3. Multiple Bonds 4. Organic Compounds 5. Resonance
4. Coupling Diagrams for Organic Compounds

The 2s2 pair of carbon can be recoupled, which makes the usual tetravalence of carbon possible. Without recoupling, one might naively expect that carbon is divalent, since it only has two singly occupied 2p orbitals in its ground state 2s2 2px1 2py1 valence electron configuration. There are states of CH and CH2 that have covalent bonds that are consistent with this expectation. Doublet CH is the ground state of CH, but singlet CH2 is the first excited state of CH2. Here is the glyph of carbon indicating the coupled s-p lobe pair that is a better representation of the element than the glyph with a 2s2 pair and unoccupied 2p orbital, along with the coupling diagrams for CH and CH2 with covalent bonds:

There is a low-lying quartet excited state of CH where the lobe pair is recoupled (see below). In principle, the triplet ground state of CH2 could involve (i) bonding H to the outer lobe orbital, which would yield a linear structure; (ii) bonding H to one of the off-axis 2p orbitals, which would yield a bent structure with a bond angle near 90° or (iii) a combination of the two, which would yield a bond angle between ~110° and 180°. The structure of triplet CH2 has a bond angle of about 160° and is therefore consistent with the third scenario. The fourth glyph has a trefoil form with three in-plane s-p lobe orbitals that are 120° apart and a fourth out-of-plane unhybridized 2p orbital, as shown below.

When a third hydrogen atom is added to triplet CH2 to form the doublet CH3 methyl radical, there are again three options: (i) bond to the third in-plane lobe orbital to form a planar structure; (ii) bond to the out-of-plane 2p orbital to form a pyramidal structure; or (iii) a combination of the two, which would yield a flat pyramidal structure. In this case, the planar form is preferred. It is only when a fourth hydrogen is added that the planarity is broken, and the first three bond pairs bend back to make room for the fourth pair. That leads to tetrahedral methane, and the fifth, tetrahedral glyph. Coupling diagrams for CH3 and CH4 are shown here, along with the singly occupied 2p orbital on CH3.

The linear motif glyph (quartet CH), planar motif glyph (doublet CH2), and tetrahedral motif glyph (CH4) can be used to construct coupling diagrams for tetravalent carbon-containing compounds. Examples that use the linear glyph include acetylene (C2H2) hydrogen cyanide (HCN), and carbon dioxide (CO2).

Examples that use the planar glyph include ethylene (C2H4) and formaldehyde (H2CO).

Finally, methanol (CH3OH) is an example that uses the tetrahedral glyph:

In the next section, we will see that it is straightfoward to depict resonance forms with coupling diagrams.

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5. Resonance and Coupling Diagrams
Copyright 2023 DE Woon, University of Illinois. All rights reserved.