is conjugate acid of butenedioic acid 116.011 116.07216 2-butenedioic acid Beilstein:8132074 C4H4O4 CHEBI:22958 InChI=1S/C4H4O4/c5-3(6)1-2-4(7)8/h1-2H,(H,5,6)(H,7,8) VZCYOOQTPOCHFL-UHFFFAOYSA-N [H]C(=C([H])C(O)=O)C(O)=O but-2-enedioic acid chebi_ontology hydrogen butenedioate C4-dicarboxylic acid