An unknown aromatic compound 'X' is treated with oleum to form compound 'Y'. Compound 'Y' is fused with NaOH at high temperature and then acidified to yield compound 'Z'. When 'Z' is treated with chloroform in the presence of aqueous sodium hydroxide, it forms salicylaldehyde as the major product. Identify the starting compound 'X'.

Step-by-step solution

Working backwards from the final product: Compound 'Z' reacts with chloroform ( CHCl ₃ ) and aqueous NaOH to form salicylaldehyde. This is the Reimer-Tiemann reaction, which is a characteristic reaction of phenol. Therefore, compound 'Z' is phenol. Phenol ('Z') is produced by the fusion of compound 'Y' with NaOH followed by acidification. This is the standard preparation method of phenol from benzenesulfonic acid. Thus, compound 'Y' is benzenesulfonic acid. Benzenesulfonic acid ('Y') is formed by treating the start