o-Toluidine (ortho-toluidine) is an organic compound with the chemical formula CH3C6H4NH2. It is the most important of the three isomeric toluidines. It is a colorless liquid although commercial samples are often yellowish. It is a precursor to the herbicides metolachlor and acetochlor. Synthesis and reactions o-Toluidine is produced industrially by nitration of toluene to give a mixture of nitrotoluenes, favoring the ortho isomer. This mixture is separated by distillation. 2-Nitrotoluene is hydrogenated to give o-toluidine. The conversion of o-toluidine to the diazonium salt gives access to the 2-bromo, 2-cyano-, and 2-chlorotoluene derivatives. N-acetylation is also demonstrated. Prilocaine, an amino amide-type local anesthetic, yields o-toluidine when metabolized by carboxylesterase enzymes. Large prilocaine doses can cause methemoglobinemia due to oxidation of hemoglobin by o-toluidine. Metabolism Absorption distribution and excretion o-Toluidine is absorbed through inhalation and dermal contact. Extensive absorption of o-toluidine from the gastrointestinal tract was observed. The main excretion pathway is through the urine where up to one-third of the administered compound was recovered unchanged. Major metabolites are 4-amino-m-cresol and to a lesser extent, N-acetyl-4-amino-m-cresol, azoxytoluene, o-nitrosotoluene, N-acetyl-o-toluidine, N-acetyl-o-aminobenzyl alcohol, anthranilic acid, N-acetyl-anthranilic acid, 2-amino-m-cresol, p-hydroxy-o-toluidine. Conjugates that were formed were predominated by sulfate conjugates over glucuronide conjugates by a ratio of 6:1. The metabolism of o-toluidine involves many competing activating and deactivating pathways, including N-acetylation, N-oxidation and N-hydroxylation, and ring oxidation. 4-Hydroxylation and N-acetylation of toluidine are the major metabolic pathways in rats. The primary metabolism of o-toluidine takes place in the endoplasmic reticulum. Exposure to o-toluidine enhances the microsomal activity of aryl hydrocarbon hydroxylase (particularly in the kidney), NADPH-cytochrome c reductase and the content of cytochrome P-450. Cytochrome P450–mediated N-hydroxylation to N-hydroxy-o-toluidine, a carcinogenic metabolite, occurs in the liver. N-Hydroxy-o-toluidine can be either metabolized to o-nitrosotoluene or conjugated with glucuronic acid or sulfate and transported to the urinary bladder via the blood. Once in the bladder, N-hydroxy-o-toluidine can be released from the conjugates in an acidic urine environment to either react directly with DNA or be bio-activated via sulfation or acetylation by cytosolic sulfotransferases or N-acetyltransferases (presumably NAT1). The postulated activated form (based on comparison with other aromatic amines), N-acetoxy-o-toluidine, is a reactive ester that forms electrophilic arylnitrenium ions that can bind to DNA. Other activation pathways (ring-oxidation pathways) for aromatic amines include peroxidase-catalyzed reactions that form reactive metabolites (quinone-imines formed from nonconjugated phenolic metabolites) in the bladder. These metabolites can produce reactive oxygen species, resulting in oxidative cellular