Aromatic hydroxy and amino carboxylic acids

Aromatic 0-hydroxy carboxylic acids behave very similarly to fi-oxo carboxylic acids they sometimes lose carbon dioxide when merely boiled with water, but this is a general reaction when they are heated with pyridine, quinoline, aniline, or dimethylaniline. This behavior can certainly be ascribed to reaction of aromatic 0-hydroxy acids as the keto tautomer  

As examples, / - and y-resorcylic acid afford resorcinol readily when boiled with water.30 

When there are two carboxyl groups in ortho-position to a hydroxyl group, both are split off at the same time thus 3,5-xylenol is obtained in 90% yield when 2-hydroxy-4,6-dimethylisophthalic acid is heated in quinoline at 160 to 210° until gas evolution ceases 31 

Pyridonecarboxylic and 0-hydroxyquinolinecarboxylic acids can be similarly decarboxylated. 4-Hydroxy-3-quinolinecarboxylic acid, for example, can be converted into 4-quinolinol by either of the two following methods 32 

Analogous p-hydroxy and p-amino carboxylic acids can be decarboxylated under similar conditions, as in their keto and imino forms they can be regarded 

---

Chlorine, bromine, and iodine can be introduced under mild conditions at free ortho- and /wra-positions to OH or NH2 groups of aromatic hydroxy and amino carboxylic acids, also into similar sulfonic acids and esters. On reaction in water an excess of bromine replaces a COOH group (see page 163). The most easily accessible derivatives of o- and / -hydroxybenzoic acid are the dihalo derivatives in which the halogen atoms are in the ortho- and para-, or in the two ortho-positions, respectively, to the OH group. When treated in... 

Iodo derivatives are particularly interesting members of the halogenated aromatic hydroxy and amino carboxylic acids, for they are intermediates for the preparation of X-ray contrast media. They have been successfully prepared by iodination with IC1, which will therefore be treated in this Section (but see also page 165). 

---