2.4- Dinitrophenylhydrazone from aldehydes

As noted above, it is not easy to purify unsubstituted hydrazones. Therefore, reactions of hydrazone derivatives with iodine monofluoride have also been studied (Table 1). A-Methyl-and iV,iV-dimcthylhydrazones react more slowly than unsubstituted hydrazones with iodine monofluoride higher tempteratures are often required for their conversion into ge/w-difluorides (Table 1). The reason for this behavior is that elimination of iodomethane from the intermediates is less favorable (see Scheme 1, R = Me). The use of pure derivatives increases the yield in cases sensitive to hydrolysis (cf. entries 25 and 26). 2,4-Dinitrophenylhydrazones only react with iodine monofluoride at temperatures above O C. The yields are generally lower than in the case of unsubslituted hydrazones, and 2,4-dinitrophenylhydrazones formed from aldehydes are unreaetive. 

Dinitrophenylhydrazones. All aldehydes and ketones readily form bright-yellow to dark-red 2,4-dinitrophenylhydrazones. Yellow derivatives are formed from isolated carbonyl groups and orange-red to red derivatives from aldehydes or ketones conjugated with double bonds or aromatic rings. 

Periodate ion has been determined by the formation of a chromophore with 2,2 -azinobis(3-ethylbenzothiazoline-6-sulphonic acid) and by means of an iodide-selective electrode. " The 2,4-dinitrophenylhydrazones derived from aldehydic fragments produced on periodate oxidation of carbohydrates have been separated by t.l.c., after which the individual hydrazones could be determined spectrophotometrically. ... 

In addition to this, Grignard reagents prepared from alkyl and acyl halo-genides can be converted by reaction with dimethylformamide and 2,4-dinitrophenylhydrazine to 2,4-dinitrophenylhydrazones of aldehydes (see p. 218) (4). 

Dinitrophenylhydra2ones usually separate in well-formed crystals. These can be filtered at the pump, washed with a diluted sample of the acid in the reagent used, then with water, and then (when the solubility allows) with a small quantity of ethanol the dried specimen is then usually pure. It should, however, be recrystallised from a suitable solvent, a process which can usually be carried out with the dinitrophenylhydrazones of the simpler aldehydes and ketones. Many other hydrazones have a very low solubility in most solvents, and a recrystallisation which involves prolonged boiling with a large volume of solvent may be accompanied by partial decomposition, and with the ultimate deposition of a sample less pure than the above washed, dried and unrecrystal-lised sample. 

The experimental procedure to be followed depends upon the products of hydrolysis. If the alcohol and aldehyde are both soluble in water, the reaction product is divided into two parts. One portion is used for the characterisation of the aldehyde by the preparation of a suitable derivative e.g., the 2 4-dinitrophenylhydrazone, semicarbazone or di-medone compound—see Sections 111,70 and 111,74). The other portion is employed for the preparation of a 3 5-dinitrobenzoate, etc. (see Section 111,27) it is advisable first to concentrate the alcohol by dis tillation or to attempt to salt out the alcohol by the addition of solid potassium carbonate. If one of the hydrolysis products is insoluble in the reaction mixture, it is separated and characterised. If both the aldehyde and the alcohol are insoluble, they are removed from the aqueous layer separation is generally most simply effected with sodium bisulphite solution (compare Section Ill,74),but fractional distillation may sometimes be employed. 

Imine formation from such reagents as hydroxylamine and 2,4-dinitro-phenylhydrazine is sometimes useful because the products of these reactions— oximes and 2,4-dinitrophenylhydrazones (2,4-DNPs), respectively—are often crystalline and easy to handle. Such crystalline derivatives are occasionally prepared as a means of purifying and characterizing liquid ketones or aldehydes. 

When present in macro quantities, aldehydes and ketones can be determined by conversion to the 2,4-dinitrophenylhydrazone which can be collected and weighed. When present in smaller quantities (10 3M or less), although hydrazone formation takes place, it does not separate from methanol solution, but if alkali is added an intense red coloration develops the reagent itself only produces a slight yellow colour. Measurement of the absorbance of the red solution thus provides a method for quantitative determination. 

Many organic chemicals are analyzed by RPC. These include various arylhydroxylamines as the N-hydroxyurea derivative with methyl isocyanate (614) alkyl- and alkoxy-disubstituted azoxybenzenes (6t5), n-alkyl-4-nitrophenylcarbonate esters ranging in length from methyl to octyl (616), 4-nitrophenol in the presence of 4-nitrophenyl phosphate (617), ben-zilic acid, and benactyzine-HCI using ion-pair chromatography (618), as well as aniline and its various metabolites (619), stereoisomers of 4,4 -dihydroxyhydrobenzoin (620), and aldehydes and ketones as the 2,4-dinitrophenylhydrazones (621). The technique has also been used to analyze propellants and hydrazine and 1,1-dimethylhydrazine were quantitita-vely determined (622, 623). 

