Hybridization and Bonding in Ethane

PROBLEM 1.21 Describe the bonding in methylsilane (HsCSiHs), assuming that it is analogous to that of ethane. What is the principal quantum number of the orbitals of silicon that are hybridized  

The orbital hybridization model of bonding is not limited to compounds in which all the bonds are single, but can be adapted to compounds with double and triple bonds, as described in the following two sections. 

The C—H and C—C bond distances in ethane are 111 and 153 pm, respectively, and the bond angles are close to tetrahedral. 

FIGURE 1.23 Orbital overlap description of the sp -sp CT bond between the two carbon atoms of ethane. 

The orbital hybridization model of covalent bonding is readily extended to carbon-carbon bonds. As Fignre 1.23 illustrates, ethane is described in terms of a carbon-carbon O bond joining two CH3 (methyl) groups. Each methyl group consists of an -hybridized carbon attached to three hydrogens by o bonds. Overlap of the 

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Introduction to Alkanes Methane, Ethane, and Propane 57 sp Hybridization and Bonding in Methane 58... 

Methane and the Biosphere 59 Bonding in Ethane 61 sp Hybridization and Bonding in Ethylene 61... 

The pATa of the hydrocarbon ethane is about 50, that of ethylene about 44, and that of acetylene is about 25. The hybridization of the C-H bond in ethane is sp (25% i character), in ethylene it... 

X-ray crystallographic analysis indicated that benzene is a planar, regular hexagon in which all the carbon-carbon bond lengths are 139 pm, intermediate between the single C-C bond in ethane (154 pm) and the C=C bond in ethene (134 pm), and therefore all have some double bond character. Thus the representation of benzene by one Kekule structure is unsatisfactory. The picture of benzene according to valence bond theory is a resonance hybrid of the two Kekule or canonical forms 4 and 9, conventionally shown as in Figure 1.2, and so each carbon-carbon bond apparently has a bond order of 1.5. 

Let s now use the principles of hybridization to determine the type of bonds in ethane, ethylene, and acetylene. 

Figure 11.9 The cr bonds in ethane (CaHe). A, Two sp hybridized C atoms and six H atoms in ethane form one C—C a bond and six C—H...

In the preceding section we have discussed the double bond in ethylene and the triple bond in acetylene as involving two or three bent bonds. These bent bonds are described as involving tetrahedral orbitals of each of the two carbon atoms. Each bent bond is similar to the carbon-carbon single bond in ethane, H3C—CHg, except that it is bent. There is another description of these molecules that has become popular in recent years. This description involves use of hybrid orbitals different from the tetrahedral sp hybrid orbitals described above and shown in Figure 6-8. 

At 146 pm the C 2—C 3 distance m 1 3 butadiene is relatively short for a carbon-carbon single bond This is most reasonably seen as a hybridization effect In ethane both carbons are sp hybridized and are separated by a distance of 153 pm The carbon-carbon single bond m propene unites sp and sp hybridized carbons and is shorter than that of ethane Both C 2 and C 3 are sp hybridized m 1 3 butadiene and a decrease m bond distance between them reflects the tendency of carbon to attract electrons more strongly as its s character increases... 

Structure. Ethylene is a planar molecule with a carbon—carbon bond distance of 0.134 nm, which is shorter than the C—C bond length of 0.153 nm found in ethane. The C—H bond distance is 0.110 nm, and the bond angles are [Pg.432]

Although it is rather certain that electrostatic interactions of polar groups, steric hindrance, and partial double bond character due to conjugation will all be of importance in selected molecules, the explanation of the barrier in ethane probably requires something else. Though far from being proven and certainly not now useful for prediction, the idea that the ethane barrier arises from repulsion of C—H bond orbitals on the carbons, due to their being more concentrated than sp hybrids, seems the most plausible picture available. 

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