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Chemical structure of imidazole
IUPAC name
Other names 1,3-diazole
CAS number 288-32-4 Yes check.svgY
EC number 206-019-2
RTECS number NI3325000
ChemSpider ID 773
Molecular formula C3H4N2
Molar mass 68.077 g/mol
Appearance white or pale yellow solid
Density 1.23 g/cm3, solid
Melting point

89-91 °C (362-364 K)

Boiling point

256 °C (529 K)

Solubility in water miscible
Acidity (pKa) pKa=14.5, pKBH+=6.993
Crystal structure monoclinic
planar 5-membered ring
Dipole moment 3.61D
MSDS External MSDS
R-phrases R20 R22 R34 R41
S-phrases S26 S36 S37 S39 S45
Flash point 146 °C
 Yes check.svgY (what is this?)  (verify)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Imidazole is an organic compound with the formula C3H4N2. This aromatic heterocyclic is classified as an alkaloid. Imidazole refers to the parent compound whereas imidazoles are a class of heterocycles with similar ring structure but varying substituents. This ring system is present in important biological building blocks such as histidine, and the related hormone histamine. Imidazole can serve as a base and as a weak acid. Many drugs contain an imidazole ring, such as antifungal drugs and nitroimidazole.[1][2][3][4][5]



Imidazole was first synthesized by Heinrich Debus in 1858, but various imidazole derivatives had been discovered as early as the 1840s. His synthesis, as shown below, used glyoxal and formaldehyde in ammonia to form imidazole.[6] This synthesis, while producing relatively low yields, is still used for creating C-substituted imidazoles.


In one microwave modification the reactants are benzil, formaldehyde and ammonia in glacial acetic acid forming 2,4,5-triphenylimidazole (Lophine).[7]

Structure and properties

Imidazole is a 5-membered planar ring, which is soluble in water and other polar solvents. It exists in two equivalent tautomeric forms because the hydrogen atom can be located on either of the two nitrogen atoms. Imidazole is a highly polar compound, as evidenced by a calculated dipole of 3.61D, and is entirely soluble in water. The compound is classified as aromatic due to the presence of a sextet of π-electrons, consisting of a pair of electrons from the protonated nitrogen atom and one from each of the remaining four atoms of the ring.

Some resonance structures of imidazole are shown below:



Imidazole is amphoteric, i.e. it can function as both an acid and as a base. As an acid, the pKa of imidazole is 14.5, making it less acidic than carboxylic acids, phenols, and imides, but slightly more acidic than alcohols. The acidic proton is located on N-1. As a base, the pKa of the conjugate acid (cited above as pKBH+ to avoid confusion between the two) is approximately 7, making imidazole approximately sixty times more basic than pyridine. The basic site is N-3.


A ball-and-stick model of imidazole, showing carbon-carbon and a carbon-nitrogen double bonds.

Imidazole can be synthesized by numerous methods besides the Debus method. Many of these syntheses can also be applied to different substituted imidazoles and imidazole derivatives simply by varying the functional groups on the reactants. In literature, these methods are commonly categorized by which and how many bonds form to make the imidazole rings. For example, the Debus method forms the (1,2), (3,4), and (1,5) bonds in imidazole, using each reactant as a fragment of the ring, and thus this method would be a three-bond-forming synthesis. A small sampling of these methods is presented below.

Formation of one bond

The (1,5) or (3,4) bond can be formed by the reaction of an imidate and an α-aminoaldehyde or α-aminoacetal, resulting in the cyclization of an amidine to imidazole. The example below applies to imidazole when R=R1=Hydrogen.

Formation of one bond
Formation of Two Bonds

The (1,2) and (2,3) bonds can be formed by treating a 1,2-diaminoalkane, at high temperatures, with an alcohol, aldehyde, or carboxylic acid. A dehydrogenating catalyst, such as platinum on alumina, is required.

Formation of two bonds

The (1,2) and (3,4) bonds can also be formed from N-substituted α-aminoketones and formamide and heat. The product will be a 1,4-disubstituted imidazole, but here since R=R1=Hydrogen, imidazole itself is the product. The yield of this reaction is moderate, but it seems to be the most effective method of making the 1,4 substitution.

Formation of three bonds
Formation of Four Bonds

This is a general method which is able to give good yields for substituted imidazoles. It is essentially an adaptation of the Debus method called the Debus-Radziszewski imidazole synthesis. The starting materials are substituted glyoxal, aldehyde, amine, and ammonia or an ammonium salt.[8]

Arduengo imidazoles
Formation from other Heterocycles

Imidazole can be synthesized by the photolysis of 1-vinyltetrazole. This reaction will only give substantial yields if the 1-vinyltetrazole is made efficiently from an organotin compound such as 2-tributylstannyltetrazole. The reaction, shown below, produces imidazole when R=R1=R2=Hydrogen.


Imidazole can also be formed in a vapor phase reaction. The reaction occurs with formamide, ethylenediamine, and hydrogen over platinum on alumina, and it must take place between 340 and 480 °C. This forms a very pure imidazole product.

Biological significance and applications

Imidazole is incorporated into many important biological molecules. The most pervasive is the amino acid histidine, which has an imidazole side chain. Histidine is present in many proteins and enzymes and plays a vital part in the structure and binding functions of hemoglobin. Histidine can be decarboxylated to histamine, which is also a common biological compound. It is a component of the toxin that causes urticaria, which is another name for allergic hives. The relationship between histidine and histamine are shown below:

Histidine decarboxylase.svg

One of the applications of imidazole is in the purification of His-tagged proteins in immobilised metal affinity chromatography(IMAC). Imidazole is used to elute tagged proteins bound to Ni ions attached to the surface of beads in the chromatography column. An excess of imidazole is passed through the column, which displaces the His-tag from nickel co-ordination, freeing the His-tagged proteins.

