Preparation of tri-iodo benzene compounds

A process for the preparation of a 2,4,6-triiodinated benzene ring containing compound which comprises iodinating a 2,4,6-unsubstituted benzene ring containing compound with an iodine halide iodinating agent in an aqueous reaction medium at a pH below 4 and precipitating the triiodinated reaction product, characterised in that precipitation of the triiodinated reaction product is effected by:

• adding an aqueous base to the reaction medium while this is at a temperature in the range of 70 to 95° C., thereby to bring the pH of the reaction medium to above 5 a value in the range of 3 to 7;
• adding a reducing agent to the reaction medium while agitating the reaction medium, adding seed crystals of the triiodinated reaction product to the reaction medium;
• cooling the reaction medium to a temperature below 35° C. over a period of 10 to 30 hours, at a cooling rate of 1.5 to 5° C./hour during the period during which precipitation occurs;
• filtering the reaction medium to collect the precipitated triiodinated reaction product;
• and washing the precipitated triiodinated reaction product with from 70 to 200% by weight relative to the weight of the precipitated triiodinated reaction product of water, whereby to yield a product having a salt content of less than 0.1% by weight.