Hydrerering

Hydrerering is a chemical term used to describe the hydrogenation of ring systems, in which molecular hydrogen (H2) is added to unsaturated carbon–carbon bonds within cyclic structures. In English-language chemistry literature, the process is usually called hydrogenation, with hydrering appearing as a variant spelling in some regional or historical sources. The broad transformation covers hydrogenation of alkenes, alkynes, and aromatic rings to more saturated derivatives.

Mechanism and catalysis

Most hydrering reactions employ metal catalysts, either heterogeneous (for example, nickel, palladium, platinum, or ruthenium on a solid support) or homogeneous (such as rhodium- or ruthenium-based complexes). The mechanism typically involves adsorption of hydrogen and substrate onto the metal, activation of H–H bonds to form metal–hydride species, and transfer of hydrogen to the substrate in a concerted fashion, often with syn addition. Reaction conditions vary widely, from room temperature and mild pressure in specialized cases to high pressures and elevated temperatures for more recalcitrant substrates. Solvent choice and catalyst design influence selectivity and rate.

Types and selectivity

Hydrogenation can be complete or partial. Partial hydrogenation is used to convert alkynes to cis-alkenes (often with a poisoned or Lindlar-type catalyst), while complete hydrogenation saturates alkenes, alkynes, or aromatic rings. Hydrogenating aromatic rings to cyclohexane derivatives requires more active catalysts and higher pressures, and is central to certain fine-chemical syntheses and pharmaceutical routes. Asymmetric hydrogenation uses chiral catalysts to produce enantioenriched products.

Applications and considerations

Hydrogenation is widely used in petrochemical processing, pharmaceutical synthesis, and the production of saturated fats in the food industry. The partial hydrogenation of vegetable oils historically led to trans fats, prompting regulatory and reformulation changes. Environmental and safety considerations include the flammability of hydrogen gas, catalyst toxicity, and energy requirements, with growing interest in greener hydrogen sources.