A quasi-planar polyaromatic compound containing an azo and a Schiff base linkage

The title compound, 2-{(E)-3-[(E)-4-chloro­phenyl­imino]-4-hydroxy­phenyl­diazenyl}benzoic acid hemihydrate, C₂₀H₁₄ClN₃O₃·0.5H₂O, forms sheets, parallel to the (100) plane, that are linked by weak aromatic C—H...O interactions. These sheets are in turn linked by strong hydrogen bonds to interstitial water mol­ecules.