Hydroxamic acids, detection

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. 

It may be noted that primary aliphatic amides are readily converted by hydro-xylamlne hydrochloride into hydroxamic acids, which may be detected by the addition of ferric chloride solution ... 

Indirect detection of an intermediate. The overall reaction of hydroxylamine with a carboxylic acid derivative yields a hydroxamic acid as the product, Eq. (3-176). 

Very recently, Mordini and coworkers" have overcome the problems associated with the long reaction times that are normally required for the synthesis of hydroxamic acids from esters by performing these transformations under MW irradiation. The protective groups are also well tolerated under these reaction conditions, though a partial deprotection of the feri-butoxycarbonyl (Boc) group was observed in the reaction with Boc-proline ester. Amidic bonds and ketals also survive without any detectable decomposition. All the reactions go to completion in about six minutes, except in the case of the conversion of Boc-protected phenylalanine methyl ester, which required longer reaction times (12 min). 

Distribution of compounds in barley and wheat tissues. Tissues of barley and wheat leaves were mechanically separated under the microscope. It was observed that in barley gramine was more concentrated in the epidermis than in the entire leaf (Table II). Hydroxamic acids in wheat were absent in epidermic tissues and were more concentrated in the vascular tissues than in the entire leaf. Neither compound was detected in xylem exudates nor in guttation drops. 

Less than 0.07% of the recovered urinary radioactivity in rats given 100 or 1000 mg/kg bw [ Clacetamide coeluted upon high-performance liquid chromatography with an N-hydroxyacetamide standard and this hydroxamic acid could not be detected after incubation of acetamide with rat liver microsomes and NADPH or in primary cultures of rat hepatocytes. [ C]Acetamide does not bind covalently to proteins in the presence of rat liver microsomes and NADPH or cytosolic fraction, whereas hepatocyte cultures contained non-extractable radioactivity. This association was inhibited by cycloheximide to the same extent as [ CJacetate incorporation into cellular proteins (Dybing et al., 1987). 

Abbasi [75] determined metavanadate in solution by a method based on the formation of a violet colour with vanadium( V) on addition of a chloroform solution of N-(p-NN dimethylanilino-3-methoxy-2-naphtho)hydroxamic acid to the acidified (4-6 mol/1 hydrochloric acid) sample. This solution was evaluated spectrophotometrically at 570 nm. The detection limit was 0.05 xg vanadium at a dilution ratio of 1 107. Very few interferences occur in this procedure. The method was also applied to extracts of soils, plants and geological samples. See also Sect. 2.29. 

Bigler, L., Baumeler, A., Werner, C., and Hesse, M. 1996. Detection of noncovalent complexes of hydroxamic-acid derivatives by means of electrospray mass spectrometry. Helv. Chim. Acta 79, 1701-1709... 

Hydroxamic acids are commonly occurring secondary metabolites in cultivated and wild Gramineae.16,27,43 This family of compounds has not been found in cereal seeds, but they can be detected easily in seedlings and mature plants of cereals such as wheat, corn or rye.3 The presence of these compounds is species dependent and is also influenced by the age of the plant, temperature, photoperiod and organ assayed.4,14,19 Their occurrence has been broadly related with the resistance of cereals (corn, wheat, wild rye, giant reed, etc) to insects and disease organisms. 1... 

Using two cooled bubblers connected in series and charged with ethanol, 0.5-1 L of air was sampled at a flow of 0.3 L/min. The vinyl acetate was converted to N-hydroxyacetamide by the addition of hydroxylamine hydrochloride and sodium hydroxide. After 30 minutes, the hydroxamic acid was complexed with iron by the addition of hydrochloric acid and ferric chloride. The intensity of the color which developed in 10 minutes was compared to the intensities of standards. The lower limit of detection for this method was reported to be 0.3 yg/mL. 

Three papers from Ghosh s group deal with the hydrolysis of benzohydroxamic acids in acidic and alkaline conditions. A pre-equihbrium protonation followed by a slow A-2 type nucleophilic attack by water is seen as the mechanism of the acid-catalysed hydrolysis of j -chlorophenylbenzohydroxamic acid (107 R = / -ClCgEU) by mineral acids (HCl, HCIO4) in 20% aqueous dioxane. An A-2 mechanism was also supported for the reaction of (107 R = Me) imder comparable conditions. The alkaline hydrolysis imder micellar conditions of (107 R = Ph) and a series of para-substituted derivatives has been investigated in the presence of cationic and anionic micelles in 5% dioxane-water medium at 55 °C. Cationic surfactants exerted a catalytic effect and anionic surfactants were inhibitory. The rate-surfactant profiles were analysed in terms of the pseudophase and Piszkiewicz models. The detection of N2O in the products of the oxidation of hydroxamic acids suggests the intermediacy of nitroxyl, HNO, in the process. Scheme 9 may represent the pathway followed. 

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). 

In 75 days old rye plants, a DIBOA concentration of 128 to 423 pg / g dry weight was determined. 35- Day old rye has a potential to release 14.3 kg / ha of DIBOA [169]. Even a release of 16.2 kg DIBOA / ha from 34 day old rye was reported [170]. Fresh rye mulch containing 20 - 50 mmol / ha hydroxamic acid had only half of this concentration after 12 days, and after 121 -168 days the compound was not detectable anymore [62]. Longer lasting phytotoxic effects of the mulch should be due to the more stable decomposition product BOA. It was foimd that fall-killed Balboa rye suppressed weed biomass by 84 % compared to controls [163]. Fall planted spring killed rye mulch was able to inhibit total weed biomass by 68 -95 % [164]. Similar results were reported using rye mulch in no-till cropping systems [174]. 

Detection of vinyl acetate part as ethyl acetate Detection as the hydroxamic acid with iron(III) chloride + IR spectrum + +... 

A test for sulphonic acids is based on conversion to the sulphonyl chloride and then reaction with hydroxylamine. The reaction product can be converted with a drop of acetaldehyde to a hydroxamic acid which is detected in the classical way through the brown-violet colour with a drop of a Fe3+ reagent264. 

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