An improved process for preparing a compound selected from among 2,4,6- triiodo-5-amino-N-alkylisophthalamic acid, salts thereof, esters thereof, 2,4,6-triiodo-5-amino-isophthalamide, 2,4,6-triiodo-5- amino-N- hydroxyalkyl-isophthalamide and 2,4,6-triiodo-5-amino-N,N'- bishydroxyalkyl-isophthalamide. A substrate selected from among 5-amino-N- alkylisophthalamic acid, salts thereof, esters thereof 5-amino- isophthalamide, 5-amino-N-hydroxyalkyl-isophthalamide and 5-amino-N,N'- bishydroxyalkylisophthalamide is reacted with an iodine halide in an aqueous reaction medium. In accordance with the improvement, the substrate and a source of the iodine halide are added to the reaction medium at such relative rates that, at any instant substantially throughout the addition cycle, the substrate is present in stoichoimetric excess over the iodine halide, but the difference between the cumulative amount of the substrate that has been added to the medium at such instant, expressed as a proportion of the total ultimate charge of the substrate, and the cumulative amount of the source of iodine halide that has been added to the medium at such instant, expressed as a proportion of the total ultimate charge of the source of iodine halide, does not exceed 10%. When the substrate is 5-amino-isophthalamide, 5-amino-N-hydroxyalkyl- isophthalamide or 5-amino-N,N'-bishydroxyalkyl-isophthalamide, the cumulative amount of the iodine halide may be in stoichiometric excess of not more than about 10% of the substrate computed on the same basis. PPFurther improvements, respectively comprising incorporation of an alkaline buffer composition and operation at relative high dilution, are also disclosed.
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