Methane tetrahedral arrangement

Methane, CH4, has four bonding pairs on the central atom. To be as far apart as possible, the four pairs must take up a tetrahedral arrangement around the C atom. Because the electron arrangement is tetrahedral and an H atom is attached to each bonding pair, we expect the molecule to be tetrahedral (see 1), with bond angles of 109.5°. Thar is the shape found experimentally. 

When we try to apply VB theory to methane we run into difficulties. A carbon atom has the configuration [HeJ2s22pvl2p l,1 with four valence electrons (34). However, two valence electrons are already paired and only the two half-filled 2/ -orbitals appear to be available for bonding. It looks as though a carbon atom should have a valence of 2 and form two perpendicular bonds, but in fact it almost always has a valence of 4 (it is commonly tetravalent ) and in CH4 has a tetrahedral arrangement of bonds. 

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. 

Figure 1.10 The tetrahedral, trigonal pyramidal, and angular geometries of the methane, ammonia, and water molecules based on the tetrahedral arrangement of four electron pairs.

Systematic and traditional names exist in abundance in organic chemistry. Many familiar names, such as chloroform (an organic solvent), have been replaced by names that more accurately reflect the structure of the molecule in this case, trichloromethane (CHCI3). This conveys the fact that the structure of chloroform is the same as that of methane (CH4 - a tetrahedral arrangement of four hydrogens around a central carbon), but with three of the hydrogens... 

For a final example, let s consider methane, CH with four bonding electrons surrounding the central carbon atom. VSEPR predicts a tetrahedral arrangement with bond angles of 109.5°. 

Water, ammonia, and methane share the common feature of an approximately tetrahedral arrangement of four electron pairs. Because we describe the shape of a molecule according to the positions of its atoms rather than the disposition of its electron pairs, however, water is said to be bent, and ammonia is trigonal pyramidal. 

Because valence electron octets are so common, particularly for second-row elements, the atoms in a great many molecules have shapes based on the tetrahedron. Methane, for example, has a tetrahedral shape, with H-C-H bond angles of 109.5°. In NH3, the nitrogen atom has a tetrahedral arrangement of its four charge clouds, but one corner of the tetrahedron is occupied by a lone pair, resulting in a trigonal pyramidal shape for the molecule. Similarly, H20 has two corners of the tetrahedron occupied by lone pairs and thus has a bent shape. 

The same kind of sp3 hybridization that describes the bonds to carbon in the tetrahedral methane molecule also describes bonds to nitrogen in the trigonal pyramidal ammonia molecule, to oxygen in the bent water molecule, and to all other atoms that VSEPR theory predicts to have a tetrahedral arrangement of four charge clouds. 

In the MO picture, there will be a bonding MO (and an antibonding MO) for each bond in die Lewis structure. Furthermore, the MO model must be in accord with experimental observations. Experiments have shown that the bonds in methane are all identical, with tetrahedral geometry. Therefore, methane must have four equivalent bonding MOs, with a tetrahedral arrangement. 

The molecular structure of methane. The tetrahedral arrangement of electron pairs produces a tetrahedral arrangement of hydrogen atoms. 

The simplest member of the saturated hydrocarbons, which are also called the alkanes, is methane (CH4). As discussed in Section 14.1, methane has a tetrahedral structure and can be described in terms of a carbon atom using an sp-J hybrid set of orbitals to bond to the four hydrogen atoms (see Fig. 22.1). The next alkane, the one containing two carbon atoms, is ethane (C2H6), as shown in Fig. 22.2. Each carbon in ethane is surrounded by four atoms and thus adopts a tetrahedral arrangement and sp3 hybridization, as predicted by the localized electron model. 

Compare ammonia with methane, which does not undergo inversion. The unshared pair plays the role of a carbon-hydrogen bond in determining the most stable shape of the molecule, tetrahedral. But, unlike a carbon-hydrogen bond, the unshared pair cannot maintain a particular tetrahedral arrangement the pair points now in one direction, and the next instant in the opposite direction. 

Methane and carbon tetrachloride, CCl4, have zero dipole moments. We certainly would expect the individual bonds—of carbon tetrachloride at least—to be nolar because of the very symmetrical tetrahedral arrangement, however, they exactly cancel each other out (Fig. 1.14). In methyl chloride, H CI, the polarity... 

Methane (CH4) consists of one carbon atom bonded to four hydrogen atoms in a tetrahedral arrangement. Here are four ways to represent a methane molecule. Refer to Table C-1 In Appendix C for a key to atom color convention. 

Even though the valence would be correct after promotion, the structure still would be wrong. Beryllium hydride would have two different kinds of bonds, and methane would have three identical bonds formed by overlap of H(ls) with the C(2p) orbitals and a different bond formed by H(ls) and C(2s). Pauling proposed that new orbitals with the proper symmetry for bond formation could be formed by hybridization of 2s and 2p orbitals after promotion. The Be(2s) and Be(2pz) orbitals would combine to form two equivalent hybrid orbitals oriented 180° apart. The C(2s) would hybridize with the three C 2p) orbitals to give four equivalent new orbitals in a tetrahedral arrangement around the carbon atom. 

Four pairs of electrons are positioned in a tetrahedral arrangement around the carbon atom, just as they are in the water and ammonia molecules. But in methane, all four pairs are shared between the carbon atom and the four hydrogen atoms. Because there are no lone pairs requiring extra space, the structure of methane is a perfect tetrahedron with bond angles of 109.5°. 

Figure 1.21 depicts some of the spatial aspects of orbital hybridization. Each sp hybrid orbital has two lobes of unequal size, making the electron density greater on one side of the nucleus than the other. In a bond to hydrogen, it is the larger lobe of a carbon sp orbital that overlaps with a hydrogen Is orbital. The orbital overlaps corresponding to the four C—H bonds of methane are portrayed in Figure 1.22. Orbital overlap along the intemuclear axis generates a bond with rotational symmetry—in this case a C(2sp )—H(l.y) CT bond. A tetrahedral arrangement of four a bonds is characteristic of sp -hybridized carbon.

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