O-Phenylenediamine

o-Phenylenediamine[1]
IUPAC name Benzene-1,2-diamine
Other names o-Phenylene diamine 1,2-Diaminobenzene 1,2-phenylenediamine
Identifiers
CAS number	95-54-5 Y
ChemSpider	13837582 Y
UNII	8B713N8Q0F N
ChEBI	CHEBI:34043 N
ChEMBL	CHEMBL70582 Y
Jmol-3D images	Image 1
SMILES Nc1ccccc1N
InChI InChI=1S/C6H8N2/c7-5-3-1-2-4-6(5)8/h1-4H,7-8H2 Y Key: GEYOCULIXLDCMW-UHFFFAOYSA-N Y
Properties
Molecular formula	C6H8N2
Molar mass	108.1 g/mol
Density	1.031 g/cm3
Melting point	102 to 104 °C; 216 to 219 °F; 375 to 377 K
Boiling point	252 °C; 486 °F; 525 K (Other sources: 256 — 258 °C.)
Solubility in water	soluble in hot water
Acidity (pKa)	0.80[2] pka2 = 4.57 [3]
Hazards
MSDS	Oxford MSDS
EU classification	Toxic (T); Dangerous for the environment (N)
NFPA 704	1 2 0
N (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references

o-Phenylenediamine is an organic compound with the formula C₆H₄(NH₂)₂. This aromatic diamine is an important precursor to many heterocyclic compounds. It is isomeric with m-phenylenediamine and p-phenylenediamine, and is commonly referred to as OPD.

Preparation

Commonly 2-nitrochlorobenzene is treated with ammonia, and the resulting 2-aminonitrobenzene is then hydrogenated:^[4]

ClC₆H₄NO₂ + 2 NH₃ → H₂NC₆H₄NO₂ + NH₄Cl

H₂NC₆H₄NO₂ + 3 H₂ → H₂NC₆H₄NH₂ + 2 H₂O

In the laboratory, the reduction of the nitroaniline is effected with zinc powder in ethanol, followed by purification of the diamine as the hydrochloride.^[5] This compound darkens in air; impurities may be removed by treating a hot aqueous solution with sodium dithionite (reducing agent) and activated carbon, and allowing the product to cool crystallize.

Reactions and uses

o-Phenylenediamine condenses with ketones and aldehydes to give rise to a variety of useful products. Reactions with carboxylic acids and their derivatives afford benzimidazoles. The herbicide benomyl is prepared in this manner. Also, quinoxalinedione may be prepared by condensation of o-phenylenediamine with dimethyl oxalate. Condensation with xanthate esters affords mercaptoimidazoles, which are used as antioxidants in rubber products. Treatment with nitrous acid give benzotriazole, a corrosion inhibitor. Condensation of substituted o-phenylenediamine with various diketones is used in the preparation of a variety of pharmaceuticals.^[6]

In coordination chemistry, phenylenediamine is an important ligand precursor. Schiff base derivatives, such as those derived from salicylaldehyde, are excellent chelating ligands. Oxidation of its metal-phenylenediamine complexes affords the diimine derivatives, which are intensely colored and often exist in multiple stable oxidation states.^[7]

Safety

With an LD₅₀ of 44 mg/L (aquatic), o-phenylenediamine is about 1000 times less toxic than the para-isomer. Anilines are typically handled as if they are carcinogenic. For many applications, OPD has been replaced by safer alternatives such as 3,3',5,5'-tetramethylbenzidine.^[8]

References

1. ↑ DuPont Specialty Intermediates: o-Phenylenediamine (OPD)
2. ↑ http://www.phs.d211.org/science/smithcw/AP%20Chemistry/Posted%20Tables/Acid%20Dissociations.pdf
3. ↑ http://www.phs.d211.org/science/smithcw/AP%20Chemistry/Posted%20Tables/Acid%20Dissociations.pdf
4. ↑ Robert A. Smiley “Phenylene- and Toluenediamines” in Ullmann's Encyclopedia of Industrial Chemistry 2002, Wiley-VCH, Weinheim. doi:10.1002/14356007.a19_405
5. ↑ E. L. Martin (1943), "o-Phenylenediamine", Org. Synth. ; Coll. Vol. 2: 501 
6. ↑ See for example, Renault, J. et al. (1981). "Heterocyclic quinones. Quinoxaline-5,6 and 5,8 diones, potential antitumoral agents". Eur. J. Med. Chem. 16: 545–550. 
7. ↑ Warren, L. F. (1977). "Synthesis of [M'-N₄] and [M'-N₆] Complexes Based on o-Benzoquinone Diimine with Cobalt, Iron, and Ruthenium". Inorg. Chem. 16 (11): 2814–2819. doi:10.1021/ic50177a028. 
8. ↑ Deshpande SS (1996). Enzyme Immunoassays: From Concept to Product Development. New York: Chapman & Hall. p. 169. ISBN 978-0-412-05601-7.