The formation of pyridine from acetaldehyde and ammonia is known as the Chichibabin pyridine synthesis. This reaction involves a series of condensation and cyclization steps to form the pyridine ring. Here is the step-by-step mechanism of the reaction:1. Nucleophilic attack of ammonia on acetaldehyde: The lone pair of electrons on the nitrogen atom of ammonia attacks the carbonyl carbon of acetaldehyde, forming an intermediate hemiaminal.2. Dehydration of the hemiaminal: The hemiaminal undergoes dehydration, losing a molecule of water to form an imine also known as a Schiff base .3. Aldol condensation: Another molecule of acetaldehyde undergoes an aldol condensation with the imine formed in step 2. This involves the nucleophilic attack of the alpha-carbon of the second acetaldehyde molecule on the imine carbonyl carbon, followed by the elimination of water to form a -aminocrotonaldehyde.4. Michael addition: The -aminocrotonaldehyde formed in step 3 undergoes a Michael addition, in which the nitrogen atom of the amino group attacks the -carbon of the ,-unsaturated carbonyl group, forming a cyclic intermediate.5. Tautomerization: The cyclic intermediate undergoes tautomerization, which involves the migration of a hydrogen atom and the formation of a double bond to yield dihydropyridine.6. Aromatization: Finally, dihydropyridine undergoes a dehydrogenation reaction, losing two hydrogen atoms to form the aromatic pyridine ring.Key intermediate compounds involved in this reaction include:- Hemiaminal- Imine Schiff base - -Aminocrotonaldehyde- Cyclic intermediate- DihydropyridineThe final product of this reaction is pyridine, which is an important heterocyclic aromatic compound used in various chemical and pharmaceutical applications.