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Department of Chemistry, Anugrah Memorial College, Gaya

B. Sc. Chemistry (Hons) Part – III Paper: VII – Organic Chemistry (Gr. – B) Unit II: Heterocyclic Compounds Topic – Pyridine Lecture: #1 By: Dr. Rajesh Ranjan Pandey (Assistant Professor)

PYRIDINE

Lecture - 1 Contents:  Introduction  Physical Properties  Aromaticity  Tautomerism  Hydrogen Bonding  Synthesis (#1 - 5)

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Pyridine is a heterocyclic organic compound with the chemical formula C5H5N. It is structurally related to benzene, with one methine group (=CH−) replaced by a nitrogen atom. The pyridine ring occurs in many important compounds, including agrochemicals, pharmaceuticals, and vitamins.

Physical Properties:  It is colourless liquid, but older or impure samples can appear yellow.  It is a highly flammable.  It has a distinctive, unpleasant fish-like smell.  It is a water-miscible liquid.  Melting point: −41.6 °C  Boiling Point: 115.2 °C

Properties:

1. Aromaticity:

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2. Tautomerism:

3. H- Bonding: Pyridine is soluble in water due to hydrogen bonding.

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Synthesis of Pyridine:

1. From acetylene: (a) Bönnemann cyclisation: The trimerisation of a part of a nitrile molecule and two parts of acetylene into pyridine in the presence of Co(I) catalyst is called Bönnemann cyclisation.

(b) By passing a mixture of acetylene and hydrogen cyanide through a red-hot iron tube:

(c) By reaction of acetylene with ammonia and formaldehyde dimethylacetal in the presence of aluminium oxide at 500°C. (Commercial Method)

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2. By dehydrogenation of piperidine: Pyridine can be prepared by dehydrogenation of piperidine with concentrated sulphuric acid at 300°C or with nitrobenzene at 260°C.

3. From Pyrrole:

(a) Ciamician–Dennstedt rearrangement: Its an extension of Reimer–Tiemann reaction and entails the ring-expansion of pyrrole with dichlorocarbene to 3-chloropyridine.

(b) When treated with sodium methoxide and methylene iodide, pyrrole undergoes ring expansion forming pyridine.

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4. Erom tetrahydrofurfuryl alcohol: Pyridine can be prepared by heating tetrahydrofurfuryl alcohol with ammonia in the presence of aluminium oxide at 500°C. (Commercial Method)

5. From glutaconic aldehyde : Pyridine can be prepared by y heating glutaconic aldehyde with ammonia.

(To be continued…)