Phenylpropiolic acid, C<sub>6</sub>H<sub>5</sub>CCCO<sub>2</sub>H, formed by the action of alcoholic potash on cinnamic acid dibromide, C<sub>6</sub>H<sub>5</sub>CHBrCHBrCO<sub>2</sub>H, crystallizes in long needles or prisms which melt at 136âÂÂ137 ðC. When heated with water to 120 ðC, it yields phenylacetylene (C<sub>6</sub>H<sub>5</sub>CCH). Chromic acid oxidizes it to benzoic acid; zinc and acetic acid reduce it to cinnamic acid, C<sub>6</sub>H<sub>5</sub>CH=CHCO<sub>2</sub>H, whilst sodium amalgam reduces it to hydrocinnamic acid, C<sub>6</sub>H<sub>5</sub>CH<sub>2</sub>CH<sub>2</sub>CO<sub>2</sub>H. Ortho-nitrophenylpropiolic acid, NO<sub>2</sub>C<sub>6</sub>H<sub>4</sub>CCCO<sub>2</sub>H, prepared by the action of alcoholic potash on ortho-nitrocinnamic acid dibromide, crystallizes in needles which decompose when heated to 155âÂÂ156 ðC. It is readily converted into indigo.