2.3     The Methyl Group (CH₃) and Methyl Compounds (CH₃X)

As soon as one of the four hydrogens surrounding the central carbon in methane is replaced with another atom, pure tetrahedral symmetry is lost. We might well anticipate that one of the four sp³ hybrid orbitals of carbon could overlap in a stabilizing way with any atom X offering an electron in any atomic orbital (Fig. 2.11). For maximum stabilization, we want to fill the new, bonding molecular orbital created from this overlap, which requires two, and only two, electrons.

All manner of derivatives of methane, called methyl compounds (CH₃―H), are possible. The atom or group replacing H is called a substituent, and one option for naming these substituted compounds is to drop the “ane” from the name of the parent molecule, methane, and append “yl.” The methyl is followed by the name of the X group as a separate word. Table 2.2 shows several methyl derivatives with their melting points, boiling points, and physical appearances.

The bonding in methyl compounds closely resembles that in methane, and it is conventional to speak of the carbon atom in any H₃C―X as being sp³ hybridized, just as it is in the more symmetrical CH₄. Strictly speaking, this is wrong because the bond from C to X is not the same length as that from C to H, and the H―C―H bond angle cannot be exactly the same as the X―C―H angle. Because sp³ hybridization yields four exactly equivalent bonds directed toward the corners of a tetrahedron, the bonds to H and X in H₃C―X cannot be exactly sp³ hybrids, only approximately sp³ (sp^2.8, say). This point is very often troubling to students, but it is really quite simple. Consider converting an sp²-hybridized carbon into an sp³-hybridized carbon, as in Figure 2.12. We start with the carbon having three sp² hybrid orbitals and a pure, unhybridized 2p orbital (H―C―H angle = 120°). As we bend the sp² hybrid orbitals, we come to a point at which the H―C―H angle has contracted to 109.5°, which is sp³ hybridization. Our orbitals, originally one-third s and two-thirds p (sp²: 33.3% s, 66.7% p character), have become one-fourth s and three-fourths p (sp³: 25% s, 75% p character). They have gained p character in the transformation from sp² to sp³. Now look at the 2p atomic orbital in Figure 2.12. As we bend the hybrid orbitals, this orbital goes from pure p to one-fourth s, three-fourths p. It has gained s character in the transformation. There is a smooth transformation as the orbitals change from sp² to sp³, and between these two extremes, the hybridization of carbon is intermediate between sp² and sp³, something like sp^2.8. The hybridizations sp² and sp³ are limiting cases, applicable only in completely symmetrical situations. The hybridization of a carbon (or other) atom is intimately related to the bond angles. If you know one, you know the other.

 

TABLE 2.2 Some Simple Derivatives of Methane, Otherwise Known as Methyl Compounds

 

In general, methyl compounds (H₃C―X) closely resemble methane in their approximately tetrahedral geometry: sp³ is a good approximation, but methyl compounds cannot be perfect tetrahedra.

In addition to CH₃ X, other substituted molecules are possible in which more than one hydrogen is replaced by another atom or group of atoms. Thus, even for one-carbon molecules, there are many possible substituted structures.