Halogenated phenols hydroxylation

When halogenated phenols or phenolic ethers are nitrated with nitric acid a halogen o- or p- to hydroxyl or alkoxyl group can also be replaced. The ease of replacement appears to be in the order Cl< Br[Pg.128]

Berger U, Herzke D, Sandanger TM. 2004. Two trace analytical methods for determination of hydroxylated PCBs and other halogenated phenolic compounds in eggs from Norwegian birds of prey. Anal Chem 76 441 -52. 

Indications. An immediate brown colour, or a green colour changing to brown, indicates the presence of an aminophenol or of a phenol having two or more hydroxy groups in adjacent positions on the ring. Monophenols, halogenated phenols, and phenols with hydroxyl groups meta to each other react more slowly or not at all. 

With the simple aryl halides such as the mono-chlor derivatives of benzene or its homologues this reaction does not take place. Tf, however, a benzene halide has also substituted in the ring two nitro, sul-phonic acid or carboxyl groups, in the ortho and para positions to the halogen, then treatment of the halide with potassium hydroxide results in replacing the halogen with hydroxyl and the corresponding substituted phenol will be obtained. 

But phenol hydroxyl groups cannot be replaced by the action of a halogen hydracid. 

The more energetic action of the phosphorus halides may also be perceived in the fact that phenol hydroxyl groups can be replaced by a halogen, by the use of the phosphorus compounds, which, as mentioned above, is impossible with the halogen hydracids, eg. ... 

The reactions of the phenolic hydroxyl group which have been included in this chapter comprise esterification with carboxylic acids and related reactants, with inorganic materials, with replacements producing nitrogen, sulphur and halogen-containing compounds, procedures for removal of the OH group and miscellaneous reactions. 

Hydroxide Ions. The photolysis of propanil, 4-CPA, and other chlorinated aromatic pesticides in water also can generate phenols through the replacement of halogen by hydroxyl groups (39, 41). Oxygen does not seem to have an important part in this reaction, if it is involved at all, and observation that the replacement proceeds upon irradiation... 

Aromatic hydroxyls Phenols, halogenated phenols, R(Hal) + 1200 salicylic acid (and derivatives) bcnzofuranols, chlorophenols (and metabolites), paracetamol, hesperidin, tyrosine... 

Halogen substitution for hydrogen in phenols can also be effected. Halogens (Cl, Br, and I) enter ortho and para with respect to the phenol hydroxyl (-OH). Chlorination with excess chlorine (CI2) in the presence of iron(III) chloride (ferric chloride, FeCla) at 70-75°C produces 2,3,4,6-tetrachlorophenol (Equation 8.27). 

Usually the above method may be applied to substances with a free phenolic hydroxyl group and a free para position or a para position that is substituted by a halogen, hydroxyl or alkoxyl, sulphonic acid or carboxylic acid group since in general these substances give a positive reaction. 

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