Dinitrophenylhydrazine Reagent

Dinitrophenylhydrazones, To a few drops of formalin, add a few drops of 2,4-dinitrophenylhydrazine reagent A (p. 261) a yellow precipitate is produced in the cold. Acetaldehyde gives an orange-coloured precipitate. Dissolve a few drops of benzaldehyde or salicylal-dehyde in 2 ml. of methanol and then add a few drops of the reagent B an orange-coloured precipitate is obtained. In each case filter and recrystallise from ethanol. Take the m.p. (M.ps., p. 539-540.)... 

K. Treat with 2,4-dinitrophenylhydrazine reagent (pp. 263, 334). Yellow or orange-yellow precipitates given by most aldehydes and ketones. 

If an unknown compound gives a positive test with the 2 4-dinitrophenylhydrazine reagent, it then becomes necessary to decide whether it is an aldehyde or a ketone. Although the dimedone reagent (Section 111,70,2) reacts only with aldehydes, it is hardly satisfactory for routine use in class reactions. It is much simpler to make use of three other reagents given below, the preparation and properties of which have already been described (Section 111,70). 

Dinitrophenylhydrazine reagent may be prepared by either of the following methods. 

To an ice-cold mixture of 1.0 ml of concentrated sulphuric acid and 5 ml of saturated aqueous potassium dichromate solution, add 2 ml of the alcohol or its concentrated aqueous solution. If the alcohol is not miscible with the reagent, shake the reaction mixture vigorously. After 5 minutes, dilute with an equal volume of water, distil and collect the first few ml of the aqueous distillate in a test tube cooled in ice. (Aldehydes and ketones are volatile in steam.) Test a portion of the distillate for a carbonyl compound with 2,4-dinitrophenylhydrazine reagent (p. 1218). If a solid derivative is obtained, indicating that the compound was a primary or secondary alcohol, test a further portion with SchifFs reagent (p. 1291) to distinguish between the two possibilities. The derivative may be recrystallised the m.p. may give a preliminary indication of the identity of the alcohol. 

Miscellaneous class reactions. (Section 9.3.) Determine the effect of the following class reagents upon small portions of the original mixture (i) bromine in dichloro-methane solution (ii) potassium permanganate solution (iii) alcoholic silver nitrate solution (iv) 2,4-dinitrophenylhydrazine reagent (v) SchifFs reagent (vi) acetyl chloride (vii) benzoyl chloride (in the presence of aqueous sodium hydroxide) (viii) sodium (only if water is absent) (ix) iron(m) chloride solution and (x) bromine water. 

The distillate may contain volatile neutral compounds as well as volatile acids and phenols. Add a slight excess of 10-20 per cent sodium hydroxide solution to this distillate and distil until it is judged that all volatile organic compounds have passed over into the distillate. If necessary, the determination of the refractive index of the distillate or the application of specific chemical tests (e.g. for carbonyl compounds use the 2,4-dinitrophenylhydrazine reagent) should be used to confirm completion of distillation. Keep this distillate (S,) for Step 4. 

Dinitrophenylhydrazine test. Place 5 drops of each substance into separately labeled clean, dry test tubes (100 x 13 mm) and add 20 drops of the 2,4-dinitrophenylhydrazine reagent to each. If no precipitate forms immediately, heat for 5 min. in a warm water bath (60°C) cool. Record your observations on the Report Sheet. 

Dinitrophenylhydrazine Reagent Add 4 mL of sulfuric acid to 0.8 g of 2,4-dinitrophenylhydrazine, then while swirling, add 6 mL of water, dropwise. When dissolution is essentially complete, add 20 mL of alcohol, mix, and filter. The filtrate is the reagent. 

Preparation of 2,4-dinitrophenylhydrazine reagent A suspension of 2,4-dinitrophenylhydrazine (0.05%, w/v) in 1 M HC1 is warmed for 1 h at 50°C, stirred, cooled to room temperature and extracted with n-hexane (2 x 50 ml) in order to extract trace carbonyl impurities. The reagent is flushed with nitrogen, stoppered and stored in the dark at ambient temperature for not more than 24 h. 

The dinitrophenylhydrazones are prepared by mixing equal volumes of sample and 2,4-dinitrophenylhydrazine reagent. After 2 h the reaction mixtures are extracted twice with an equal volume of dichloro-methane, centrifuged at 2000 xg for 10 min, and the combined solvent evaporated under nitrogen at reduced pressure at 35°C in a rotary evaporator. 

Dinitrophenylhydrazone. See under Group 1(c), Aldehydes. All ketones react with 2,4-dinitrophenylhydrazine reagent. 

The products shown in Table I, column 4, were obtained and identified by the following procedure. In each case an aliquot equal to that used for active oxygen determinations (usually 0.5 to 1.0 cc.) was added at 0° C. to a five- to tenfold excess of sodium bisulfite in 10 cc. of water. The solution was then allowed to warm to room temperature and, if no volatile compounds were expected, it was warmed slightly. Then a twofold excess of 2,4-dinitrophenylhydrazine reagent was added and the mixture was heated on the water bath for 0.5 hour. After cooling and adding water if necessary, the precipitate formed was separated, washed with water, and dried in the desiccator under reduced pressure. 

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