| Alkylhydrazine (example) |
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| Arylhydrazines (examples) |
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Hydrazines (R2N−NR2) are a class of chemical compounds with two nitrogen atoms linked via a covalent bond and which carry from one up to four alkyl or aryl substituents. Hydrazines can be considered as derivatives of the inorganic hydrazine (H2N−NH2), in which one or more hydrogen atoms have been replaced by hydrocarbon groups.[1]
Hydrazines can be divided into three groups according to the degree of substitution. Hydrazines belonging to the same group behave similarly in their chemical properties. Monosubstituted hydrazines and so-called asymmetrically disubstituted hydrazines, where (only) two hydrocarbon groups are bonded to the same nitrogen atom are colorless liquids. Aliphatic monosubstituted and asymmetrically disubstituted hydrazines are very water soluble, strongly alkaline and good reducing agents. Aromatic monosubstituted and asymmetrically disubstituted hydrazines are poorly soluble in water, less basic and weaker reducing agents. For the preparation of aliphatic hydrazines, the reaction of hydrazine with alkylating compounds such as alkyl halides is used, or by reduction of nitroso derivatives. Aromatic hydrazines are prepared by reducing aromatic diazonium salts.[3][4]
In symmetric disubstituted hydrazines, a hydrocarbon group is bonded to each of the hydrazine nitrogen atoms. Like asymmetrically disubstituted hydrazines, they are liquids, but their boiling points are typically higher. In particular, the aliphatic compounds are basic and reducing agents and are soluble in water. Aromatic symmetric disubstituted hydrazines are not soluble in water. Symmetrically disubstituted hydrazines are prepared by reducing nitro compounds under basic conditions or by reducing the azines.
Tri- or tetrasubstituted aliphatic hydrazines are water-insoluble weakly basic compounds. The corresponding arylhydrazines are solid colorless substances which are insoluble in water and substantially not basic. They react with concentrated sulfuric acid to form violet or dark blue compounds.
Phenylhydrazine and 2,4-dinitrophenylhydrazine have were used historically in analytical chemistry to detect and identify compounds with carbonyl groups. Phenylhydrazine was used to study the structure of carbohydrates, because the reaction of the sugar's aldehyde groups lead to well crystallizing phenylhydrazonesorosazones.
Organohydrazines and their derivatives are numerous, especially when hydrazones are included.
