Production of omega chloro- and bromo-tiglaldehyde and its acetal and acylate

4 - Chloro - and - bromo - 2 - formylbutene-(2) or acetals and acylates thereof are produced by treating 2-formyl-2-hydroxybutene-(3) or its acetals or acylates with thionyl chloride, thionyl bromide or phosgene in the presence of N,N-dialkyl substituted amides of low molecular weight fatty acids, including compound in which the N alkyl groups are connected to each other direct or via oxygen, or N-alkyl substituted lactams, optionally in the presence of anhydrous solvents when a concentration of 0.01 mol of the amide relative to the formyl-hydroxybutene is preferred. 2 - Formyl - 2 - hydroxybutene - (3) is prepared by hydrogenation, by the process of Specification 871,804, of 2 - formyl - 2 - hydroxybutyne-(3), itself obtained by reacting methylglyoxal acetal with acetylene. It is stated that the compounds prepared by the process of the invention find use in the preparation of the corresponding 4-hydroxy-2-formyl-butene-(2) compounds.