Replacement of halogen by the hydroxyl group

Hydrolysis of alkyl halides is carried out in practice by boiling them with aqueous alkali, with a suspension of alkaline-earth carbonates, or with a suspension of lead(n) oxide. A two-stage process is often used, in which the halide is converted by treatment with an alkali acetate into the ester which is then hydrolysed. This route is particularly useful for synthesis of 1,2-diols because hydrolysis of 1,2-dihalides usually gives poor yields owing to formation of by-products. 

Hydrolysis of at-halo carbonyl compounds must be effected very cautiously since the resulting -hydroxy carbonyl compounds are sensitive to alkali. An aqueous suspension of barium carbonate has been used for preparation of 2-hydroxy-3-phenylpropionaldehyde.521 The preparation of a-hydroxy carboxylic acids is important Witzemann522 has given details for preparation of glycolic acid, and Bischoff and Walden523 recorded that of 2-hydroxybutyric acid  

2-Bromobutyric acid (100 g) was boiled for 5-6 h with an equivalent amount (83 g) of potassium carbonate in water (500 ml). The solution was then evaporated, the residue was treated with the calculated amount of hydrochloric acid, and the viscous solution was filtered from the precipitated mixture of potassium chloride and bromide and extracted with ether. Drying of the extract over sodium sulfate, removal of the ether, and several fractionations in a vacuum gave 2-hydroxybutyric acid, m.p. 42°, b.p. 140°/12 mm. The yields were not stated. 

Loss of hydrogen halide may occur as a side reaction to hydrolysis of aliphatic halides, but this can often be prevented by working with freshly precipitated silver oxide, which reacts even in the cold. Olefin formation can, however, be avoided with certainty by hydrolysing the olefin by way of the acetate. 

It is difficult to replace aromatically bound halogen by a hydroxyl group. Hydrolysis of halobenzenes has been effected under pressure in the presence of alkali or after addition of catalytically active substances.526 There are numerous patents describing, e.g., the conversion of chlorobenzene into phenol 527 and phenols have been obtained from bromobenzene derivatives in an autoclave in the presence of copper catalysts.528 Hydrolysis of halo-benzene derivatives is easier if they contain electron-attracting substituents in the ortho- or /wra-position to the halogen. 

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