In a reaction, reagents X and Y are respectively used to convert salicylic acid: Reagents X and Y are respectively:

Step 1: Identify X — Salicylic acid → Aspirin

Aspirin = acetylsalicylic acid = the phenolic -OH of salicylic acid is acetylated (not the carboxylic acid -OH).

Acetylation of the phenolic -OH: $\ce{o-HO-C6H4-COOH + (CH3CO)2O ->[ H+ ] o-CH3COO-C6H4-COOH + CH3COOH}$

Reagent X = acetic anhydride $(\ce{CH3CO)_2O}$ with $\ce{H+}$ catalyst ✓

Step 2: Identify Y — Methyl salicylate → Salicylic acid (reverse esterification) OR Salicylic acid → Methyl salicylate

The arrow shows Salicylic acid ←Y— Methyl salicylate, meaning Y converts methyl salicylate back to salicylic acid (hydrolysis). But in synthesis, we prepare methyl salicylate FROM salicylic acid:

$\ce{o-HO-C6H4-COOH + CH3OH <=>[H+][\Delta] o-HO-C6H4-COOCH3 + H2O}$

Reagent Y = methanol + H+ (acid catalyst), heat — Fischer esterification of the carboxylic acid group ✓

Key distinction: Aspirin synthesis uses acetic anhydride (acetylates -OH phenol); methyl salicylate synthesis uses methanol (esterifies -COOH).

Key Points to Remember:

• Acetic anhydride: acetylates phenolic -OH → aspirin.
• Fischer esterification (ROH + H+): esterifies carboxylic acid → ester.
• Aspirin synthesis: the product contains both -OCOCH3 (phenol ester) and -COOH (free carboxylic acid).
• Aspirin is an analgesic, antipyretic, anti-inflammatory drug.