Hydroxamic acid test

Both succinic and phthalic anhydride respond to the hydroxamic acid test (see 5 above). 

When the compound for identification fails to respond to test 4 (aldehyde or ketone), the next class reactions to apply are the hydroxamic acid test and saponification, i.e., hydrolysis in alkaline solution. These are the class reactions for esters and anhydrides the rarely-encountered lactones react similarly. 

A. Carry out the following preliminary test. Dissolve a drop or a few small crystals of the compound in 1 ml. of rectified spirit (95 per cent, ethanol) and add 1 ml. of hydrochloric acid. Note the colour produced when 1 drop of 5 per cent, ferric chloride solution is added to the solution. If a pronounced violet, blue, red or orange colour is produced, the hydroxamic acid test described below is not applicable and should not be used. 

Equilibrium diagrams, 22-26 construction of, 26-29 Equivalent weight of an acid, 1071 Ester-amides, table of, 425 Esterification, 379 -382, 1000, 1001 Esters, hydroxamic acid test for, 1062, 1063 ... 

The presence of any of these functional types may be established chemically by applying the hydroxamic acid test. These compounds react with hydroxyl-amine in the presence of sodium hydroxide to form the sodium salt of the corresponding hydroxamic acid. On acidification and addition of iron(m) chloride solution the magenta coloured iron(m) complex of the hydroxamic acid is formed. 

The hydroxamic acid test is also given by acid chlorides and some primary aliphatic amides which are readily converted by hydroxylamine hydrochloride into hydroxamic acids. Some esters, mainly of carbonic, carbamic, sulphuric and other inorganic acids, give only a yellow colour. 

It is always advisable to ensure that the sample does not give a colour with iron(m) chloride before carrying out the hydroxamic acid test. 

The hydroxamic acid forms a red-blue complex with iron(III) ion. The unknown must first give a negative test with hydroxylamine at lower temperature (Group 1(f), Alkaline Hydroxamic Acid Test) before trying this test. 

Ammonium salts on treatment with alkali liberate ammonia, which can be detected by its odor and the fact that it will turn red litmus, blue. A more sensitive test utilizes the copper(II) ion, which is blue in the presence of ammonia [see Group VIII a(i)]. Ammonium salts will not give a positive hydroxamic acid test (Ih) as given by amides. 

Unsubstituted amides are detected by the hydroxamic acid test. Group 1(h). 

It is necessary to establish that esters are absent before using the hydroxamic acid test for any other class of compound. 

Test for aliphatic anodes. Aliphatic amides, but not aromatic amides, (with the exception of salicylamide, which contains a free phenolic group and reacts directly with ferric chloride solution) give the hydroxamic acid test under the following conditions Add 0 1 g of the compound to 1 ml of 5% methanoUc hydro-xylamine hydrochloride reagent and boil for three minutes. When a drop of ferric chloride solution is added to the cooled solution, a wine-red or purple colour is produced. 

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