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The long and short of it: how extreme carbon–carbon bond lengths keep surprising chemists

June 8, 2018

The carbon–carbon single bond is the most conventional connection in organic chemistry. It is formed simply by the equal sharing of two electrons from each carbon atom. However, despite this apparent simplicity, it is fair to say that the carbon–carbon single bond is the basis for some of the most complex and important structural components of life.

Carbon is the foundation for all organic molecules as it is one of the few elements that can form long chains of its own atoms. This ability, coupled with the high strength of the single carbon bond, gives rise to an array molecular forms with a variety of differing properties. In this blog post, we will look at why this bond length can vary, and how carbon–carbon bond-forming reactions have become key steps in many syntheses of organic chemicals and natural products.

C–C bond length: testing the limits  

C–C bond length can vary dramatically. This is because the bond length between two atoms in a molecule depends not only on the bonded atoms themselves, but also the effects from other atoms and functional groups within the molecule that can stretch or squeeze the bond. Consequently, C–C bond lengths can alter in different organic molecules, sometimes deviating considerably from the standard length of 154 picometers (set by the bond length in diamond).

For example, shorter single C–C bonds lengths occur in molecules like propionitrile. The electronegative cyano group in the molecule draws electrons from the C–C bond towards itself, reducing the bond length to 144 pm. Other molecules can also apply strain to the bond to shorten it. In the unusual organic compound in-methylcyclophane, the C–C single bond is squeezed between a triptycene and a phenyl group, resulting in a shorter bond distance of 147 pm.

The molecular structure of In-methylcyclophane

The molecular structure of In-methylcyclophane

Conversely, many longer C–C bonds can be formed with interestingly arranged organic molecules. In fact, researchers at Hokkaido University led by Takanori Suzuki and Yusuke Ishigaki have very recently synthesized a molecule that has the longest reported C–C bond among neutral hydrocarbons. Measuring a whopping 180.6 pm in length, this bond breaks the theoretical limit of 180.3 pm previously calculated for caged dimer compounds.

Synthesizing extraordinary molecules that push the length limits of the C–C bond is not merely a contest to set new records. The various organic structures with extremely long or short bonds can help refine the understanding of chemical bonding. It may even help us answer the questions of when and how a bond becomes a bond.

How are C–C bonds formed?

The innumerable variety of molecules that the C–C bond contributes to also means that there are many ways that C–C bonds can be synthesized. Named after their discoverers, some examples of the useful ways to form C–C bonds are Grignard reactions, Friedel-Crafts alkylation, Gattermann-Koch formylation, and the Houben-Hoesch synthesis. These reactions have now become so commonplace in organic chemistry that many of today’s products would not exist without them. For example, the Grignard reaction is a key step in the industrial production of tamoxifen, which is used for the treatment of breast cancer.

A particularly widespread way of forming C–C bonds is known as the Suzuki reaction. The Suzuki reaction creates a new C–C bond by using a boronic acid and an organohalide in the presence of a palladium catalyst. After its discovery, the Suzuki reaction contributed so much to the field that its inventor was awarded the Nobel Prize in Chemistry in 2010.

The great advantage of the Suzuki reaction is that it uses boronic acids as a reagent. The variety within boronic acids is almost limitless, which gives organic chemists the widest range of tools available for synthesizing complex molecules. In addition, boronic acids are abundant, better for the environment, and less toxic than the tin-based or zinc-based compounds that are used in other similar reactions.

At Alfa Aesar, we produce boronic acids to cater to the needs of today’s organic chemists. Whether used for C–C bond-forming reactions or other applications, we have a broad range of boronic acids, esters, and related compounds, and a selection of novel coupling catalysts and ligands to assist you in your syntheses.

 

With our catalogue of over 33,000 products, we’re confident we can support your chemical research and development. For more information on how we can help contact us today.

 


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