Replacement of halogen by carbonyl-oxygen

Benzylidene chloride obtained by chlorination of toluene is converted into benzaldehyde by dissolving iron(n) benzoate in the chloride, covering the solution with a layer of water, and heating the mixture at 90-95° until evolution of hydrogen chloride ceases. Saturation with slaked lime and distillation in steam affords the aldehyde.530 

4-Chlorobenzylidene chloride is hydrolysed by concentrated sulfuric acid in 5 hours at room temperature,531 and 4-chloro-3,3,4-trifluoro-l-phenyl-cyclo-butene gives 3-phenyl-3-cyclobutene-l,2-dione (75% yield) when treated with 

It is not always necessary to start with gem-dihalides for the synthesis of aldehydes or ketones. There is a process by which carbonyl compounds can be obtained from monohalides, sometimes in excellent yield for instance, when 3-chlorocyclopentene is stirred vigorously with aqueous sodium dichromate solution at 0°, a chromium complex of 2-cyclopenten-l-one is formed, and this is decomposed to give a 60-68% yield of the ketone if 50% sulfuric acid is dropped in carefully with cooling 513 and 1,2-cyclopentanedione can be obtained in 80% yield by dropwise addition of aqueous iron(m) chloride solution to 2-chlorocyclopentanone in water with rapid stirring at 100°.539 

A process due to Sommelet540 is generally applicable (halomethyl)aryl compounds react with hexamethylenetetramine in alcohol or chloroform to yield quaternary ammonium salts, which decompose to aldehydes, formaldehyde, methylamine, and ammonia when boiled with water or dilute acid it is usually unnecessary to isolate the intermediate compound if the whole process is carried out in 1 1 acetic acid-water. 

Korschak and G. S. Kolesnikow, Synthese organischer Verbindungen, Vol. 2, 1956, VEB Verlag, Berlin, and Porta Verlag, Munich, p. 136. 

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