3D model (JSmol)
CompTox Dashboard (EPA)
|Molar mass||59.072 g·mol−1|
|Melting point||50 °C (122 °F; 323 K)|
Std enthalpy of
|−57 – −55 kJ mol−1|
Std enthalpy of
|−1.0511 – −1.0531 MJ mol−1|
|Lethal dose or concentration (LD, LC):|
LD50 (median dose)
|475 mg/kg (oral, rat)|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Guanidine is the compound with the formula HNC(NH2)2. It is a colourless solid that dissolves in polar solvents. It is a strong base that is used in the production of plastics and explosives. It is found in urine as a normal product of protein metabolism. A guanidine moiety also appears in larger organic molecules, including on the side chain of arginine.
Guanidine can be thought of as a nitrogenous analogue of carbonic acid. That is, the C=O group in carbonic acid is replaced by a C=NH group, and each OH is replaced by a NH
2 group. A detailed crystallographic analysis of guanidine was elucidated 148 years after its first synthesis, despite the simplicity of the molecule. In 2013, the positions of the hydrogen atoms and their displacement parameters were accurately determined using single-crystal neutron diffraction.
The compound was first synthesized in 1861 by the oxidative degradation of an aromatic natural product, guanine, isolated from Peruvian guano. The commercial route involves a two step process starting with the reaction of dicyandiamide with ammonium salts. Via the intermediacy of biguanidine, this ammonolysis step affords salts of the guanidinium cation (see below). In the second step, the salt is treated with base, such as sodium methoxide.
With a pKb of 0.4, guanidine is a strong base. Most guanidine derivatives are in fact salts containing the conjugate acid.
The conjugate acid is called the guanidinium cation, (C(NH
3). This planar, symmetric ion consists of three amino groups each bonded to the central carbon atom with a covalent bond of order 4/3. It is a highly stable +1 cation in aqueous solution due to the efficient resonance stabilization of the charge and efficient solvation by water molecules. As a result, its pKa is 13.6 meaning that guanidine is a very strong base in water; in neutral water, it exists almost exclusively as guanidinium.
Guanidine exists protonated, as guanidinium, in solution at physiological pH.
Guanidinium chloride (also known as guanidine hydrochloride) has chaotropic properties and is used to denature proteins. Guanidinium chloride is known to denature proteins with a linear relationship between concentration and free energy of unfolding. In aqueous solutions containing 6 M guanidinium chloride, almost all proteins lose their entire secondary structure and become randomly coiled peptide chains. Guanidinium thiocyanate is also used for its denaturing effect on various biological samples. Guanidinium chloride is used as an adjuvant in treatment of botulism, introduced in 1968, but now its role is considered controversial – because in some patients there was no improvement after this drug administration.
Guanidinium hydroxide is the active ingredient in some non-lye hair relaxers.
Guanidines are a group of organic compounds sharing a common functional group with the general structure (R
5. The central bond within this group is that of an imine, and the group is related structurally to amidines and ureas. Examples of guanidines are arginine, triazabicyclodecene, saxitoxin, and creatine.
Galegine is isoamylene guanidine.
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