Pyrrolidine is a saturated five-membered cyclic compound in which nitrogen is part of the ring. Pyrrolidine is  also known as tetrahydropyrrole or azacyclopentane and is basic in nature. It is practically planar and structurally  flexible. All the carbon atoms in pyrrolidine are sp3  hybridized. The unshared lone pair of electrons on nitrogen constitutes the fourth group linked to the nitrogen and can be considered as sp3 -hybridized nitrogen with a tetrahedral  bond angle of approximately 109 degrees.

Physical Properties 

Pyrrolidine is a colorless to pale yellow, pungent-smelling liquid, highly soluble in water and various organic  solvents. It is a cyclic secondary amine with ammonia-like odor, and is heavier than air with a bp of 89°C. During  combustion it produces toxic oxide. Its conjugate acid has a pKa  of 11.27.

Chemical Properties 

Pyrrolidine being a good nucleophile easily undergoes electrophilic substitution reactions with different electrophiles such alkyl halides and acyl halides, and forms N-substituted pyrrolidines. N-Alkylpyrrolidine on further  reaction with alkyl halide provided quaternary salts. On reaction with cyclic ketones, enamines are obtained but  reaction with nitrous acid N-nitrosopyrrolidine has been isolated. Pyrrolidine on reaction with ketenedithioacetals  gave mono- and dipyrrolidino derivatives. Reaction of parent pyrrolidine with alkyl/aryl isocyanates or isothiocyanates provided 1,3-disubstituted ureas/thioureas.

Uses

Various modifications of pyrrolidines are present in nature as well as in synthetic chemistry. Numerous natural  alkaloids such as nicotine, hygrine, atropinone, proline, and many more have a pyrrolidine ring as a substructure in  their molecular makeup.

Pyrrolidine is present in alcoholic beverages and widely distributed in foodstuffs like bread, milk, cheese, carrots,  beer, and caffeine in traces. Its presence in foodstuffs is probably due to decarboxylation of the carboxyl group of  proline by bacteria. It is also present in various clinically used drugs and pharmaceuticals. Pyrrolidine has been found to display radical scavenger, antiinflammatory, antitumor, hepatoprotective, and also  metal-chelating properties.

Synthesis

Synthesis Parent pyrrolidine has been prepared by heating a mixture of tetrahydrothiophene or THF and ammonia in the  presence of alumina at 300°C.