Ketone 2,4-dinitrophenylhydrazine

If a group-specific reagent is now used, e.g. one that is chosen to react specifically with the reducing properties of aldehydes (ammoniacal silver nitrate solution) or to react with ketones (2,4-dinitrophenylhydrazine [9]) it is very simple to determine which form of alcohol is present in the sample. 

Ketones 2,4-Dinitrophenylhydrazine (2,4-DNP) Hydrazine Iodoform Oxime Semicarbazone Positive for all ketones (and aldehydes) Positive for all ketones (and aldehydes) Positive for methyl ketones Positive for all ketones (and aldehydes) Positive for all ketones (and aldehydes)... 

The arylhydrazines are valuable reagents for both classifying and forming derivatives because the solid products of these test reactions may be used as derivatives of the aldehyde or ketone 2,4-dinitrophenylhydrazine is particularly useful in this regard. 

Dinitrophenylhydrazine is a very important reagent for the identification of aldehydes and ketones (pp. 342, 346). It is readily prepared from chloro-2,4-dinitrobenzene (I). In the latter compound the chlorine is very reactive in... 

Reagent A is particularly useful for the treatment of the lower aliphatic aldehydes and ketones which are soluble in water cf. acetaldehyde, p. 342 acetone, p. 346). The Recent is a very dilute solution of the dinitrophenylhydrazine, and therefore is used more to detect the presence of a carbonyl group in a compound than to isolate sufficient of the hydrazone for effective recrystallisation and melting-point determination. 

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

Method 2. Dissolve 0-25 g. of 2 4-dinitrophenylhydrazine in a mixture of 42 ml. of concentrated hydrochloric acid and 50 ml. of water by warming on a water bath dilute the cold solution to 250 ml. with distilled water. This reagent is more suitable for water-soluble aldehydes and ketones since alcohol is absent. 

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). 

Aldehydes, ketones Apply sample solution and moisten with 2 N 2,4-dinitrophenylhydrazine in acetic acid After reacting, dry and chromatograph the 2,4-DNPH derivatives [14]... 

The scope of this reaction was investigated by Djerassi, °° who showed that 4-bromo ketones in the series and 2-bromo ketones in the 5a series give unsaturated 2,4-dinitrophenylhydrazones in 80-90% yield on warming under nitrogen with 1.1 moles of 2,4-dinitrophenylhydrazine in acetic acid. Cleavage with pyruvic acid affords the pure unsaturated ketones in 60-70 % yield. 

Reagents. Methanol. This must be free from aldehydes and ketones if necessary reflux 1L of the purest material available for 2 hours with 5 g of 2,4-dinitrophenylhydrazine and five drops of concentrated hydrochloric acid. Then distil the methanol through a fractionating column and collect the fraction boiling at 64.5-65.5 °C. 

The most popular reagent for the formation of aldehyde and ketone derivatives is 2,4-dinitrophenylhydrazine which forms hydrazones containing strong chromophores. 

Secondary alcohols yield ketones on oxidation and these can be reacted with 2,4-dinitrophenylhydrazine to yield the corresponding colored hydrazones. 

Aldehydes and ketones can be located as orange or yellow zones after reaction with 2,4-dinitrophenylhydrazine hydrochloride (200 mg dissolved in 2 ml of aqueous HCl). In some cases, heating to 100°C is necessary. 

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 ... 

A mixture of 20 g (0.1 mol) of aluminum isopropoxide, 0.1 mol of an aldehyde or a ketone and 100 ml of dry isopropyl alcohol is placed in a 250 ml flask surmounted by an efficient column fitted with a column head providing for variable reflux. The mixture is heated in an oil bath or by a heating mantle until the by-product of the reaction - acetone - starts distilling. The reflux ratio is adjusted so that the temperature in the column head is kept at about 55° (b.p. of acetone) and acetone only is collected while the rest of the condensate, mainly isopropyl cohol (b.p. 82°), flows down to the reaction flask. When no more acetone is noticeable in the condensate based on the test for acetone by 2,4-dinitrophenylhydrazine the reflux regulating stopcock is opened and most of the isopropyl alcohol is distilled off through the column. The residue in the distilling flask is cooled, treated with 200 ml of 7% hydrochloric acid and extracted with benzene the benzene extract is washed with water, dried and either distilled if the product of the reduction is volatile or evaporated in vacuo in the case of non-volatile or solid products. Yields of the alcohols are 80-90%. 

The sodium bisulfite addition compounds must have a free (or potentially free) ketone-type carbonyl group, since they readily form derivatives with typical ketone reagents such as semicarbazide and 2,4-dinitrophenylhydrazine.174 Decomposition of these derivatives with alkali gives the corresponding adrenochrome derivatives e.g., adrenochrome monosemicarbazone would be obtained from the semicarbazone of the adrenochrome-sodium bisulfite complex.174 If one accepts Tse and Oesterling s formulation of the adrenochrome-sodium bisulfite complex, the semicarbazone would probably have a basically similar structure (i.e. 82). This type of structure is more... 

Results from the two experiments conducted on the aldehydes and ketones are listed in Table III as separate sets of data to illustrate the care that must be exercised in conducting and evaluating these runs. Both C02 extractions were performed under similar conditions. However, in the second run, the U-tube traps were contacted with the 2,4-dinitrophenylhydrazine derivatizing solution for longer periods of time. This modification in the analytical procedure permitted higher total mass accountabilities in the second experiment, ranging from 64.9% for isophorone to 28.7% for methyl isobutyl ketone. The recoveries from the raffinate for each of these compounds remained relatively constant. This result suggested that the trap recoveries in the first case were artificially low. 

Derivatization is a procedure in which analyte is chemically modified to make it easier to detect or separate. For example, formaldehyde and other aldehydes and ketones in air, breath, or cigarette smoke25 can be trapped and derivatized by passing air through a tiny cartridge containing 0.35 g of silica coated with 0.3 wt% 2,4-dinitrophenylhydrazine. Carbonyls are converted into the 2,4-dinitrophenylhydrazone derivative, which is eluted with 5 mL of acetonitrile and analyzed by HPLC. The products are readily detected by their strong ultraviolet absorbance near 360 nm. 

The procedures used to determine ambient carbonyl concentrations involve a collection step with silica or C18 cartridges impregnated with 2,4-dinitrophenylhydrazine. Contamination is inevitable with this system, and blanks must be used to compensate for the degree of contamination. Selection of the appropriate blank values to subtract is a difficult and uncertain process. Consequently, development of a gas chromatographic system that will resolve and respond to the low-molecular-weight aldehydes and ketones is needed. The mercuric oxide and atomic emission detectors should provide adequate response for the carbonyls. 

Figure D2.4.2 Development of the quinoidal ion from the reaction of 2,4-dinitrophenylhydrazine (DNPH) with an aldehyde or ketone. Adapted from Meyer and Rebrovic (1995) with permission from AOCS Press.

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