2.4- Dinitrophenylhydrazone preparation

The UV absorptions and colors of 2,4-dinitrophenylhydrazone derivatives of aldehydes and ketones depend sensitively on the structure of the carbonyl compound. Suppose that you are asked to identify the contents of three bottles whose labels have fallen off. The labels indicate that one bottle contained butanal, one contained trans-2-hutenal, and one contained trans-3-phenyl-2-propenal. The 2,4-dinitrophenylhydrazones prepared from the contents of the bottles have the following characteristics. 

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. 

Dinitrophenylhydrazones. Small quantities may be prepared with the class reagent described in Section XI,7,4. A more satisfactory procedure is given under Aliphatic Ketones, Section 111,74,7. 

For the preparation of 2 4-dinitrophenylhydrazones, dissolve the carbonyl compound (say, 0-5 g.) in 5 ml. of ethanol and add the cal culated volume of the reagent. If a precipitate does not form immediately, dilute with a little water. Collect the derivative and recrystalhse it as above. 

Dinitrophenylhydrazones. The following procedure for the preparation of 2 4-dinitrophenylhydrazones is alternative to those given in Section 111,74,7. 

An improved method for the preparation of A" -3-ketones from 4-bromo compounds was described by Mattox and Kendall. This procedure involves dehydrobromination of the 2,4-dinitrophenylhydrazone and subsequent cleavage of the hydrazone with pyruvic acid ... 

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. 

Cyclobutanecarboxaldehyde has been prepared in very low yield by the Rosenmund reduction procedure.6 A 46% yield of the 2,4-dinitrophenylhydrazone derivative has also been reported, with the aldehyde formed as an intermediate, in the reaction of the acid chloride and lithium tri-Mmtoxyaluminum hydride at —78° in diglyme.7... 

Hajipour and coworkers prepared benzyltriphenylphosphonium peroxymonosulfate (BnPhsPHSOs) in a very high yield (95%) and purity (99%). This new oxidizing reagent was applied successfully in various deprotection reactions such as the conversion of oximes, phenylhydrazones, 2,4-dinitrophenylhydrazones and semicarbazones to the corresponding carbonyl compounds in the presence of bismuth chloride under nonaqueous conditions . Oxidative deprotection of trimethylsilyl ethers, tetrahydropyranyl ethers and ethylene acetals with BnPh3PHS05 under microwave irradiation affords the corresponding carbonyl compounds in very high yields (equation 71). The same reaction also proceeds under nonaqueous conditions ". 

These model studies permitted demonstration of metabolic a-hydroxylation of NPy by isolating 2-hydroxytetrahydrofuran as a metabolite of NPy. This was accoitplished by trapping 4-hydroxy-butyraldehyde as its 2,4-dinitrophenylhydrazone (DNP) derivative. For in vitvo studies, NPy-2,5-l C was incubated with rat liver microsomes, 02i and an NADPH generating system. After the incubation was complete, DNP reagent was added to the mixture and the products were extracted and examined by preparative TLC. A radioactive band corresponding to the DNP of 4-hydroxybutyralde-hyde was observed. This band was not present in controls in which NADPH was omitted or in which boiled enzyme was used. The mass spectrum was identical to that of a reference sairple. In addition, a minor metabolite with mass spectrum identical to that of the DNP of 2-butenal was also isolated. These results showed conclusively that NPy underwent metabolic a-hydroxylation in this in vitro system. 

Fraction 1 was found to have the same retention time as that of authentic cyclohexanone, and the infrared spectrum showed a strong absorption band at 1720 cm."1 (C=0). The 2,4-dinitrophenylhydrazone derivative was prepared, and a mixed melting point with an authentic sample was not depressed, m.p., 156°-157°C. [literature value 160.2°C. 

Pyridine base eliminations of a-bromo ketones cannot be recommended for general use because of the side reactions already discussed. The semi-carbazone-pyruvic acid method should be employed if strict absence of isomerization is required in the dehydrobromination of 2- or 4-bromo-3-ke-tones. This procedure is not applicable for the preparation of A1,4-3-ketones, due to rearrangement. The use of semicarbazide for the dehydrobromination of 6-bromo-A4-3-ketones is reported to be satisfactory for the preparation of A4,6-3-ketones65 but appears to be a neglected approach. (The corresponding 2,4-dinitrophenylhydrazones cannot be cleaved.)... 

The first reported preparation of this ring system involves an interesting two-step preparation starting with the dinitrophenylhydrazone (287). Reaction of this hydrazone with the amino ester (286) gives the intermediate hydrazone (288) in near quantitative yield. The intermediate can be cyclized to the ring closed product (135) by reaction with Raney nickel (62AP121). 

Separation of 2,4-dinitrophenylhydrazones. The solutions are prepared by dissolving 10 mg of each of the 2,4-dinitrophenylhydrazones of acetone, butan-2-one and hexan-3-one (or hexan-2-one) in 0.5 ml of ethyl acetate. Prepare a flexible silica gel sheet of dimensions 20 x 5 cm in the manner already described and apply c. 0.5 pi of each of the three solutions to give the marker spots of a diameter of between 2 and 3 mm. A mixed spot is conveniently obtained by loading sequentially to the same area further 0.5 pi aliquot portions of each of the solutions and allowing the solvent to evaporate completely between each addition. 

The most widely used protective groups for aliphatic and alicyclic aldehydes and ketones are (a) cyclic acetals and (b) cyclic dithioacetals and hemithio-acetals. The use of semicarbazones, oximes and 2,4-dinitrophenylhydrazones (for preparation see Section 9.6.13, p. 1257) is less common but has found application in certain instances where the more vigorous methods required for deprotection are not detrimental. 

A 0.1% solution of the reagent is prepared by dissolving 0.12 g, of 2,4-dinitrophenylliydrazine in 25 cc. of distilled water and 21 cc. of concentrated hydrochloric acid by warming on the steam bath. The clear yellow solution is then cooled and diluted to 125 cc. with distilled water. For use, several drops of the reagent and a few drops of the acetone-isopropyl alcohol distillate are mixed in a small test tube. The presence of acetone is shown by the formation of a yellow cloudy suspension or precipitate of acetone 2,4-dinitrophenylhydrazone. The test is considered negative if no cloudiness forms within one-half minute when 3 cc. of the reagent is added to 5 drops of distillate. After a negative test is obtained, it is advisable to reflux the mixture for five to fifteen minutes with complete condensation and then to force over a few drops of distillate for another test. If no acetone is observed, reduction is complete. Otherwise the process is continued until no more acetone can be detected. This procedure indicates complete reduction even with ketones which are reduced very slowly. 

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. 

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... 

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