Lecture Notes from CHM 1341
11 July 1996

Organic Chemistry

Inorganic Carbon Compounds

Carbon lies in the center of the Periodic Table in Group IV and has a middle-of-the-road electronegativity. It is so flexible in its bonding that you can find carbon compounds with virtually any formal oxidation state from -IV to +IV:
C ox #:  -IV    -III    -II    -I      0      I       II     III     IV
Example:  CH4   C2H6    CH3OH   C2H2   CH2O   C2H2F4  HCO2H    C2F6    CO2
And it can hybridize its valence orbitals into linear, trigonal, or tetrahedral bonding:
Hybrid:   sp + p2      sp2 + p      sp3
                     H            H
                      \            \
Example:  HC=-CH       C = O    H - C - H
                      /            /
                     H            H
And it can bond to itself endlessly!
                                H   H   H
                                 \ /   /
  H       H   H   H   H   H   H   C - C - H
   \     /     \ /   /   /     \ /     \
H - C - C - H   C - C - C - H   C - H   etc. forever
   /     \     /     \   \     /
  H   H - C - C - H   H   C - C - H
         /     \         / \   \
        H       H       H   H   H
And with any bond order up to triple!
H       H            H           H   H
 \     /              \         /     \ 
  C = C                C = C = C   H - C - etc. forever
 /     \              /         \     /
H       C - C =- C - C - H       C - C - H
       / \            \         / \   \
      H   H            H       H   H   H
And while torsion about the rigid C = C double bond is not possible, end groups off the single and triple bonds can spin "freely" with respect to one another producing another endless variety of conformations of the same molecule.

Those torsions aren't completely free if one thinks of any flexible chain; it cannot cross through itself without breaking...so not every twist is possible. Moreover, imagining a thick chain with knobs on the links, clearly the flexibility is limited to torsions which do not require that the knobs hinder one another.

So carbon chemistry is characterized by variety! And it is this which has rendered carbon so valuable in the service of Life...the literal genetic engineering of (largely) carbon structures into the architectural wonders of proteins have made Life as successful and efficient as it is. While those clever combinations of carbon, hydrogen, oxygen, and nitrogen (with important roles...some already hinted at...for phosphorus, calcium, sulfur, potassium, and a wealth of trace players) will be discussed in General Chemistry II, Organic, and Biochemistry, we concentrate in this lecture on carbon compounds which needn't have vital (as in "from Life") sources. These are called inorganic carbon compounds as if there is some mystical difference from the organic compounds of Life. Actually it was once thought that, arising "only" from Life, organic compounds embodied some of Life's vitality. Since we can now synthesize them from the elements, that worshipful aspect is gone.

As with sulfur and phosphorus (and other atoms), carbon has more than one allotrope as an element. The most stable is graphite with which you write every time you use a pencil. It writes because graphite solidifies as infinite sheets of hexagons which glide over one another and onto your paper easily. (Pencils are called "lead pencils" because the soft metal lead will leave a similar dark mark, but there's no lead in a pencil...only graphite.) For the same reason, graphite makes an excellent lubricant; locksmiths, for example, puff some graphite dust into recalcitrant locks to free them.


\     /     \     /     \     /     \     /     \     /
 C = C       C = C       C = C       C = C       C = C
/     \     /     \     /     \     /     \     /     \    
       C = C       C = C       C = C       C = C       C = C
\     /     \     /     \     /     \     /     \     /
 C = C       C = C       C = C       C = C       C = C          etc.
/     \     /     \     /     \     /     \     /     \    
       C = C       C = C       C = C       C = C       C = C
\     /     \     /     \     /     \     /     \     /
 C = C       C = C       C = C       C = C       C = C
/     \     /     \     /     \     /     \     /     \    
       C = C       C = C       C = C       C = C       C = C
Don't pay a lot of attention to those double bonds. All the bond lengths in graphite planes are the same. Imagine rotating that picture 60 degrees and then again by 60 degrees; the double bonds migrate to different opposite bonds, yes? Well the electrons aren't fooled by this; those pi electrons spend their time all around the hexagon, making each bond neither single nor double but rather about 4/3 of a single bond. See bond order in Chapter 9.

A mere 2 kJ/mol less stable (see enthalpy next chapter) at room temperature than soft graphite is the hardest known (naturally occuring) mineral, diamond! What a difference a hybridization makes. Instead of slippery planes, we get a super-rigid 3d crystal.

Instead of graphite's sp2 bonding to 2 neighbors, diamond bonds to 3 via sp3 bonding in a tetrahedral lattice like that at left.

One of the properties of tetrahedrons is that your can fill space with them leaving no holes...so the diamond structure expands to render the entire non-ionic elemental crystal a single molecule!

Within your lifetime, a new allotrope of carbon has been discovered...in soot, of all places. Well, actually, that was a good place to look, since soot is incompletely burned hydrocarbon and consists of graphite fragments and the surprising molecule C60, Buckminsterfullerene (or "buckyball"), sort of shown at right. Actually that's a soccer ball, but the two have exactly the same geometry: a dozen pentagons (black) force the 20 hexagons (white) to bend away from the plane graphite would prefer and back on itself in a rough (polyhedral approximation to a) sphere. It is a remarkably stable molecule for reasons we'll see in a moment about the strain in hydrocarbon cycles. And it may be the world's smallest test tube; one can capture small molecules inside and use the relative chemical inertness of buckyball to protect and transport them.

Other inorganic carbon compounds include the carbides, vicous Brøsted bases capable of ripping protons from water to make the triple-bonded acetylene, H-C=-C-H. And at the other end of the spectrum, the mild Brøsted acid, carbonic, which results from hydrolysis of carbon dioxide:
CO2(aq) + H2O  <=====>  H2CO3(aq)  <=====>  H+(aq) + HCO3-(aq)
That second equilibrium to form hydronium (add the water molecule mentally) and bicarbonate ion is critical in maintaining the acid-base (or pH) balance of the blood. Normal blood has 20 times as much bicarbonate ion as carbonic acid, but that balance can be disturbed. Hyperventilate and you exhaust carbon dioxide quicker than usual driving both equilibria to the left and consuming hydronium ion in the process. That leads to alkaloid tetnus (too little acid) which locks up your muscles. (Fortunately, your diaphragm locks before your heart; so you can't breathe, your carbon dioxide level rises, and all returns to normal...save for the tired muscles!)

Other inorganic carbon compounds include combinations with nitrogen, such as hydrogen cyanide, H - C =- N, and sulfur, such as thiols, - - - C - S - H.

Fascinating as these inorganic carbon species may be, they are soon eclipsed by the wonder and variety of the "organic" carbon compounds.

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Chris Parr
University of Texas at Dallas
Programs in Chemistry, Room BE3.506
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Richardson, TX  75083-0688

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Last modified 11 July 1996.