Imidazole has become an important part of many pharmaceuticals. Synthetic imidazoles are present in many fungicides and antifungal, antiprotozoal, and antihypertensive medications. Imidazole is part of the theophylline molecule, found in tea leaves and coffee beans, which stimulates the central nervous system. It is present in the anticancer medication mercaptopurine, which combats leukemia by interfering with DNA activities.

Pharmaceutical derivatives

The substituted imidazole derivatives are valuable in treatment of many systemic fungal infections[9].

Industrial applications

Imidazole has been used extensively as a corrosion inhibitor on certain transition metals, such as copper. Preventing copper corrosion is important, especially in aqueous systems, where the conductivity of the copper decreases due to corrosion.

Many compounds of industrial and technological importance contain imidazole derivatives. The thermostable polybenzimidazole PBI contains imidazole fused to a benzene ring and linked to a benzene, and acts as a fire retardant. Imidazole can also be found in various compounds which are used for photography and electronics.

Salts of imidazole

Simple imidazolium cation

Salts of imidazole where the imidazole ring is in the cation are known as imidazolium salts (for example, imidazolium chloride). These salts are formed from the protonation or substitution at nitrogen of imidazole. These salts have been used as ionic liquids and precursors to stable carbenes. Salts where a deprotonated imidazole is an anion are also possible; these salts are known as imidazolide salts (for example, sodium imidazolide).

Related heterocycles


  1. ^ Alan R. Katritzky; Rees. Comprehensive Heterocyclic Chemistry. Vol. 5, p.469-498, (1984).
  2. ^ Grimmett, M. Ross. Imidazole and Benzimidazole Synthesis. Academic Press, (1997).
  3. ^ Brown, E.G. Ring Nitrogen and Key Biomolecules. Kluwer Academic Press, (1998).
  4. ^ Pozharskii, A.F, et al. Heterocycles in Life and Society. John Wiley & Sons, (1997).
  5. ^ Heterocyclic Chemistry TL Gilchrist, The Bath press 1985 ISBN 0-582-01421-2
  6. ^ Heinrich Debus (1858). "Ueber die Einwirkung des Ammoniaks auf Glyoxal". Annalen der Chemie und Pharmacie 107 (2): 199 – 208. doi:10.1002/jlac.18581070209.  
  7. ^ Microwave-Mediated Synthesis of Lophine: Developing a Mechanism To Explain a Product Crouch, R. David; Howard, Jessica L.; Zile, Jennifer L.; Barker, Kathryn H. J. Chem. Educ. 2006 83 1658
  8. ^ US6,177,575 (PDF version) (2001-01-23) A. J. Arduengo, Process for Manufacture of Imidazoles.  
  9. ^ Comprehensive Pharmacy Review, Leon Shargel, 6th edition, p930.

1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

IMIDAZOLES, or Glyoxalines, organic chemical compounds CH=CH containing the ring system HN< I Imidazole itself was CH=N first prepared by H. Debus(A nn. 1858, 107, p. 2 54) by the action of ammonia on glyoxal, 2C2H202-1-2NH3=C3H4N2- { -H2C02-}-2H20.

The compounds of this series may be prepared by the condensation of ortho-diketones with ammonia and aldehydes R CO CO R+2NH 3 +R' CHO=3H 2 O+ fl; R C-NHS from thioimidazolones by oxidation with dilute nitric acid (W. Marckwald, Ber., 1892, 2 5, p. 2361); by distillation of hydrobenzamide and similarly constituted bodies; and by the action of phosphorus pentachloride on symmetrical dimethyloxamide, a methylchlorglyoxaline being formed (0. Wallach, Ann., 1877, 184, p. 500) .

The glyoxalines are basic in character, and the imide hydrogen is replaceable by metals and alkyl groups. They are stable towards reducing agents, and acidyl groups are only introduced with difficulty.

Imidazole (glyoxaline), C 3 H 4 N 2, crystallizes in thick prisms which melt at 88-89° C. and boil at 253° C., and are readily soluble in alcohol and in water. It is unaffected by chromic acid, but potassium permanganate oxidizes it to formic acid. It forms salts with acids.

CcH5 C-N Loplcine (triphenylglyoxaline), >C C6H5, is formed C6H5 C-NH by the dry distillation of hydrobenzamide, or by saturating an alcoholic solution of benzil and benzaldehyde (at a temperature of 40° C.) with ammonia. It crystallizes in needles which melt at 275° C. It is a weak base. When heated to 300° C. with hydriodic acid and hydrochloric acid, in the presence of some red phosphorus, it yields benzoic acid.

The keto-glyoxalines are known as imidazolones and are prepared by the action of acids on acetalyl thioureas (W. Marckwald, Ber., 18 9 2, 25, p. 2 357). Benzimidazole, C€H4 NHCH, is the simplest representative of the benzoglyoxalines and is prepared by the condensation of formic acid with ortho-phenylene diamine. It forms rhombic crystals which melt at 170° C. It is basic in character, and on oxidation with potassium permanganate yields a [[Hooc C-N N]] small amount of glyoxaline dicarboxylic acid, II 5CH.

[[Hooc C-Nh']] (E. Bamberger, Ann., 18 93, 2 73, p. 338).

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