Pyridine

Pyridine, also known as azine, is an organic compound consisting of a benzene ring with one =CH- group being replaced by a nitrogen atom, giving the chemical formula C₅H₅N.

Properties

Chemical

Pyridine is a weakly basic due to the lone pair on its nitrogen atom, and forms a stable hydrochloride salt when reacted with hydrochloric acid. It readily forms many coordination complexes with transition metal ions, as do its derivatives, 2,2'-bipyridine and terpyridine. Stronger Lewis bases can be subsituted for these ligands, making these complexes useful precursors.

Pyridine is a useful Lewis base in organic chemistry. It is a precursor to piperidine, another organic building block, which can be prepared by hydrogenation in the presence of a catalyst. In addition, pyridine can function as a polar solvent within which reactions can occur. Pyridinium chlorochromate, produced from pyridine and chromic and hydrochloric acids, can be used to oxidize primary alcohols to aldehydes and secondary alcohols to ketones.

Physical

Pyridine is a clear liquid at room temperature with a disagreeable "fishy" odor. Pyridine has historically been added to ethanol to denature it, owing to its very bitter taste. It is miscible with water and polar solvents. It is also highly flammable, with a flash point of only 17 °C, but an autoignition temperature far outside the range of those commonly encountered.

Acquisition

Pyridine can be purchased, though not cheaply, from several online chemistry suppliers in nearly pure form.

Preparation

Pyridine can be prepared in one route useful to the home chemist by the catalytic decarboyxlation of niacin(nictotinic acid) at the boiling point of pyridine(115.2 °C) using copper chromite, in which pyridine vapor distills off and can be condensed. The copper chromite used is easily produced from copper ammonium chromate(produced from copper sulfate and an ammonium dichromate) through strong heating to cause decomposition. The same synthesis may be accomplished at temperatures in excess of 300°C using calcium oxide instead. It is also created by a catalyzed vapor phase reaction between ammonia, formaldehyde, and acetaldehyde.

Pyridine can also be derived by distillation from either coal tar or, historically, bone oil, a product of the destructive distillation of bones which contains several useful reagents if properly separated.

Projects

• Piperidine synthesis
• General solvent

Safety

Pyridine is toxic and possibly carcinogenic, causing a myriad of health issues if exposure is high. Because it is easily detectable by its smell and unlikely to be consumed due to its very bitter taste, chances of serious or lethal exposure are low. Pyridine is also very flammable, with a flash point of only 17 degrees. As such, any reactions or use of pyridine should be done away from ignition sources and with proper fire safety equipment on hand.

It is often said that pyridine can cause instant male sterility on intake, but these claims are difficult to substantiate.

References

Relevant Sciencemadness threads

• Preparation of Pyridine