Carbohydrates osazone formation

Phenylhydrazine on exposure to light slowly darkens and eventually becomes deep red in colour salts of the base share this property but to a lesser degree, the sulphate and acetate (of the common salts) being most stable to light. Phenylhydrazine is largely used in organic chemistry to characterise aldehydes and ketones as their phenyl-hydrazones (pp. 342, 345), and carbohydrates as their osazones (pp. 136-140). It is readily reduced thus in the process of osazone formation some of the phenylhydrazine is reduced to aniline and ammonia. On the... 

Osazone formation is not limited to carbohydrates, but is typical of a-hydroxy aldehydes and a-hydroxy ketones in general (e.g., benzoin, C5H5CHOHCOC6H5). 

Fischer found osazone formation to be useful not only in identifying carbohydrates, but also—and this was much more intportant—in determining their configurations. For example, the two diastereomeric aldohexoses (+)-glucose and... 

Other reactions of carbohydrates include those of alcohols, carboxylic acids, and their derivatives. Alkylation of carbohydrate hydroxyl groups leads to ethers. Acylation of their hydroxyl groups produces esters. Alkylation and acylation reactions are sometimes used to protect carbohydrate hydroxyl groups from reaction while a transformation occurs elsewhere. Hydrolysis reactions are involved in converting ester and lactone derivatives of carbohydrates back to their polyhydroxy form. Enolization of aldehydes and ketones leads to epimerization and interconversion of aldoses and ketoses. Addition reactions of aldehydes and ketones are useful, too, such as the addition of ammonia derivatives in osazone formation, and of cyanide in the Kiliani-Fischer synthesis. Hydrolysis of nitriles from the Kiliani-Fischer synthesis leads to carboxylic acids. 

Interesting in this context is the fact that Fischer had already included D-fruc-tose in his studies on the osazone formation [16,17] and, consequently, would have conducted the Heyns rearrangement quite early in the history of carbohydrate chemistry (see Sect. 1.1). 

Historically, techniques such as the formation of osazones and the demonstration of fermentation have contributed significantly to the separation and identification of carbohydrates. Observation of the characteristic crystalline structure and melting point of the osazone derivative, prepared by reaction of the monosaccharide with phenylhydrazine, was used in identification. This method is not completely specific, however, because the reaction involves both carbon atoms 1 and 2 with the result that the three hexoses, glucose, fructose and mannose (Figure 9.19), will yield identical osazones owing to their common enediol form. 

Carbohydrate M.P. 20°. [Pg.457]

The mechanism for the formation of hydrazone or osazone of carbohydrate with hydrazine or phenylhydrazine was described in detail for the Fischer Phenylhydrazone and Osazone Synthesis, so only a part of the mechanism for the preparation of quinoxa-line derivative is illustrated here. The presence of boric acid and acetic acid may help the formation of hydrazone. 

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