Chemists Discover New Way to Crack Tough Carbon-Hydrogen Bond

A researcher at Southern Methodist University, Dallas, has stated that a new catalyst for breaking the tough molecular bond between hydrogen and carbon proves to be promising in developing a cheaper, easier and cleaner way to derive products from petroleum.

Some of the most useful building blocks we have in the world are simple, plentiful hydrocarbons like methane, which we extract from the ground. They can be used as starting materials for complex chemical products such as plastics and pharmaceuticals. But the first step of the process is very, very difficult -- breaking that carbon-hydrogen bond. The stronger the bond, the more difficult it is to oxidize.

Isaac Garcia-Bosch, Harold A. Jeskey Endowed Chair assistant professor in the Department of Chemistry at SMU.

The tenacious bond between carbon and hydrogen molecules must be broken by the chemical industry in order to synthesize oxidative products such as phenols and methanol. It is called oxidizing because it allows the molecule to go through a reaction in which it combines with oxygen, thus breaking C-H bonds and developing new carbon-oxygen bonds.

The standard chemical recipe calls for expensive and inefficient oxidants to break the C-H bond. That method is inexpensive, complex and leaves behind dirty waste products.

A cost-effective and cleaner way to crack the stubborn C-H bond was discovered by chemists at SMU in association with The Johns Hopkins University.

Copper catalysts capable of converting C-H bonds to C-O bonds in combination with hydrogen peroxide (oxygen source) was used by Garcia-Bosch and chemist Maxime A. Siegler, director of the X-ray Crystallography Facility at The Johns Hopkins University.

This is a very important discovery because it's the first time it's been proven that copper can carry out this kind of oxidation outside of nature in an efficient way. The prep is very simple, so labs anywhere can do it. Copper is relatively cheap compared to other metals such as palladium, gold or silver, and hydrogen peroxide is readily available, relatively cheap and very clean. One of the byproducts of oxidations with hydrogen peroxide (H₂O₂) is water (H₂O), which is the cleanest waste product you could have.

Garcia-Bosch

The researchers further identified the right ligand, which is a nitrogen-based material that binds to the copper so that the oxidation process can take place with close to perfect efficiency.

It is essential to have the right amount of hydrogen peroxide, the right metal, and the right ligand in order to oxidize these challenging C-H bonds.

"We found that combination," Garcia-Bosch said.

Garcia-Bosch further stated that chemistry is like a puzzle, where you build new molecules out of other molecules.

A number of C-H bonds are present in any one molecule. For example, in octanes, such as those found in gasoline, there is a carbon chain of eight carbons with multiple C-H bonds with varied chemical properties, Garcia-Bosch stated, and from the oxidation of each of the C-H bonds, a different product results.

Chemists design catalysts that have the potential to break and form bonds in order to construct complex chemical structures.

Catalysts have to be able to select between different C-H bonds and form new carbon-oxygen, carbon-nitrogen or carbon-fluoride bonds, for example. Biological processes use metals to do this all the time, for example in our bodies when our liver processes a pharmaceutical that we ingest using iron. Minerals such as iron, copper, manganese, calcium and potassium are critical for the natural catalytic process. For example, trees use manganese (photosynthesis) to transform water into the oxygen that we breathe.

Garcia-Bosch

Garcia-Bosch and Siegler reported their findings in the article "Copper-Catalyzed Oxidation of Alkanes with H₂O₂ under a Fenton-like Regime," published in the international edition of the journal Angewandte Chemie.

First time for using copper for C-H oxidation

Organic chemistry does not have many examples of copper as a catalyst for carbon-hydrogen oxidation. Several of the examples are based on iron.

"This is the first time in our field that we've used copper to do this C-H oxidation in a very efficient way," Garcia-Bosch said.

"Copper is very versatile in nature," he said. "With small changes in the environment of copper, you can do very diverse chemistry. That's why we picked it."

That environment is the ligand, which provides properties to the copper to spark the chemical reaction when the chemical ingredients are added in a vial or round bottom flask.

The researchers identified that these catalysts, copper in the form of a white salt and the ligand as an oil, are capable of oxidizing C-H bonds in an extremely efficient manner in combination with hydrogen peroxide, which is a reduced form of oxygen used by nature.

"You can find hydrogen peroxide anywhere, even at home in your medicine cabinet. So it's a mild oxidant," Garcia-Bosch said. "It's convenient also, because it's a liquid, rather than, say, a gas, which might require special storage. You mix everything together in a solvent and it reacts. It's like making a soup, a recipe, then you analyze the result to see what you get."

Garcia-Bosch and Siegler used a gas chromatography instrument to analyze the final solution in order to observe the results of the reaction. This allowed them to quantify the amount of oxidation product that was developed during the reaction.

Next step -- targeting a specific C-H bond

We tested this catalytic system for different substrates and we saw that it's not very selective. That's a problem. So if we have molecules that have many different C-H bonds, then it's going to oxidize all of them in a non-selective manner. In our lab, we would like to find selective catalysts. That's the next project.

Garcia-Bosch