Preparation and Reactions of Pyridine
Pyridine is six membered nitrogen containing aromatic compound with the chemical formula C5H5N. It is structurally related to benzene, with one methine group (=CH−) replaced by a nitrogen atom. It is a highly flammable, weakly alkaline, water-miscible liquid with a distinctive, unpleasant fish-like smell.
Pyridine is a stronger base than pyrrole because the lone pair of nitrogen in pyrrole is involved in the formation of the delocalized π molecular orbital, so not available for the formation of a new N-H bond with proton or simply you can say the basicity decreases due to delocalization of nitrogen lone pair of electron.
Pyridine undergoes electrophilic substitution reaction at position C-3. However, attack of electrophile at position C-2 or C-4, more electronegative nitrogen bears a +ve charge in one the resonating structures and makes it unstable. Because of the unstable nature of one of the resonating structure of the intermediate ion formed during the attack of electrophile at position C-2 or C-4, the electrophilic substitution in pyridine at position C-3 is always favoured.
Preparation of Pyridine
Preparation of Pyridine from acroline
Pyridine can be prepared by the reaction of acroline(Acrylaldehyde) and ammonia according to following reaction steps.Preparation of Pyridine From Pyrrole
Pyrrole when heated with methylene chloride in presence of sodium ethoxide, pyridine is formed.Preparation of Pyridine From Picoline
Beta-picoline on oxidation with potassium dichromate and sulphuric acid gives nicotinic acid, which on decarboxylation with calcium oxide gives pyridine.Industrial Method for the Preparation of Pyridine
Pyridine is prepared by heating the acetylene, ammonia and formaldehyde dimethylacetal in the presence of alumina at 500°C.Hantzsch Pyridine Synthesis
Condensation of two moles of β keto esters with one molecule of an aldehyde in the presence of ammonia is known as the Hantzsch pyridine synthesis. Pyridine derivatives can easily be prepared by this method.Physical Properties of Pyridine
Pyridine is a colourless liquid. Its boiling point is 115.5° C. It has a characteristic unpleasant odor. It is soluble in water and most organic solvents.Chemical Properties of Furan/Reactions of Pyridine
Resonance in Pyridine
Basic Character of Pyridine
Pyridine behaves as a base and forms fairly stable salts with acids. The lone of nitrogen is not involved in the formation of delocalized π molecular orbital, so is available for the formation of a new N-H bond with proton.Pyridine is a stronger base than pyrrole because the lone pair of nitrogen in pyrrole is involved in the formation of the delocalized π molecular orbital, so not available for the formation of a new N-H bond with proton or simply you can say the basicity decreases due to delocalization of nitrogen lone pair of electron.
Electrophylic Substitution Reactions in Pyridine
Pyridine is an aromatic compound and is less aromatic than benzene and pyrrole. It highly deactivated aromatic nucleus towards electrophilic substitution reactions. So, highly vigorous reaction conditions should be used for these reactions to take place. The low reactivity of pyridine towards the electrophilic substitution reactions is due to the higher electro negativity of nitrogen atom reduces electron density on the ring, thus deactivate the ring and also it is highly sensitive to acidic medium, it readily forms pyridinium cation with a positive charge on nitrogen atom. Similarly, electrophile itself may also react with pyridine to form corresponding pyridinium ion. This positive charge on nitrogen atom decreases electron density on nitrogen atom, consequently, the electron density on ring also decreases.Pyridine undergoes electrophilic substitution reaction at position C-3. However, attack of electrophile at position C-2 or C-4, more electronegative nitrogen bears a +ve charge in one the resonating structures and makes it unstable. Because of the unstable nature of one of the resonating structure of the intermediate ion formed during the attack of electrophile at position C-2 or C-4, the electrophilic substitution in pyridine at position C-3 is always favoured.
