Piperazine (/paɪˈpɛrəziːn/) is an organic compound with the formula (CH₂CH₂NH)₂. In terms of its structure, it can be described as cyclohexane with the 1- and 4-CH₂ groups replaced by NH. Piperazine exists as deliquescent solid with a saline taste. Piperazine is freely soluble in water and ethylene glycol, but poorly soluble in diethyl ether. Piperazine is commonly available industrially as the hexahydrate, (CH₂CH₂NH)₂·6H₂O, which melts at 44 °C and boils at 125–130 °C.

Substituted derivatives of piperazine are a broad class of chemical compounds. Many piperazines have useful pharmacological properties, prominent examples include sildenafil, ciprofloxacin, and ziprasidone.

Piperazines were originally named because of their chemical similarity with piperidine, part of the structure of piperine in the black pepper plant (Piper nigrum). The -az- infix added to "piperazine" refers to the extra nitrogen atom, compared to piperidine. However, piperazines are not in general derived from plants in the genus Piper .

Piperazine is formed by the ammoniation of either 1,2-dichloroethane or ethanolamine. This reaction is mainly used for production of ethylene diamine, but piperazine is a side product. The piperazine is separated from the product stream, which, in addition to ethylenediamine, also contains various derivatives containing CH₂CH₂NH subunits, e.g. diethylenetriamine, aminoethylpiperazine, and other related linear and cyclic chemicals of this type.

Piperazine can also be synthesized by reduction of pyrazine with sodium in ethanol.^{[citation needed]}

As confirmed by X-ray crystallography, piperidine is a centrosymmetric molecule. The ring adopts a chair conformation and the two N-H groups are equatorial.

Its basicity is that of a typical amine. The pH of a 10% aqueous solution of piperazine is 10.8–11.8. The two pK_a's are 5.35 and 9.73 at 25 °C.

Piperazine readily absorbs water and carbon dioxide from the air. Carbon dioxide produces a series of carbamates. Some of the relevant equilibria are: