Definition:
“Benzimidazole, a heterocyclic aromatic organic compound consisting of a fusion of benzene and imidazole, in an extension of the well-elaborated imidazole system, has been used as a carbon skeleton for N-heterocyclic carbenes, usually used as ligand for transition metal complexes.”
Molecular formula:
Its molecular formula is C7H6N2.
Structural formula:
Physical properties
Physical properties- Benzimidazoles have high melting points. The melting point is given as 170-172oC.
- The boiling point of benzimidazole is360 °C.
- Benzimidazoles are usually soluble in polar solvents and sparingly soluble in non polar solvents.
- Preparation of benzimidazole:Benzimidazoles are weakly basic, being somewhat less basic than imidazole.
- Benzimidazoles are also sufficiently acidic to be generally soluble in aqueous alkali and form N-metallic compounds. The acidic properties of benzimidazoles, like those of imidazole, seem to be due to stabilization of the ion by resonance.
- The molar mass of benzimidazole is 118.14 g mol−1
Preparation:
Benzimidazole is usually prepared by the reaction of o-Phenylenediamine (C6H4(NH2)2) with formic acid (HCO2H).
Reactions of benzimidazole:
Benzimidazoles can undergo the following reactions:
- Alkylation
- Acylation
- Halogenations
- Nitration
- Reaction with Grignard reagents
- sulfuric acid or chloro sulfonic acid
- Reactions of benzimidazoline 2- thione
Use of benzimidazole:
Benzimidazole is mainly used in the following ways:
- It is used as a pesticide.
- It is used as an antiviral and antibiotic.
- Some veterinary benzimidazoles are also used as anthelmintics in human medicines. Anthelmintics are drugs that expel parasitic worms (helminthes) from the body, by either stunning or killing them.
- It is also used in chemotherapy.
- Most benzimidazole derivatives are only minimally soluble in water, so they may be applied as a paste or in a liquid suspension.
Benzimidazole Fungicides
Benzimidazole fungicides are low use, broad-spectrum fungicides, widely used in agriculture for pre-harvest and post-harvest treatment for control of a wide range of pathogens. Benzimidazole fungicides were the first fungicides to be used to control eyespot, an important fungal disease of wheat caused by the necrotrophic fungus, in Europe from around 1974. At the time of their introduction, they represented a ground-breaking class of fungicides with unique properties that included systemic and curative activity that allowed extended spray intervals. World-wide, benzimidazole fungicides are registered in many countries on more than 70 crops including cereals and grapes.
Examples:
Few examples of benzimidazoles are as follows:
- Albendazole
- Benomyl
- Carbendazim
- Thiophanate-methyl
Resistance status:
Benzimidazoles represent the beginning of serious resistance problems in fungicides. A few years after commercialization, loss of disease control with benzimidazole fungicides was recorded in many crops, especially with pathogens having numerous cycles per year, such as Botrytis cinerea(a necrotrophic fungus). For those funguses, such as Tapesia spp, that have 1-2 life cycles per year, the resistance developed after 10 years.
Environmental fate
Terrestrial fate:
Benzimidazole is expected to have high mobility in soil. Benzimidazole is not expected to volatilize from dry soil surfaces, based upon an estimated vapor pressure of 7.6X10-5 mm Hg. The benzene based fungicides; Benlate and thiabendazole do not migrate any appreciable distance in soils. When applied to soil surfaces they did not move more than 25 mm into the soil. Benzimidazole does not support the growth of mixed cultures of microbes that could grow on benzimidazole-based fungicides. When repeatedly applied benzimidazole results in the increased degradation of these microbes.
Aquatic fate:
An estimated Koc value-a measure of a material's tendency to adsorb to soil particles-of 110, determined from an estimation method, indicates that benzimidazole is not expected to adsorb to suspended solids and sediment in water. At low pH (pKa = 5.3(8)), it is expected to adsorb to suspended solids and sediments. Volatilization from water surfaces is not expected. The potential for bioconcentration in aquatic organisms is low.
Atmospheric fate:
Benzimidazole, which has an estimated vapor pressure of 7.6X10-5 mm Hg at 250C, is expected to exist in the both the vapor and particulate phases in the atmosphere. Particulate-phase benzimidazole may be removed from the air by wet and dry deposition. Benzimidazole absorbs light at 245, 271 and 278 nm, so it has little potential for direct photolysis in the atmosphere.
Reference
http://extoxnet.orst.edu/tibs/glossary.htm
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