Esters ferric hydroxamate test

Although esters, like aldehydes and ketones, are neutral compounds that have a carbonyl group, they do not usually give a positive 2,4-dinitrophenylhydrazine test result. The two most common tests for identifying esters are the basic hydrolysis and ferric hydroxamate tests. 

Procedure Acetyl chloride (0.05 ml), dimethylaniline (0.05 ml), and 0.1 ml of the tested alcohol are shaken occasionally in a micro test tube. After 3—4 min the mixture is diluted with 0.5 ml of ice-cold water, stirred thoroughly (decomposition of acetyl chloride), and the ester allowed to separate. One to two drops of the ester layer are transferred with a micropipette into another test tube, where the hydroxamate test is then carried out 0.5 ml of a saturated methanolic hydroxylamine hydrochloride solution is added, the mixture is shaken and then mixed with 0.5 ml of a saturated KOH solution in methanol. The mixture is shaken again and heated over a water bath or over a microflame just to the boiling point, and after cooling it is acidified with 2 n HCl and additioned with 1 —2 drops of a 1% ferric chloride solution. Pink, red, or violet color is proof of the presence of alcohol in the original sample. 

Sulfosuccinate esters can be determined by the ferric hydroxamate procedure (54). This is a general test for esters. The esters are reacted with hydroxylamine hydrochloride in the presence of base to form the hydroxamic acids (RCONHOH). The hydroxamic acids are then detected by the color formed when ferric chloride is added to the mixture. The competing reaction, base-catalyzed hydrolysis of the esters to form the carboxylic acid, is inhibited by running the hydroxlaminolysis at ice-bath temperature. Other esters will interfere with this determination, and the various sulfosuccinates can not be distinguished from each other. 

As esters are usually difficult to detect, this test is of considerable value. In general esters react when heated with hydroxylamine to give a hydroxamic acid (I). The latter gives a coloured complex (II) with ferric salts in acid solution. 

This test identifies the presence of esters, and if the test is negative the assumption is made that a polyether is present. The test is based on the reaction of the ester with hydroxylamine, with the formation of the hydroxamic salt and the subsequent reaction with acidic ferric chloride to give a complex salt possessing an intense violet or purple colour. 

This test for an ester group involves the use of hydroxylamine and ferric chloride. The former converts the ester to a hydroxamic acid, which then complexes with Fe to give a colored species (Eqs. 25.62 and 25.63). 

Lactones will give a positive ester test, forming a hydroxamic acid when treated with alkaline hydroxylamine this acid gives an intense color with ferric chloride. This test has been used as a means of detecting lactones on a paper chromatogram (28). Free acids do not give it, and the paper must be treated with diazomethane first in order to convert the acids present to the esters. This method has also been used for the quantitative determination of gluconic 6-lactone (28a). 

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