formylating

Formylating is a chemical transformation that introduces a formyl group (-CHO) into a molecule. This process can yield aldehydes and formylated derivatives, including formylated aromatics and formamides, depending on the substrate and reagents used. Formylation is a fundamental tool in organic synthesis for building one-carbon units and enabling further transformations.

In aromatic chemistry, several established formylation methods are used. The Gattermann–Koch formylation introduces a formyl group onto activated arenes using carbon monoxide under acidic conditions with Lewis acids such as AlCl3 and CuCl, yielding benzaldehydes. The Vilsmeier–Haack formylation employs a formylating system generated from dimethylformamide (DMF) and phosphorus oxychloride (POCl3) to formylate electron-rich rings, often producing aldehydes at specific positions after hydrolysis. The Reimer–Tiemann formylation targets phenols to give ortho-formylated products (salicylaldehydes) under basic conditions with chloroform. Each method has particular scope, regioselectivity, and safety considerations, and all require careful handling of reagents and byproducts.

Beyond direct aldehyde formation on rings, formylation of amines to produce formamides is common. This uses formylating agents such as formyl chloride or formic acid derivatives and serves both to protect amino groups and to introduce formamide functionality for subsequent chemistry.

Applications and limitations: formylation provides access to benzaldehydes, salicylaldehydes, and other aldehydes, as well as protected amines, enabling pharmaceuticals, fragrances, and materials synthesis. Reagents and conditions can be harsh or hazardous, and selectivity must be managed to balance yield, scope, and environmental impact.