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Physical Chemistry

Chemists break C–C bond record

New hydrocarbon compound contains a 1.806-Å-long bond that stretches beyond theoretical limit, researchers claim

by Tien Nguyen
March 8, 2018 | APPEARED IN VOLUME 96, ISSUE 11

In two steps, chemists synthesized a compound with the longest reported C–C bond among neutral hydrocarbons.

The single carbon-carbon bond is among the most familiar covalent connections in organic compounds. But a new study suggests that chemists have yet to fully explore the limits of this basic bond.

Scientists have previously found that these ubiquitous bonds, which typically measure about 1.54 Å in length, can be elongated through the use of bulky substituents to endow molecules with special properties. Now, researchers led by Takanori Suzuki and Yusuke Ishigaki at Hokkaido University have synthesized a molecule they claim has the longest reported C–C bond among neutral hydrocarbons, measuring 1.806 Å in length (Chem 2018, DOI: 10.1016/j.chempr.2018.01.011).

The team set out to break the record by designing a stable dihydropyracylene compound with a highly strained core and two spirocyclic units that are forced to face each other, helping stretch out the central carbon-carbon bond. The chemists confirmed the presence of the C–C bond by observing its stretching vibration through Raman spectroscopy, and they measured the molecule’s record bond length using X-ray crystallography.

The bond breaks the theoretical limit of 1.803 Å, previously calculated for caged dimer compounds. This limit is set at the point at which the bond has a dissociation energy of zero and would thus dissociate. Chemists calculated the limit by assuming a linear relationship between bond length and bond dissociation energy. The Hokkaido researchers suggest that molecules like theirs with extralong bonds may deviate from this linear relationship.

Structures with extremely long or short bonds help us refine our understanding of chemical bonding, Peter R. Schreiner of Justus Liebig University Giessen says. He thinks work like this, which pushes the limits of bonding, is worth careful consideration. “It also urges us to keep asking the question, When is a bond a bond?”

CORRECTION: This story was updated on April 4, 2018, to correct the specifics of the record broken in this work. The researchers claim their molecule has the longest C–C bond among neutral hydrocarbons. The bond is not an alkane bond. Also other teams have reported longer C–C bonds in charged molecules and in nonhydrocarbons.



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Stanton de Riel (March 14, 2018 8:13 PM)
It seems that they imagined a strain which would strain the imagination?
S. N. Balasubrahmanyam (March 15, 2018 2:42 AM)
How does zinc do its job? I stretched whatever imagination I have to the Hooke Law limit but I "broke down". can you supply me with a mechanism? Does zinc supply mere electrons while standing aside? Not having access, I haven't read the Suzuki-Ishigaki paper.
Tien Nguyen (March 21, 2018 11:08 PM)
Ah, you're working with incomplete information. My apologies. It's an acid Zn reduction, triflic acid then Zn.

Raji Heyrovska (March 15, 2018 12:50 PM)
The various radii of carbon have been related to the Bohr radius (0.639 Å) obtained from the first ionization potential of carbon in: . The radius corresponding to the newly found long CC distance, R(C,long) = d(CC)/2 = 1.806/2 = 0.903 Å = 1.414*0.639, the diagonal of a square with Bohr radius as a side.
Paul Krebaum (March 17, 2018 9:25 PM)
Could the fact that each of the carbon atoms involved is "triply benzylic" have anything to do with it ?
Vernon G. Box (March 20, 2018 1:07 PM)
I measured some of these C-C bonds from the published diffraction data, and they seemed to be about 172-174 pm long. 180.3 pm? Hmm.
Raji Heyrovska (March 24, 2018 4:15 AM)
The longest radius of carbon was explained above by the present author as equal to the diagonal of a square with the Bohr radius of carbon as a side. Here the author shows that the C-C distance is also equal to the sum of the Bohr radii of C and O, d(C1C2) = aB,C + aB,O + aB,C = 0.639 + 0.529 + 0.639 = 1.807 Å, when Zn takes away the two hydroxyl groups. A short article with the corresponding Figure is in the preprint server Vixra.

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