To the clear solution obtained by warming 0-5 g. of 2 4-dinitro-phenylhydrazine, 1 ml. of concentrated hydrochloric acid and 8-10 ml. of ethanol, add 0-25 g. of the aldehyde and heat just to boiling. Allow to cool to room temperature, filter off the 2 4-dinitrophenylhydrazone and reorystallise it from ethanol or glacial acetic acid. 

A red 2.4 dinitrophenylhydrazone is obtained from the reaction of an unknown with an amine. Of the following four structures, which ketone or aldehyde could have formed this iinine ... 

You should already be familiar with approximately half of the reactions listed in Table 9.2 from your introductory class. Moreover, you have probably tried to prepare an oxime, a phenylhydrazone, a 2,4-dinitrophenylhydrazone, or a semicarbazone. These compounds serve as crystalline derivatives with sharp and characteristic melting points for identifying aldehydes and ketones and for distinguishing them. When spectroscopic methods for structure elucidation were not available, such a means of identification was very important. 

Fig. 5.3. Gas chromatogram of 2,4-dinitrophenylhydrazones of ten aliphatic aldehydes. Peaks 1 = formaldehyde 2 = acetaldehyde 3 = propionaldehyde 4 = acrolein 5 = isobutyraldehyde 6 = n-butyraldehyde 7 = isovaleraldehyde 8 = n-valeraldehyde 9 = crotonaldehyde 10 = n-capronaldehyde. For conditions see text. (Reproduced from / Chromatogr., 120 (1976) 379, by courtesy of Y. Hoshika.)...

Dinitrophenylhydrazones (DNPHs) were applied to the GC analysis of keto acids. As with carbonyl compounds, they are prepared by reaction with 2,4-dinitrophenylhydrazine and are also used mainly for the preliminary isolation of keto acids. They can be isolated from a dilute aqueous sample by adsorption on activated carbon and selective desorption [178] hydrazones of aldehydes with a methyl formate-dichloromethane mixture and hydrazones of keto acids with a pyridine-water azeotropic mixture. Hydrazones of acids are released from their pyridine salts with methanol containing hydrogen chloride. After... 

Treatment of the intermediate aldehyde 69 with 2,4-dinitrophenylhydrazine in ethanol for 5 min yielded, after recrystallization from water bright yellow-orange crystals of Af-(2-oxoethyl)-2-(2-nitroimidazol-l-yl)acetamide 2,4-dinitrophenylhydrazone, 70, for identification by NMR. 

Colorometric procedures involving reaction of aldehydes with hydrazines, semicarbazide, or piperidine/nitroprusside solutions are also non-specific and lack sensitivity (15, 35, 36). Schmidt et al. (33) have proposed an HPLC method for analyzing the 2,4-dinitrophenylhydrazone (DNPH) derivatives of specific aldehydes. This procedure allows for a number of ddehydes to be separated and measured simultaneously, however, HPLC methods in general suffer from poor resolving power and may have low sensitivity (37). In addition, hydrazine derivatizations are often performed under acidic conditions for maximal reactivity these conditions would not provide quantitative information on total aldehyde content. 

The ethanol-insoluble mixture of phenylhydrazones was extracted several times with hot chloroform and the solvent removed. The reddish residue was then recrystallized from pyridine yield of levulinic aldehyde, 2,4-dinitrophenylhydrazone, 90.5% melting point, 232° C. Literature melting point, 235-6° C. (5). 

According to the studies performed by the Bundesanstalt fur Materi-alprufung, Germany (BAM), this compound may explode when dry, but in the presence of 20% water there is no longer any danger of explosion. It is widely used in analytical organic chemistry for the preparation of dinitrophenylhydrazon and its derivates from ketones and aldehydes. 

Regeneration of carbonyl compounds from certain derivatives.1 Ketones can be recovered in satisfactory yield from the phenylhydrazones, p-nitrophenyl-hydrazones, tosylhydrazones, oximes, and semicarbazones by reaction with 1 equivalent of (C6H5SeO)20. 2,4-Dinitrophenylhydrazones and N,N-dimethyl-hydrazones are inert under even rather vigorous conditions. The reagent can also be used to regenerate aldehydes from oximes or tosylhydrazones. 

PKesi 0,6, Dry the aldehyde over Na2S04 and fractionate it in a vacuum. The 2,4-dinitrophenylhydrazone forms red crystals from MeOH with m 238-240°, and the semicarbazone has m 210-212° (from MeOH). [Eme et al. Helv Chim Acta 34 148 1951, Beilstein 27 IIEIV 2615.]... 

Figure 69. Aldehydes and acetone derivatives from 2,4-dinitrophenylhydrazone (10 mgL 1).

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