(A) Acid chlorides on treatment with H2, Pd-BaSO 4 form aldehyde. This reaction is called "Rosenmund reaction".
(B) Benzene can be converted to benzaldehyde by reacting it with CO,HCl, anhydrous AlCl3. This reaction is known as "Gatterman-Koch reaction".
(C) Alkene (acetylene) can be converted to aldehyde (acetaldehyde) by reacting it with HgSO4 and H2SO4. This reaction is called "oxymercuration-demercuration reaction".
(D) DIBAL-H reagent is used to convert esters into aldehydes.