussed in Chapter 3 (see Section 3.2.2). Methane (CH4) is a nonpolar molecule, and has four covalent carbon-hydrogen bonds. In methane, aU four C—H bonds have the same length (1.10 A), and all the bond angles (109.5°) are the same. Therefore, all four covalent bonds in methane are identical. Three different ways to represent a methane molecule are shown here. In a perspective formula, bonds in the plane of the paper are drawn as solid hues, bonds sticking out of the plane of the paper towards you are... 

Fig. 11-3.1. A set of vectors v1( va, V , and v4 representing the four o hybrid orbitals used by carbon to bond the four hydrogens in methane.

The first three geometries involve the tetrahedral, trigonal, and digonal hybrids discussed above and the fourth involves the use of pure s and p orbitals as discussed on page 149. The last structure contains three equivalent bonds at mutual angles of 60 and a fourth bond at an angle of approximately 145° to the others. U is impossible to construct s-p hybrid orbitals with angles less than 90°, and so structure V is ruled out. In this sense it may be sard that hybridization does not allow" structure V, but it may not be said that it "chooses ore of the others. Carbon hybridizes sp, sp2, and spJ in various compounds, und the choice of sp3 in methane is a result of the foot that the tetrahedral structure is the most stable possible. 

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