Scientists at University College London have developed a simpler method to produce reactive hydrogen atoms using light, avoiding toxic materials and extreme conditions. The technique, led by Roopender Kumar, could revolutionize chemical manufacturing by making hydrogen radicals more accessible and safer to handle.
The discovery marks a significant shift in how hydrogen radicals—extremely reactive atoms consisting of a single proton and electron—are generated. For over a century, methods relied on harsh conditions like high heat, mercury vapour, or intense radiation. Kumar’s team, however, has shown that a combination of hydrazine, thiophenol, and near-UV light can produce these radicals at room temperature, eliminating the need for costly or hazardous materials.
A Gentler Approach to Hydrogen Radical Production
The new method uses hydrazine and thiophenol, two inexpensive chemicals, along with acetonitrile as a solvent and molecular sieves to remove moisture. When exposed to specific wavelengths of light, the mixture generates hydrogen radicals through a process involving Rydberg atoms—highly excited, short-lived particles that split to release the desired hydrogen atoms. This reaction is highly sensitive to light wavelength, with deviations causing a sharp drop in radical production.
Researchers confirmed the process using electron paramagnetic resonance (EPR), density functional theory (DFT) modelling, and UV-visible spectroscopy. These techniques validated the formation of hydrogen radicals and the proposed chemical pathway. Unlike older methods, which often left metal residues or required specialized equipment, Kumar’s approach works with common lab materials and mild conditions.
Historical Context and Previous Challenges
Scientists Find Simple New Way To Make Reactive Hydrogen
The first successful method for creating atomic hydrogen dates back to 1912, when Langmuir used a tungsten filament heated to over 2000 Kelvin to split hydrogen molecules. Subsequent techniques relied on radiation, microwaves, or mercury vapour under ultraviolet light. While these methods advanced understanding, none proved practical for industrial or laboratory use due to complexity, cost, or safety risks.
Hydrogen radicals are notoriously unstable, reacting instantly with nearby molecules. This reactivity limits their use, as traditional methods often failed to produce them in controlled quantities. Kumar’s team addressed these challenges by designing a system where light acts as the sole catalyst, enabling precise control over the reaction.
Why This Method Matters for Chemistry and Industry
The technique has broad applications, particularly in pharmaceutical manufacturing, where metal contamination is a critical concern. By eliminating metal catalysts, the method reduces waste and improves product purity. Kumar’s team demonstrated its effectiveness in hydrogenating a wide range of organic compounds, including alcohols, esters, and protected amines, achieving yields up to 96% in some cases.
Experts like Maxie Roessler of Imperial College London highlight the breakthrough’s potential to democratize access to hydrogen radicals. This makes hydrogen radical reactions far more accessible, allowing a broader range of researchers to explore and exploit this species,
she said. The method also enables reactions previously confined to physics labs, such as alkene hydrogenation, which now works reliably under mild conditions.
Potential Implications and Unanswered Questions
Simple technique makes it easier to access and exploit
While the method shows promise, challenges remain. Some reactive alkenes produce lower yields due to side reactions like dimerization, suggesting further optimization is needed. Additionally, the long-term stability of the process and its scalability for industrial use require investigation.
Roopender Kumar envisions broader applications beyond chemistry, including biological research. This could change how chemists think about radical reactions, he said. However, the full impact of the discovery will depend on how quickly the scientific community adopts the technique and adapts it to new contexts.
The development underscores a growing trend toward greener, more sustainable chemical processes. By simplifying the production of hydrogen radicals, Kumar’s team has opened a new avenue for innovation—one that balances scientific rigor with practicality, and safety with efficiency.
According to Kumar, the method’s use of a violet LED, a sulfur-based organocatalyst (thiophenol), and hydrazine eliminates the need for precious or toxic metals, no hydrogen gas, no pressure equipment.
The process involves hydrazine and the sulfur catalyst combining to produce an intermediate that reacts when irradiated with ultraviolet light, generating a protonated hydrazine with an extra electron—a neutral Rydberg radical. This unstable species splits back into hydrazine and the hydrogen radical, ready to react with alkenes and halides.
Kumar emphasized that the method’s simplicity allows any chemist to run it, contrasting with earlier “brute force” approaches like white-hot filaments, electrical discharges, mercury lamps or ionising radiation.
The team tested the technique on compounds structurally related to fluoxetine (Prozac) and menthol, as well as terpenoids and amino acids. Notably, the radical reaction hydrogenated allyl glycine without scrambling stereochemistry,
Kumar noted.
Maxie Roessler, an expert in radical chemistry and electron paramagnetic resonance spectroscopy at Imperial College London, described the breakthrough as making hydrogen radical reactions far more accessible, allowing a broader range of researchers to explore and exploit this species.
She added that the discovery unlocks reactions that, until now, had remained confined to physics laboratories, out of reach for synthetic chemists.
The study, titled A Synthetic Method to Hydrogen Radicals,
was published. Kumar, a lecturer in Organic Chemistry and Chemical Biology at UCL’s Chemistry department, highlighted that the method’s ability to avoid metal catalysts offers the first real hope of more accessible alternatives
for greener radical reactions.