Methane tetrahedral structure

The axes of the sp orbitals point toward the corners of a tetrahedron Therefore sp hybridization of carbon is consistent with the tetrahedral structure of methane Each C—H bond is a ct bond m which a half filled Is orbital of hydrogen over laps with a half filled sp orbital of carbon along a line drawn between them... 

At this stage, it looks as though electron promotion should result in two different types of bonds in methane, one bond from the overlap of a hydrogen ls-orbital and a carbon 2s-orbital, and three more bonds from the overlap of hydrogen Is-orbitals with each of the three carbon 2/ -orbitals. The overlap with the 2p-orbitals should result in three cr-bonds at 90° to one another. However, this arrangement is inconsistent with the known tetrahedral structure of methane with four equivalent bonds. 

Figure 1.2 The tetrahedral structure of methane. Bonding electrons in methane principally occupy the space within the wire mesh.

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. 

Figure 11.12 The three-dimensional tetrahedral structure of carbon (e.g., in methane, CH4), with an angle between the bonds of 109.5°. The simple straight lines are in the plane of the paper, the solid tapered line points towards the observer and the dashed line is into the paper.

Like ammonia, the structure is similar to the tetrahedral structure of methane. The two lone pairs repel each other in order to be as far apart as possible. The squeezing of the hydrogens in water is even greater than that in ammonia. The H-O-H bond angle in water is 104.5°. 

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. 

The final molecule of this series is methane, the tetrahedral structure of which follows if a fourth unit positive charge is removed from the nucleus in the ammonia lone-pair direction. There are now four equivalent bonding orbitals, which may be represented approximately as linear combinations of carbon s-p hybrid and hydrogen Is functions. The transformation from molecular orbitals into equivalent orbitals or vice versa is exactly the same as for the neon atom. 

Some illustrative examples are shown above. Despite his disclaimer Kekule s structural formulae are clearly the harbingers of their modern equivalents. The proposed tetrahedral structure of carbon, which followed, ignored the good advice and amounts to no more than a geometrical rearrangement of Kekule s diagrams, as shown here to represent the actual size and shape of carbon in methane. 

Methane has a tetrahedral structure with each C-H bond 109 pm and all the bond angles 109.5°. To simplify tilings, we shall draw a molecule of methane enclosed in a cube. It is possible to do this since the opposite corners of a cube describe a perfect tetrahedron. The carbon atom is at the centre of the cube and the four hydrogen atoms are at four of the corners. 

Now we shall seek analogies between transition metal complexes and simple, well-studied organic molecules or fragments. In principle, any hydrocarbon can be constructed from methyl groups (CH3), methylenes (CH2), methynes (CH), and quaternary carbon atoms. They can be imagined as being derived from the methane molecule itself which has a tetrahedral structure ... 

Now that we have obtained the electron-pair arrangement that gives the least repulsions, we can determine the positions of the atoms and thus the molecular structure of CH4. In methane each of the four electron pairs is shared between the carbon atom and a hydrogen atom. Thus the hydrogen atoms are placed as shown in Fig. 13.14, giving the molecule a tetrahedral structure with the carbon atom at the center. 

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. 

Experimentally, methane has been found to have the higU v tetrahedral structure we have assembled. Each carbon-hydre exactly the same length, 1.10 A the angle between any pair of boi c hedral angle 109.5°. It lakes 104 kcal/mole to break one of the bor... 

The tetrahedral structure of methane has been verified by electron diffraction (Fig. 2.1c), which shows beyond question the arrangement of atoms in such siihple molecules. Later on, we shall examine some of the evidence that led chemists to accept this tetrahedral structure long before quantum mechanics or electron diffraction was known. 

Thus, only the tetrahedral structure for methane agrees with the evidence of isomer number. It is true that this is negative evidence one might argue that isomers exist which have never been isolated or detected simply because the experimental techniques are not good enough. But, as we said before, any compound that contains carbon bonded to four other atoms can be considered to be a derivative of methane in the preparation of hundreds of thousands of. compounds of this sort, the number of isomers obtained has always been consistent with the concept of the tetrahedral carbon atom. 

Methane, CH4, has steric number 4, and VSEPR predicts a tetrahedral structure, which is confirmed by experimental results. Starting with the electron configuration C (ls) (2s) (2p), the VB model cannot account for the formation of CH4 and predicts that CH2 would be the stable hydride, which is again contrary to the... 

This paragraph, coupled with the next but one before it, clearly implies a r ular tetrahedral structure for methane, a fact that has been denied by some. (Gf. A. Findlay, A Hundred Tears of Chemistiyf New York Macmillan, 1938, p. 73. The author suggests that in van t Hoff s mind arose the idea of the tetrahedral carbon atom, in le Bel s the idea of the asymmetric carbon atom. In fact, each paper contains both ideas. Cf. also A. Sementsov, American Scientist 43, 97 (1955).)—O.T.B.]... 

Analysis of this type of expression shows that LC is just another one-electron function with w = 0 or 1, directed at a special angle, that depends on the coefficients a, b and c. An infinite number of such linear combinations is possible, each defining another one-electron eigenfunction directed at one of an infinite number of angles, measured with respect to the original laboratory coordinate system. The important conclusion is that each linear combination corresponds to a new choice of axes. Selection of the polar axis along any Z always leaves Px and Py undefined as separate entities. In particular, there is no hope ever to simulate the tetrahedral structure of methane in terms of a linear combination of carbon electron eigenfunctions. That requires four linear combinations, each with a different polar axis, which is physically impossible. 

Methane has four shared pairs of electrons. Here, minimum electron repulsion is achieved by arranging the electrons at the corners of a tetrahedron (Figure 4.6). Each H—C—H bond angle is 109.5°. Methane has a three-dimensional tetrahedral structure. Silicon, in the same group as carbon, forms compounds such as SiCl4 and SiH4 that also have tetrahedral structures. 

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