Bond Lengths in Organic Compounds
The actual bond length between two atoms in a molecule depends on such factors as the orbital hybridization and the electronic and steric nature of the substituents. The carbon-carbon bond length in diamond is 154 pm which is also the largest bond length that exists for ordinary carbon covalent bonds.
Unusually long bond lengths do exist. In one, tricyclobutabenzene, a bond length of 160 pm is reported. The current record holder is another cyclobutabenzene with length 174 pm based on X-ray crystallography. In this type of compounds the cyclobutane ring would force 90° angles on the carbon atoms connected to the benzene ring where they ordinarily have angles of 120°.
The existence of a very long C-C bond length of up to 290 pm is claimed in a dimer of two tetracyanoethylenedianions although this concerns a 2-electron-4-center bond. This type of bonding has also been observed in posi manager dimers of neutral phenalene dimers. The bond lengths of these so-called pancake bonds are up to 305 pm.
Shorter than average carbon carbon bonds distances are also possible, alkenes and alkynes have bond lengths of respectively 133 and 120 pm due to increased s-character of the sigma bond. In benzene all bonds have the same length: 139 pm. In carbon carbon single bonds increased s-character is also notable in the central bond of diacetylene (137 pm) and that of a certain tetrahedrane dimer (144 pm).
In propionitrile the cyano group withdraws electrons also resulting in a reduced bond length (144 pm). Squeezing a CC bond is also possible by application of strain. An unusual organic compound exists called In-Methylcyclophane with a very short bond distance of 147 pm for the methyl group being squeezed between a triptycene and a phenyl group. In an in silico experiment a bond distance of 136 pm is estimated for neopentane locked up in fullerene. The smallest theoretical CC single bond obtained in this study is 131 pm for a hypothetical tetrahedrane derivative.
The same study also estimated that stretching or squeezing the CC bond in an ethane molecule by 5 pm required 2.8 or 3.5 kJ/mol, respectively. Stretching or squeezing the same bond width by 15 pm required an estimated 21.9 or 37.7 kJ/mol.
| C–H | Length (pm) | C–C | Length (pm) | Multiple-bonds | Length (pm) |
|---|---|---|---|---|---|
| sp3–H | 110 | sp3–sp3 | 154 | Benzene | 140 |
| sp2–H | 109 | sp3–sp2 | 150 | Alkene | 134 |
| sp–H | 108 | sp2–sp2 | 147 | Alkyne | 120 |
| sp3–sp | 146 | Allene | 130 | ||
| sp2–sp | 143 | ||||
| sp–sp | 137 |
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