The synthesis of pyridine from acetaldehyde and ammonia is known as the Chichibabin pyridine synthesis. Here is a detailed step-by-step mechanism for the reaction:1. Nucleophilic attack of ammonia on acetaldehyde:Ammonia NH3 acts as a nucleophile and attacks the carbonyl carbon of acetaldehyde CH3CHO , forming an intermediate hemiaminal.Intermediate 1: Hemiaminal CH3CH OH NH2 2. Dehydration of the hemiaminal:The hemiaminal undergoes dehydration, losing a water molecule H2O to form an imine.Intermediate 2: Imine CH3CH=NH 3. Formation of an enamine:Another molecule of acetaldehyde reacts with the imine, with the nitrogen atom acting as a nucleophile and attacking the carbonyl carbon of the acetaldehyde. This forms an intermediate iminium ion, which then undergoes a 1,2-hydride shift to form an enamine.Intermediate 3: Enamine CH3CH=CH-NH2 4. Nucleophilic attack of ammonia on the enamine:Ammonia NH3 acts as a nucleophile again and attacks the -carbon of the enamine, forming an intermediate amino dienamine.Intermediate 4: Amino dienamine CH3CH NH2 CH=CH-NH2 5. Cyclization and dehydration:The amino dienamine undergoes intramolecular cyclization, with the nitrogen atom attacking the -carbon of the dienamine, forming a cyclic intermediate. This intermediate then undergoes dehydration, losing a water molecule H2O to form dihydropyridine.Intermediate 5: Dihydropyridine C5H5N 6. Aromatization:Dihydropyridine undergoes dehydrogenation, losing two hydrogen atoms H2 to form the final product, pyridine.Product: Pyridine C5H5N In summary, the Chichibabin pyridine synthesis involves a series of nucleophilic attacks, dehydration, cyclization, and aromatization steps to form pyridine from acetaldehyde and ammonia.