A freshly prepared Vilsmier-Haack Reagent.

This stuff was originally described by  Anton Vilsmeier and Albrecht Haack in 1927. It is made up by mixing phosphorous oxychloride and a formamide at low temperature. It is a quite practical thing that is widely used for the formylation of various compounds including sensitive compounds e.g.: pyrroles, furanes and various heterocyclic systems. 

The reagent is formed and reacts as the following mechanism illustrates:

Formylation (aldehyde group insertion to the molecule) with the Vilsmeier–Haack reagent. 

This pretty useful reaction is named after Anton Vilsmeier and Albrecht Haack who discovered, that if phosphorous-oxychloride is added to a substituted amide (they used originally N-Methylformanilide, but N,N-dimethylformamide works also great) a chloroiminium ion will form. If an activated aromatic compound is present, an electrophilic aromatic substitution produces an iminium ion intermediate, which can be hydrolyzed to give an aryl aldehyde. This happens: 

-image from wikipedia.