Osazon

They are formed by treatinga-diketones, a-hyd-roxyaldehydes, hydroxyketones, aminoalde-hydes or aminoketones with arylhydrazines. Sugars can be identified by their osazones which have characteristic melting-points, formation times or crystal appearance. 

Fructose (V) under similar conditions gives first the phenylhydrazonc (Va) by the direct condensation of the >C 0 group of carbon atom 2 with one molecule of phenylhydrazine. The second molecule of phenylhydrazine then oxidises the primary alcohol group of carbon atom 1 to the -CHO group by removal of two atoms of hydrogen, which as before serve to reduce the phenyl-hydrazine to aniline and ammonia. The compound (Vb) which is thus produced then undergoes direct condensation with the third molecule of phenylhydrazine, giving the osazone of fructose, or fructosazone (Vc). 

Now since the configuration of carbon atoms 3, 4 and 5 of glucose and fructose art identical, it folhjws that glucosazone and fructosazone are identical in all respects. The osazone is formed however more rapidly from fructose than from glucose, and this difference in rate of formation may be used to distinguish the two sugars, provided the reactions are carried out under strictly parallel conditions (pp. 138, 338). 

Both maltose and lactose, being reducing sugars, give osazones which differ from one another and from glucosazone in crystalline form. Sucrose (G-r-r-F), having no potential aldehyde or ketone grouping, does not form an osazone. 

Phenylhydrazine is, however, frequently supplied in the form of its hydro chloride or sulphate, since these salts on exposure to light darken less rapidly than the free base. If these salts are used, however, osazone formation is unsatisfactory, partly because the mineral acid formed by hydrolysis of... 

If it is desired to observe the crystalline form of the osazone, draw up in a glass tube a few drops of the cold filtrate containing the fine crystals, and transfer to a microscope slide. Cover the drops with a slip and examine under the microscope unless the filtrate has been cooled very slowly and thus given moderately-sized crystals, the high power of the microscope will probably be required. Note the fine yellow needles aggregated in the form of sheaves. Compare with Fig. 63(A). 

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... 

Phenylhydrazine is usually dissolved in acetic acid for hydrazone formation if a salt of phenylhydrazine with an inorganic acid is used, it must be mixed with an excess of sodium acetate (see preparation of osazones, p. 137). 

Benzil Osazone. CeH5C( N NHCeH5)C( N NHCeH5)CeH5. 

Osazone formation. The preparation of glucosazone has already been given (p. 137). It may be carried out on a small scale by either of the following methods, according as (a) the phenyl hydrazine base, or (Z>) one of its salts, is used. 

Osazone formation. Fructose forms an osazone identical with that obtained from glucose. It is, however, usually formed more quickly... 

Osazone formation. Forms an osazone, m.p. 206 (see however footnote, p. 140) this osazone, unlike glucosazone, is soluble in hot water. See p. 139 for preparation. Examine the crystals under the microscope and note the sheaves of plates, not needles (Fig. 63(B),... 

Osazone formation. Forms a yellow osazone, m.p. 208° soluble in hot water. See p. 137 for preparation. If examined under the microscope very characteristic clusters of hedge-hog crystals will be seen (Fig. 63(c), p. 139). The difference in the crystalline appearance of lactosazonc and maltosazone should be very carefully noted, as this difference forms the chief and most reliable method of differentiating between these two sugars. 

Does not reduce ammoniacal silver nitrate or Fehling s solution, and does not form an osazone. 

In view of the indefinite melting-points of many sugars and of their readily obtainable derivatives e.g., osazones, cf. p. 139), their identification should be based primarily on their chemical properties. Their rotatory power can often be used for identification purposes, but is not considered in this book. 

Glucose and fructose give the same osazone. This will be evident from the following schemes representing the reactions which are believed to occur ... 

The approximate times of osazone formation in minutes are given in Table 111,139. The product from mannose is the simple hydrazone and is practically white. Arabinose osazone separates first as an oil, whilst that from galactose is highly crystalline. Lactose and maltose give no precipitate from hot solution. 

The photographs of the osazones were kindly supplied by Thomas Kerfoot and Co. Lid., of Vala of Bardsley, Ashton.under-Lyue, the weU-known manufacturers of sugars. 

Methylphenylosazones. cw-Methylphenylhydrazine does not form osazones with aldoses presumably because the base or more probably the methylphenylhydrazonium ion [CgH5NCH3NH3]+ will oxidise a—CHjOH but not a >CHOH group it readily forms osazones with ketoses, thus providing an excellent reagent for fructose. 

Another reaction that is characteristic of a-hydroxy aldehydes or ketones, which has been found of value for the characterisation of sugars, is the formation of osazones with phenylhydrazine. This reagent reacts with either an aldose... 

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