2.4- Dinitrophenylhydrazine test reagent

The 2,4-dinitrophenylhydrazine test easily detects 1 part of acetone in 500 to 1000 parts of isopropyl alcohol. Since the reagent solution, even though kept in a stoppered bottle, may deteriorate in the course of several months, old solutions should be checked before use. For the use of the test with toluene instead of isopropyl alcohol see the note on p. 204. 

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. 

Primary and secondary alcohols are also oxidized by this reagent (see Experiment 52H). Therefore, this test is not useful in identifying aldehydes unless a positive identification of the carbonyl group has already been made. Aldehydes give a 2,4-dinitrophenylhydrazine test result, whereas alcohols do not. 

Alcohols are neutral compounds. The only other classes of neutral compounds used in this experiment are the aldehydes, ketones, and esters. Alcohols and esters usually do not give a positive 2,4-dinitrophenylhydrazine test aldehydes and ketones do. Esters do not react with Ce(lV) or acetyl chloride or with Lucas reagent, as alcohols do, and they... 

An unknown organic liquid, compound A, was found to bum with a yellow, sooty flame and give a positive Lucas test ( 5 min). Upon treatment with sodium dichromate-sulfuric acid solution it produced compound B, which also burned with a yellow, sooty flame. Compound B gave a positive 2,4-dinitrophenylhydrazine test, but a negative result when treated with the Tollens reagent. However, compound B did give a positive iodoform test. 

The 2,4 dinitrophenylhydrazine test does not distinguish between aldehydes and ketones. However, there are two simple tests which can do so, based on the feet that aldehydes can be oxidized whereas ketones cannot. Fehling s solution and Tollens reagent are both mild oxidizing reagents that react with aldehydes to produce carboxylic acids (Figure 10.80). 

Precautions, (i) The above tests must be carried out with discretion. If the substance is only moderately soluble in the solvent selected, and a comparatively large volume of the latter is required, the consequent dilution of the acid in the reagent may cause the separation of the free 2,4 dinitrophenylhydrazine (although this is more likely to happen with Reagent B than with A). Furthermore, if the compound under investigation should have basic properties, the neutralisation of part of the acid in the reagent may have the same result. 

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). 

Both aldehydes and ketones contain the carbonyl group, hence a general test for carbonyl compounds will Immediately characterise both classes of compoimds. The preferred reagent is 2 4-dinitrophenylhydrazine, which gives sparingly soluble phenylhydrazones with carbonyl compounds ... 

Steam distillation is discontinued when the distillate gives a negative test with 2,4-dinitrophenylhydrazine reagent. 

Reaction with 2 4-dinitrophenylhydrazine. Add 2 drops (or 0 05-0-1 g.) of the substance to be tested to 3 ml. of the 2 4-dinitro-phenylhydrazine reagent, and shake. If no precipitate forms immediately... 

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 9.5.) Determine the eifect of the following class reagents upon small portions of the original mixture (for some of the tests an aqueous solution or suspension may be used) (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) iron(m) chloride solution and (ix) 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. 

To 10 mL of the stock solution of 2,4-dinitrophenylhydrazine in phosphoric acid add about 0.1 g of the compound to be tested. Ten milliliters of the 0.1 M solution contains 1 millimole (0.001 mole) of the reagent. If the compound to be tested has a molecular weight of 100 then 0.1 g is 1 millimole. Warm the reaction mixture for a few minutes in a water bath and then let crystallization proceed. Collect the product by suction filtration (Fig. 1), wash the crystals with a large amount of water to remove all phosphoric acid, press a piece of moist litmus paper on to the crystals, and if they are acidic wash them with more water. Press the product as dry as possible between sheets of filter paper and recrystallize from ethanol. Occasionally a high-molecular-weight derivative won t dissolve in a reasonable quantity (20 mL) of ethanol. In that case cool the hot suspension and isolate the crystals by suction filtration. The boiling ethanol treatment removes impurities so that an accurate melting point can be obtained on the isolated material. 

A 0.1% solution of the reagent is prepared Iqr dissolving 0.12 g. of 2,4-dinitrophenylhydrazine in 26 cc. of distilled water and 21 cc. of concentrated hydrochloric acid Iqr warming on the steam bath. The clear yellow solution is then cooled and diluted to 126 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 6 drops of distillate. After a negative test is obtained, it is advisable to reflux the mixture for flve to flfteen 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. 

Aldehydes are characterized, and in particular arc differentiated from ketones, through their ease of oxidation aldehydes give a positive test with Tollens reagent (Sec. 19.9) ketones do not. A positive Tollens test is also given by a few other kinds of easily oxidized compounds, e.g., certain phenols and amines these compounds do not, however, give positive tests with 2,4-dinitrophenylhydrazine. 

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