Compounds that contain carbon are called organic compounds because they are associated with living material. Here we will consider six small organic compounds that can be made from some of the CH_n compounds we characterized in the previous section. The three intermediates of interest are the version of CH with three singly occupied orbitals, the version of CH₂ with two singly occupied orbitals, and the CH₃ radical, which are known as the methylidyne, methylene, and methyl radicals, respectively. We will examine each of these cases in turn. For reference, here are the 2D and 3D diagrams for each radical:

24.1

CH radical as a starting point. With its lobe orbital aligned with the CH bond axis and two off-axis singly occupied 2p orbitals, CH resembles the N atom to a degree. Back in Section 16 we found that we could join two N atoms together with a triple bond. The bonds consisted of a sigma bond and two pi bonds.

Since the singly occupied lobe orbital of CH is 180° away from the the CH bond pair, we would expect HCN to be linear, answer (c). HCN or hydrogen cyanide is indeed linear, as shown in the structure below. The formation of 2D and 3D bonding diagrams is also depicted.

24.2

Note that HCN will not form unless each of the three electron pairs is an antiparallel spin combination. The next thing we can do is to combine two CH radicals, just as we combined two N atoms, to form C₂H₂, which is also known as acetylene:

24.3

Like HCN, acetylene is linear. This triple bonded form of carbon is found frequently in organic compounds, whether it is as the linear R₁C≡CR₂ or the R₁C≡N motifs (where R₁ and R₂ can be varied considerably).

CH₂ radical as a starting point. CH₂ has an in-plane lobe-like orbital and an out-of-plane 2p orbital and has nominal bond angles of 120°. One atom we can bond to it is O, as shown below:

24.4

Here we form a double bond between C and O, with one sigma bond and one pi bond. This forms H₂CO or formaldehyde. Note that the bond angles are only slightly distorted from the nominal 120° bond angles. As with CH, we can assemble two CH₂ radicals to form C₂H₄, also known as ethylene:

12.5

The double bonded form of carbon is also found frequently in organic compounds. The two motifs here are both planar: R₁R₂C=CR₃R₄ and R₁R₂C=O (where any of the R_i substituents can be varied considerably).

CH₃ radical as a starting point. The methyl radical, CH₃, is planar until a fourth bond is formed, when it becomes tetrahedral as in methane. Here we show replaced the fourth H with the hydroxyl radical, OH, to yield CH₃OH or methanol:

24.6

The bond angles around the carbon are again only slightly distorted from the tetrahedral angles of 109.5° (and note that the C-O-H angle again opens up considerable from the nominal 90° angle). The last compound we will look at in this section is formed by combining two methyl radicals to yield C₂H₆, which is known as ethane:

24.7

An enormous number of other compounds can be formed by replacing any of the H atoms in either methanol or ethane with other groups or hydrocarbon chains.

Summary. In this section we've barely dipped into the literally limitless combinations of carbon-containing compounds that are possible. We've seen three different motifs for CC or CX bonds (where X can be N, O, and other elements): triple, double, and single bonds. We saw how each of these motifs arises straightforwardly from one of the CH_n compounds.

In the next section, we will return to the oxygen, combining two O atoms to form O₂ and then exploring the addition of H.