Bond length, alkanes hybridization

The simplest alkane is methane, CH4. Methane is perfectly tetrahedral, with the 109.5° bond angles predicted for an. sy 3 hybridized carbon. Four hydrogen atoms are covalently bonded to the central carbon atom, with bond lengths of 1.09 A. 

The effect of introducing /j -hybridized atoms into acyclic molecules was discussed in Section 2.2.1, and it was noted that torsional barriers in 1-alkenes and aldehydes are somewhat smaller than in alkanes. Similar effects are seen when sp centers are incorporated into six-membered rings. Whereas the energy barrier for ring inversion in cyclohexane is 10.3 kcal/mol, it is reduced to 7.7 kcal/mol in methylenecy-clohexane ° and to 4.9 kcal/mol in cyclohexanone. The conformation of cyclohexene is described as a half-chair. Structural parameters determined on the basis of electron diffraction and microwave spectroscopy reveal that the double bond can be accommodated into the ring without serious distortion. The C(l)—C(2) bond length is 1.335 A, and the C(l)-C(2)-C(3) bond angle is 123°. The substituents at C(3) and C(6) are tilted from the usual axial and equatorial directions and are referred to as pseudoaxial and pseudoequatorial. 

As illustrated, sp orbitals have more s character than the other hybridized atomic orbitals, and therefore, sp orbitals more closely resemble s orbitals. Compare the shapes of the hybridized atomic orbitals, and note that the electron density of an sp orbital is closest to the nucleus (much like an s orbital). As a result, a —H bond will be shorter than other C—H bonds. Since it has the shortest bond length, it will therefore be the strongest bond. In contrast, the Csp —bond has the longest bond length, and is therefore the weakest bond. Compare the spectra of an alkane, an alkene, and an alkyne ... 

The same relationship between torsional angle and vicinal coupling constant holds for substituted alkanes if appropriate values are used for J and These limiting values depend on the electronegativity and orientation of substituents, the hybridization of carbon atoms, bond lengths, and bond angles. 

Note first that the C-C bond is more than 50% stronger than the Si-Si bond. This is due to the increased bond length and therefore less effective overlap of sp -hybrid orbitals to form the (t bonds in the silanes. (The C-C internuclear distance is 1.54 A, whereas the Si-Si distance is 2.34 A, some 50% longer.) Second, C-H bonds are about 40% stronger than the corresponding Si-H bonds, again due to less effective orbital overlap in the latter. These shorter and therefore stronger C-C and C-H bonds are certainly major factors in the enhanced stability of the chains found in the alkanes as compared with the silanes. 

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... 

---