Hydrogenacetylenedicarboxylate (often abbreviated as Hadc or HADC) is a monovalent anion with the formula C4HO4− or HO2C-C≡C-CO2−. The anion can be derived from acetylenedicarboxylic acid by removal of a single proton, or from the acetylenedicarboxylate dianion by addition of a proton.
The name is also used for any salt of this anion; which may also be called acid, hydrogen, or monobasic acetylenedicarboxylate.
The parent acid was synthetized by E. Bandrowski in 1877. Salts of this anion are of interest in crystallography because they contain unusually short and strong hydrogen bonds.
Structure
In many crystalline salts (with the exception of the lithium one), the HADC units form linear chains connected by strong hydrogen bonds. Each carboxylate group is usually planar; but the two groups may lie in different planes due to rotation about the C-C bonds. They are coplanar in the hydrated salts NaHC4O4·2H20 and CsHC4O4·2H20, nearly coplanar in the guanidinium salt [C(NH2)3]+ · C4HO4−, but off by 60 degrees or more in other salts such as anhydrous KHC4O4.