The Hafner synthesis, often called the Ziegler–Hafner azulene synthesis, constructs an azulene framework by reacting a cyclopentadienyl anion with a seven-carbon electrophilic chain derived from a pyridinium or pyrylium salt. Nucleophilic substitution or addition first joins the five-carbon cyclopentadienyl unit to the activated C7 fragment; subsequent electrocyclic ring closure and elimination establish the fused five- and seven-membered rings of azulene. Hafner's original synthesis used glutacondialdehyde-derived electrophiles and furnished azulene without a separate dehydrogenation step ^[1]. Modern accounts distinguish pyridinium-derived Zincke iminium routes and related pyrylium variants while retaining the same cyclopentadienyl-plus-C7 construction ^[2]. The term is constitutionally and mechanistically specific: it is not a general palladium cross-coupling, nor does it include unrelated annulations that produce azulenes from different carbon frameworks.