Dinitrophenylhydrazone

Brady s reagent A solution of 2,4-dinitro-phenylhydrazine sulphate in methanol. Gives characteristic crystalline yellow to deep red 2,4-dinitrophenylhydrazone products with aldehydes and ketones. 

Form phenylhydrazones, 2,4-dinitrophenylhydrazones and semicarbazones. (Many oximes are too soluble for ready isolation.)... 

A) Phenylhydrazones and 2,4-Dinitrophenylhydrazones (see 2 abo e). The latter are often to be preferred to phenylhydrazones because (a) the phenylhydrazones may separate as syrups, and also may decompose in hot solvents cf. p. 257), ( ) the 2,4 dinitrophenylhydrazones are often formed in the cold or with only brief warming, are much less soluble, and have higher m.ps. (p. 263). 

Note. The 2,4-dinitrophenylhydrazones of many higher aldehydes and ketones may be insoluble in most solvents. In this case, Mter them off, wash with ethanol, dry and take the m.p. attempted recrystallisation may cause partial decomposition. (M.ps., pp. 530-540.)... 

The conversion of the compounds under investigation into coloured derivatives (e.g., the separation of carbonyl compounds by conversion into their 2 4-dinitrophenylhydrazones, etc. of hydrocarbons through their picrates of alcohols through their 3 5-dinitrobenzoates of glucose, fructose and other simple sugars through their p-phenylazobenzoyl esters). 

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. For experimental details, see under Aliphatic ketones. Section 111,74,7. 

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

Compounds related to phenylhydrazine react in an analogous way p Nitrophenylhydrazine yields p nitrophenylhydrazones 2 4 dinitro phenylhydrazine yields 2 4 dinitrophenylhydrazones... 

Interaction of formaldehyde with 2,4-dinitrophenylhydrazine in acid media causes 2,4-dinitrophenylhydrazone (DNPhydrazone) formaldehyde formation. Gas-chromatographic analysis of 2,4-DNP-hydrazone formaldehyde toluene extract with an electron holding detector makes it possible to detect it at the level of 0,001 mg/dm. Phenol is detected in the form of tribromphenol yield, the hexane extract of which undergoes chromatography with an electron holding detector which provides the level of phenol detection of 0.001 mg/dm (the limit of quantitative detection). 

Dinitrophenylhydrazones are cleaved by various oxidizing and reducing agents, and by exchange reactions. 

Purified by recrystn from diisopropyl ether or pet ether and dried in air. If too impure then dissolve in Et20, wash with aq NaHC03, then H2O, dried (MgS04), filtd, evapd and the residue recrystd as before. [Cardwell J Chem Soc 715 1949.] Tbe oxime can be recrystd from CHCl3-pet ether (at -20°) and has m 84-85° [Barkenbus et al. J Org Chem 20 871 7955]. The 2,4-dinitrophenylhydrazone has m 186° (from EtOAc) [Barkenbus et al. J Org Chem 16 232 7957]. The S-dioxide is recrystd from AcOH, m 173-174° [Fehnel and Carmack J Am Chem 5oc70 1813 1948]. 

Other possibilities are the reduction of nitro groups by applying the sample solutions to adsorbent layers containing zinc dust and then exposing to hydrochloric acid vapors [110] 3,5-Dinitrobenzoates and 2,4-dinitrophenylhydrazones can also be reduced in the same way on tin-containing silica gel phases [111] Cellulose layers are also suitable for such reactions [112] Seiler and Rothweiler have described a method of trans-salting the alkali metal sulfates to alkali metal acetates [113]... 

---