Methods for the introduction of a hydroxyl group into an aromatic ring

1 METHODS FOR THE INTRODUCTION OF A HYDROXYL GROUP INTO AN AROMATIC RING 

A fairly general procedure, which has also been used on the industrial scale, involves heating the alkali metal sulphonate with either sodium or potassium hydroxide in the presence of a small amount of water to aid the fusion process. The reaction mechanism may be formulated as a bimolecular nucleophilic addition-elimination sequence. 

Occasionally in the synthesis of phenols by this route oxidation products are formed. A particular example is provided by the alkali fusion of sodium anthraquinone-2-sulphonate during which a second hydroxyl group is introduced into the 1-position, forming the dyestuff alizarin (1) (cognate preparation in Expt 6.99). In the procedure described the oxidation step is promoted by the deliberate introduction of potassium chlorate as an oxidant. 

Direct nucleophilic displacement of the halogen in an aryl halide is difficult and hydrolysis to phenols requires high temperatures and pressures the method is therefore only suitable on the large scale. The presence of a nitro group in the ortho or para position, however, makes the halogen more labile since electron withdrawal by the nitro group in these positions stabilises the intermediate anion by electron delocalisation. p-Chloronitrobenzene, for example, is hydrolysed to p-nitrophenol when heated with 15 per cent sodium hydroxide solution at about 150 °C. 

When two activating nitro groups are present hydrolysis takes place readily with dilute aqueous alkali solution (e.g. 2,4-dinitrophenol, Expt 6.100). 

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Methods for the introduction of a hydroxyl group into an aromatic ring by (a) replacement of a sulphonic acid group (Expt 6.99) (b) replacement of a halogen (Expts 6.100 and 6.101) and (c) replacement of a diazo group (see Expt 6.69). 

Methods for the introduction of a hydroxyl group into an aromatic ring 970... 

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