Replacement of nitrogen with hydrogen (reduction)

The amino functional group can in effect be removed by diazotization followed by reduction.  The most common reagent is hypophosphorous acid (H₃PO₂), but NaBH₄ also works.  Ethanol can also effect the reduction, and this is commoner in the older literature.

Removal of the NH₂ in this way can be useful in synthesis, because the amino group is a powerful activator and ortho-para director.  For example, the NH₂ can be used to introduce three bromines rapidly ortho/para to itself, then the NH₂ can be removed to leave the bromines intact.  A compound such as 2,4,6-tribromobenzoic acid can be easily prepared in this way.

Substitution by an -OH group

The NH₂ can be replaced by OH simply by warming the acidic solution of the benzenediazonium salt, and water acts as the incoming nucleophile.  Since chloride ion can act as a competing nucleophile at higher temperatures, we avoid a halogenation side reaction by using a non-nucleophilic anion such as hydrogen sulfate, HSO₄¯.  The diazonium hydrogen sulfate salt is prepared simply using NaNO₂ in cold dilute sulfuric acid, and then warmed to produce the phenol.  In some cases, the entire reaction is done in warm aqueous H₂SO₄.

Since there is no easy way to introduce an OH group directly onto an aromatic ring via electrophilic aromatic substitution, this reaction is the most commonly used way to prepare phenols in synthesis.  Given the importance of the hydroxyl group in organic chemistry – many phenols are found in nature, for example – means that this reaction is widely used.