Triiodobenzoyl chloride

3 4 6-Triiodobonzoyl chloride may be prepared by the following series of reactions  

It is an excellent reagent for the characterisation of alcohol-ethers (see Section 111,27). 

4-Amino-3 5-diiodobenzoic acid. In a 2 litre beaker, provided with a mechanical stirrer, dissolve 10 g. of pure p-aminobenzoic acid, m.p. 192 (Section IX,5) in 450 ml. of warm (75 ) 12-5 per cent, hydrochloric acid. Add a solution of 48 g. of iodine monochloride (1) in 40 ml. of 25 per cent, hydrochloric acid and stir the mixture for one minute during this time a yellow precipitate commences to appear. Dilute the reaction mixtiu e with 1 litre of water whereupon a copious precipitate is deposited. Raise the temperature of the weU-stirred mixture gradually and maintain it at 90 for 15 minutes. Allow to cool to room temperature, filter, wash thoroughly with water and dry in the air the yield of crude acid is 24 g. Piuify the product by dissolving it in dilute sodium hydroxide solution and precipitote with dilute hydrochloric acid the yield of air-dried 4-amino-3 5-diiodobenzoic acid, m.p. 350 , is 23 g. 

3 4 5-Triiodobeiizoic acid. Dissolve 6-8 g. of 3 5-diiodo-4-aminobenzoic acid in 30 ml. of cold concentrated sulphuric acid, add a large excess (3-0 g.) of powdered sodium nitrite, and allow the mixture to stand at 0 for 2 hours. Treat the cold diazonium solution with a solution of 17-0 g. of potassium iodide in 40 ml. of water a dark red precipitate separates. Warm the mixture on a water bath until the evolution of nitrogen ceases, and remove any residual iodine with a httle 

3 4 5-Triiodobenzoyl chloride. Reflux 5 g. of 3 4 5-triibdo-benzoic acid, m.p. 289-290°, gently with 10 ml. of redistilled thionyl chloride for 2 hours. Distil off the excess of thionyl chloride on a water bath, and recrystallise the residue from carbon tetrachloride - light petroleum with the use of a little decolourising charcoal. The yield of the acid chloride (bright yellow needles, m.p. 138°) is 3-8 g. it keeps well in a stoppered bottle. 

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Triiodobenzoates. The derivatives enumerated above are unsatisfactory for alcohol - ethers, e.g., the mono-ethers of ethyleneglycol ( cellosolves ) and the mono-ethers of diethyleneglycol ( car-bitols ) (see Table 111,27). Crystalline derivatives of alcohol - ethers are readily obtained with 3 4 5-triiodobenzoyl chloride (for preparation, see Section VII,21), for example ... 

Place 0 -5 g. of 3 4 5 triiodobenzoyl chloride in a small test-tube, add 0 -25 ml. of the alcohol - ether and heat the mixture gently over a micro burner until the evolution of hydrogen chloride ceases (3-5 minutes). Pour the molten mass into 10 ml. of 20 per cent, alcohol to which crushed ice has been added. Some derivatives solidify instantly those which separate as oils change to solids in a few minutes without further manipulation. Recrystallise from rectified spirit (use 50 per cent, alcohol for esters of methyl and butyl carbitol ). 

Triiodobenzoyl chloride is prepared by refluxing 5 g of 3,4,5-triiodoben-zoic acid (Expt 6.70) with 10 ml of thionyl chloride for 2 hours. The excess thio-nyl chloride is removed by distillation and the residue recrystallised from carbon tetrachloride-light petroleum. The acid chloride has m.p. 138 °C the yield is 3.8 g. It should be kept in a well-stoppered bottle. 

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