: 180
Aldehydes are important but tricky to synthesize because they are easily over-oxidized to acids or over-reduced to alcohols. Key methods: (1) Rosenmund reduction: RCOCl + H2/Pd-BaSO4 → RCHO (poisoned catalyst stops at aldehyde). (2) Stephen's reduction: RCN + SnCl2/HCl → RCHO. (3) DIBAL-H at -78°C: RCOOR' or RCN → RCHO (temperature-controlled). (4) PCC or PDC oxidation: RCH2OH → RCHO (anhydrous conditions prevent over-oxidation). (5) Etard reaction: ArCH3 + CrO2Cl2/CS2 → ArCHO. (6) Gattermann-Koch: ArH + CO/HCl/AlCl3-CuCl → ArCHO (formylation). (7) Reimer-Tiemann: ArOH + CHCl3/NaOH → o-HO-ArCHO (phenol formylation). (8) Ozonolysis: Alkene + O3/Zn → aldehyde fragments. (9) Hydroboration of alkynes: RC≡CH + disiamylborane then H2O2 → RCHO. Each method has specific substrate scope and selectivity — choosing the right one is a common JEE synthesis challenge.