Acid-catalyzed formation of mixed carboxylic anhydride

Steps:

• Identify the reaction as dehydration between two carboxylic acids under acidic conditions to form a mixed anhydride.
• Ethanoic acid is CH₃COOH (C₂H₄O₂); the product structure implies involvement of formic acid (HCOOH).
• Formic acid (CH₂O₂) + ethanoic acid → mixed anhydride CH₃C(O)OCHO + H₂O.
• Confirm molecular formula of X matches formic acid, CH₂O₂, based on product's carbon-oxygen framework.

Why D is correct:

• CH₂O₂ is the formula for formic acid (HCOOH), which forms the mixed anhydride with ethanoic acid, matching the product's structure and reaction type.

Why the others are wrong:

• A: C₂H₄O (e.g., acetaldehyde) forms acetals or other products, not the shown anhydride.
• B: C₂H₄O₂ is ethanoic acid itself; self-reaction yields symmetric anhydride, not the mixed product shown.
• C: C₂H₆O (ethanol) undergoes esterification to ethyl ethanoate, not the anhydride structure.

Final answer: D