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Topic:- PYRIDINE (C5H5N) SHIPRA GUPTA 140990105016
OCUP Topic:- PYRIDINE (C5H5N) SHIPRA GUPTA
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Pyridine Pyridine occurs in coal tar and in the distillate from bones and has been produced industrially from these sources.
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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.
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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.
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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.
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Preparation …
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Cont.. 2. By heating a mixture of acetylene, ammonia and formaldehyde dimethylacetal in the presence of aluminum at 500 degree.
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Physical properties Pyridine is a colourless liquid, bp 115.5⁰C, having a characteristic unpleasant odour. It is soluble in water and most organic solvents.
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Chemical properties Basic character; formation of salts.
Electrophilic substitution reaction. Nucleophilic substitution reaction. Reduction.
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Basic character; formation of salts
Pyridine is basic (pKb= 8.75). It reacts with strong acid to form salts.
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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.
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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.
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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.
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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.
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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.
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Reduction reaction Pyridine undergoes reduction with H₂/Ni or Na/C₂H₅OH to form piperidine.
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Uses As a basic solvent in organic reactions. To denature alcohol.
For preparing sulfapyridine.
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