3-Methylpyridine or 3-picoline, is an organic compound with formula 3-CH3C5H4N. It is one of three positional isomers of methylpyridine, whose structures vary according to where the methyl group is attached around the pyridine ring. This colorless liquid is a precursor to pyridine derivatives that have applications in the pharmaceutical and agricultural industries. Like pyridine, 3-methylpyridine is a colorless liquid with a strong odor and is classified as a weak base.
3-Methylpyridine is produced industrially by the reaction of acrolein, with ammonia. These ingredients are combined as gases which flows over an oxide-based heterogeneous catalyst. The reaction is multistep, culminating in cyclisation.
2 CH2CHCHO + NH3 → CH3C5H4N + 2H2OThis process also affords substantial amounts of pyridine, which arises by demethylation of the 3-methylpyridine. A route that gives better control of the product starts with acrolein, propionaldehyde, and ammonia:
CH2CHCHO + CH3CH2CHO + NH3 → 3-CH3C5H4N + 2 H2O + H2It may also be obtained as a co-product of pyridine synthesis from acetaldehyde, formaldehyde, and ammonia via Chichibabin pyridine synthesis. Approximately 9,000,000 kilograms were produced worldwide in 1989. It has also been prepared by dehydrogenation of 3-methylpiperidine, derived from hydrogenation of 2-Methylglutaronitrile.[1]
3-Picoline is a useful precursor to agrochemicals, such as chlorpyrifos. Chlorpyrifos is produced from 3,5,6-trichloro-2-pyridinol, which is generated from 3-picoline by way of cyanopyridine. This conversion involves the ammoxidation of 3-methylpyridine:
CH3C5H4N + 1.5O2 + NH3 → NCC5H4N + 3H2O
3-Cyanopyridine is also a precursor to 3-pyridinecarboxamide,[2] [3] [4] which is a precursor to pyridinecarbaldehydes:
3-NCC5H3N + [H] + catalyst → 3-HC(O)C5H4N Pyridinecarbaldehydes are used to make antidotes for poisoning by organophosphate acetylcholinesterase inhibitors.
Pyridine derivatives (including 3-methylpyridine) are environmental contaminants, generally associated with processing fossil fuels, such as oil shale or coal.[5] They are also found in the soluble fractions of crude oil spills. They have also been detected at legacy wood treatment sites. The high water solubility of 3-methyl pyridine increases the potential for the compound to contaminate water sources. 3-methyl pyridine is biodegradable, although it degrades more slowly and volatilize more readily from water samples than either 2-methyl- or 4-methyl-pyridine.,[6] [7]
3-Methylpyridine is the main precursor to niacin, one of the B vitamins. Approximately 10,000 tons of niacin are produced annually worldwide.[8]
Like most alkylpyridines, the LD50 of 2-methylpyridine is modest, being 400 mg/kg (oral, rat).[8]