The formation of furan from 1,4-butanediol under acidic conditions involves a series of reactions, including dehydration, cyclization, and dehydrogenation. Here's a step-by-step description of the mechanism:1. Protonation: In the presence of an acid, one of the hydroxyl groups of 1,4-butanediol gets protonated, forming a positively charged oxonium ion.2. Dehydration: The oxonium ion undergoes a dehydration reaction, where a water molecule is eliminated, leading to the formation of a carbocation at the position where the hydroxyl group was protonated.3. Cyclization: The carbocation undergoes an intramolecular nucleophilic attack by the lone pair of electrons on the oxygen atom of the other hydroxyl group, forming a 5-membered ring. This results in the formation of an oxonium ion in the ring.4. Deprotonation: The oxonium ion loses a proton, leading to the formation of a tetrahydrofuran THF intermediate.5. Protonation and Dehydration: The THF intermediate undergoes a second protonation at one of the carbon atoms adjacent to the oxygen atom, followed by a dehydration reaction to form a dihydrofuran intermediate with a double bond.6. Aromatization: The dihydrofuran intermediate undergoes a final dehydrogenation step, where two hydrogen atoms are removed, and a second double bond is formed, resulting in the formation of the aromatic compound furan.In summary, the mechanism of furan formation from 1,4-butanediol under acidic conditions involves protonation, dehydration, cyclization, and dehydrogenation steps.