Synthesis of Aromatic Aldehydes Formylation Reactions

In order to synthesize benzaldehyde using the Friedel-Crafts acylation, formyl chloride would be required. Since this acid chloride is too reactive to be prepared, several synthetic equivalents have been developed to accomplish this formylation reaction. 

Friedel-Crafts Reaction Fails with Formyl Chloride 

The Gattermann-Koch reaction provides one such formylation method. In this reaction, benzene (or an alkylbenzene) is treated with carbon monoxide (CO), HCl, and AICI3. The CO and HCl combine to produce formyl chloride in situ. The usual reaction of the acid chloride with AICI3 enables the Friedel-Crafts acylation to proceed, affording benzaldehyde. 

In the Vilsmeier-Haack reaction, a reagent such as A(A-dimethylformamide (DMF) is the source of the formyl group. Reaction of DMF with phosphorus oxychloride (POCI3), creates an electrophilic iminium ion in situ that reacts with activated aromatic compounds. Hydrolysis of the resulting imininm ion results in a benzaldehyde derivative. 

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