Aromatic Hydrocarbons Reactions Mechanism

Hint: Phenol is formed when benzene diazonium chloride reacts with warm water.
Explanation:

(1) When acetylene (C₂H₂) is passed through a red hot iron tube (typically at around 873 K), it undergoes a trimerization reaction, where three molecules of acetylene combine to form benzene. This process is a classic example of cyclic polymerization and is used industrially for the preparation of benzene from acetylene.



(2) When benzene diazonium chloride is warmed with water, it undergoes hydrolysis to form phenol, nitrogen gas, and hydrochloric acid. The reaction proceeds as follows:



(3) When benzene carboxylate (benzoic acid or its sodium salt, sodium benzoate) is heated with soda lime (a mixture of sodium hydroxide and calcium oxide), it undergoes decarboxylation. In this reaction, the carboxyl group (–COOH) is removed as carbon dioxide, and a hydrogen atom replaces it on the benzene ring, forming benzene as the main product.

(4) When n-hexane is heated with Mo₂O₃, it undergoes a process called aromatization, resulting in the formation of benzene. This reaction occurs at high temperatures (e.g., 500°C or 773 K) and under pressure (e.g., 10-20 atm). The Mo₂O₃ acts as a catalyst to facilitate the cyclization and dehydrogenation steps, transforming the linear alkane n-hexane into a cyclic aromatic compound, benzene. 
 
Hence, option 2 is the correct answer.