But-3-enoic acid (or 3-butenoic acid) is a short-chain unsaturated carboxylic acid, with the linear formula .
The acid is obtained via a reaction of allyl cyanide and concentrated hydrochloric acid. The mixture is gently heated with occasional shaking. The reaction produces a voluminous precipitate of ammonium chloride. Then water is introduced, and the upper acid layer is separated.
Other methods are also known.
The acid forms a clear light yellow liquid. Causes skin burns.
The compound is well-soluble in water, ethanol, and diethyl ether.
The acid has been employed in preparing specialized coatings and complex organic compounds, including those involving Diels-Alder reactions.