Topic:- PYRIDINE (C5H5N) SHIPRA GUPTA 140990105016 OCUP Topic:- PYRIDINE (C5H5N) SHIPRA GUPTA 140990105016

Pyridine Pyridine occurs in coal tar and in the distillate from bones and has been produced industrially from these sources.

STRUCTURE All rings atom in pyridine (5 carbons + 1 nitrogen) are sp2 hybridized. Two of the sp2 orbitals on each atom overlap with each other to form C-C and C-N σ bond. The third sp2 orbital on each carbon overlap with an s orbit from hydrogen to form the C-H σ bond. The unshared pair of electrons in this orbital is referred to as a nitrogen pair. It makes an angle of 120 degree.

Each ring atom in pyridine also has an unhybridized p orbital containing an electron. This p orbital are perpendicular to plan containing σ bond. The lateral overlap of p orbital produces a delocalization ∏ molecular orbital containing six electron. One half of this ∏ MO lies above and the other half lies below the plan of the σ bonds.

Short Hand representation A common short hand representation of pyridine is simply a hexagon with a circle inside. This can represented as delocalized molecular orbital.

Preparation …

Cont.. 2. By heating a mixture of acetylene, ammonia and formaldehyde dimethylacetal in the presence of aluminum at 500 degree.

Physical properties Pyridine is a colourless liquid, bp 115.5⁰C, having a characteristic unpleasant odour. It is soluble in water and most organic solvents.

Chemical properties Basic character; formation of salts. Electrophilic substitution reaction. Nucleophilic substitution reaction. Reduction.

Basic character; formation of salts Pyridine is basic (pKb= 8.75). It reacts with strong acid to form salts.

Explanation : The reason for the basic character is that the nitrogen lone pair electrons are in sp² hybrid orbital and are not involved in the formation of the delocalized ∏ molecular orbital. It is readily available for the formation of a new N-H bond with proton.

Pyridine is more basic than pyrrole: This is because the nitrogen lone pair electrons in pyrrole are in p orbital and form part of the delocalised ∏ molecular orbital. They are not available for the formation of new N-H bond with proton.

Electrophilic substitution reaction: Pyridine undergoes electrophilic substitution reaction at 3-position only under vigorous conditions. Explanation : Attack of the electrophile at 2-position (or 4-position) in pyridine leads to an intermediate with only two important resonance contributing structures.

Three resonance structures are possible for the intermediate produced by attack at 3-position. That is , the intermediate produced by the attack at 3-position is more stable. This is the reason that electophile attack occurs at 3-position.

Nucleophilic substitution reaction Pyridine undergoes nucleophilic substitution reaction at 2-position. Explanation : Attack of the nucleophile at 3-position in pyridine leads to an intermediate with three resonance contributing structures. Attack of the nucleophile at 2-position (or 4-position) also gives an intermediate with three resonating structures.

Reduction reaction Pyridine undergoes reduction with H₂/Ni or Na/C₂H₅OH to form piperidine.

Uses As a basic solvent in organic reactions. To denature alcohol. For preparing sulfapyridine.

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