The carbon-hydrogen bonds of alkanes, particularly those at the ends of molecules, where each carbon has three hydrogen atoms attached, are very difficult to “break” if you want to replace the hydrogen atoms with other atoms. Half (CH4) and ethane (CH3CH3) are formed exclusively by these tightly bonded hydrogen atoms. In the magazine Applied chemistry, a team of researchers has now described how they break these bonds as they form new carbon-nitrogen (amidation) bonds.
If it were possible to easily break C – H bonds in hydrocarbons, it would be possible to synthesize complex organic molecules, such as pharmaceuticals, in a much more convenient and direct way than oil. This strategy could also provide more avenues for recycling plastic waste. The formation of carbon-nitrogen bonds is of particular interest because they play an important role in natural products. For example, amide bonds bind individual amino acids to proteins.
Although there has been some success in the functionalization of heavy hydrocarbons, even in the end positions, the particularly strong C – H bonds of light alkanes, especially methane, can hardly be divided. The use of these primary components of natural gas as synthetic building blocks is especially desirable, as it would allow the use of this often wasted by-product of oil extraction.
A team led by Ana Caballero and Pedro J. Pérez (University of Huelva, Spain), as well as John F. Hartwig (University of California, Berkeley, USA), have now successfully coupled amides (nitrogen-containing organic compounds) to alkanes. light with loss of a hydrogen atom. The products of these dehydrogenative amidations are known as N-alkyl amides.
The starting point of this approach was the measurement of C – H bonds in heavy alkanes with a catalyst based on copper and di-tert-butyl peroxide as an oxidizing agent, as developed several years ago by the group. Hartwig. Catalyst variation led to success. If copper has phenanthroline-type ligands (an aromatic system containing three-membered six-membered nitrogen), it is possible to produce high yields in the reaction of ethane with benzamide — and also with other amides — using benzene as the solvent. The reaction also worked when supercritical carbon dioxide — a more environmentally friendly option — was used as the solvent. The reaction with age is an unusual formation of CN bonds with an inactivated primary CH bond.
Propane, n-butane and iso-butane gave similar results. In light alkanes, reactivity correlates significantly with the dissociation energy of CH bonds than in higher alkanes.
And methane? Even the hardest candidate, whose methane amidation had never been observed, could attach to the amide. Isotopic experiments were used to show that methane reacts to form N-methylbenzamide.
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M. Ángeles Fuentes et al, Catalyzed dehydrogenative amidation of light alkanes copper, Angewandte Chemie International Edition (2021). DOI: 10.1002 / anie.202104737
Citation: Breaking CH bonds in hydrocarbons to synthesize complex organic molecules (2021, July 19) retrieved July 19, 2021 at https://phys.org/news/2021-07-ch-bonds-hydrocarbons-complex-molecules .html
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