Part 27

Maquenne has studied the action of phenylhydrazin on sugars and considers that this reaction offers the only known means of precipitating these bodies from solutions where they are found mixed with other substances. The osazones, which are thus obtained, are usually very slightly soluble in the ordinary reagents, for which reason it is easy to obtain them pure when there is at the disposition of the analyst a sufficient quantity of the material. But if the sugar to be studied is rare and if it contain, moreover, several distinct reducing bodies, the task is more delicate. It is easy then to confound several osazones which have almost identical points of fusion; for example, glucosazone with galactosazone. Finally, it becomes impossible by the employment of phenylhydrazin to distinguish glucose, dextrose or mannose from levulose alone or mixed with its isomers. Indeed, these three sugars give, with the acetate of phenylhydrazin the same phenylglucosazone which melts at about 205°. It is noticed that the weights of osazones which are precipitated when different sugars are heated for the same time with the same quantity of the phenylhydrazin, vary within extremely wide limits. It is constant for each kind of sugar if the conditions under which the precipitation is made are rigorously the same. There is then, in the weight of the osazones produced, a new characteristic of particular value. The following numbers have been obtained by heating for one hour at 100°, one gram of sugar with 100 cubic centimeters of water and five cubic centimeters of a solution containing forty grams of phenylhydrazin and forty grams of acetic acid per hundred. After cooling the liquid, the osazones are received upon a weighed filter, washed with 100 cubic centimeters of water, dried at 110° and weighed. The weights of osazones obtained are given in the following table:

Weight of the osazones. Character of the sugar. gram.

Sorbine, crystallized 0.82 Levulose ” 0.70 Xylose ” 0.40 Glucose, anhydrous 0.32 Arabinose, crystallized 0.27 Galactose ” 0.23 Rhamnose ” 0.15 Lactose ” 0.11 Maltose ” 0.11

With solutions twice as dilute as those above, the relative conditions are still more sensible, and the different sugars arrange themselves in the same order, with the exception of levulose, which shows a slight advantage over sorbine and acquires the first rank. From the above determinations, it is shown that levulose and sorbine give vastly greater quantities of osazones, under given conditions, than the other reducing sugars. It would be easy, therefore, to distinguish them by this reaction and to recognize their presence also even in very complex mixtures, where the polarimetric examination alone would furnish only uncertain indications.

It is remarkable that these two sugars are the only ones among the isomers or the homologues of dextrose, actually known, which possess the functions of an acetone. They are not, however, easily confounded, since the glucosazone forms beautiful needles which are ordinarily visible to the naked eye, while the sorbinosazone is still oily and when heated never gives perfectly distinct crystals.

This method also enables us to distinguish between dextrose and galactose, of which the osazone is well crystallized and melts at almost the same temperature as the phenylglucosazone. Finally, it is observed that the reducing sugars give less of osazones than the sugars which are not capable of hydrolysis, and consequently differ in their inversion products. It is specially noticed in this study of the polyglucoses (bioses, trioses), that this new method of employing the phenylhydrazin appears very advantageous. It is sufficient to compare the weights of the osazones to that which is given under the same conditions by a known glucose, in order to have a very certain verification of the probabilities of the result of the chemical or optical examination of the mixture which is under study. All the polyglucoses which have been examined from this point of view give very decided results. The numbers which follow have reference to one gram of sugar completely inverted by dilute sulfuric acid, dissolved in 100 cubic centimeters of water, and treated with two grams of phenylhydrazin, the same quantity of acetic acid, and five grams of crystallized sodium acetate. All these solutions have been compared with the artificial mixtures and corresponding glucoses, with the same quantities of the same reagents. The following are the results of the examination:

Weight of the osazone. Character of the sugar. gram.

1 {Saccharose, ordinary 0.71 {Glucose and levulose (.526 g each) 0.73

2 {Maltose 0.55 {Glucose (1.052 g) 0.58

3 {Raffinose, crystallized 0.48 {Levulose, glucose and galactose (.333 g each) 0.53

4 {Lactose, crystallized 0.38 {Glucose and galactose (.500 g each) 0.39

It is noticed that the agreement for each saccharose is as satisfactory as possible. Numbers obtained with the products of inversion are always a little low by reason of the destructive action of sulfuric acid, and in particular, upon levulose. This is, moreover, quite sensible when the product has to be heated for a long time with sulfuric acid in order to secure a complete inversion. It is evident from the data cited from the papers of Fischer, Maquenne, and others, that the determination of sugars by this method is not a very difficult analytical process and may, in the near future, become of great practical importance.

=148. Molecular Weights of Carbohydrates.=—In the examination of carbohydrates the determination of the molecular weights is often of the highest analytical value.

The uncertainty in respect of the true molecular weights of the carbohydrates is gradually disappearing by reason of the insight into the composition of these bodies, which recently discovered physical relations have permitted.

Raoult, many years ago, proposed a method of determining molecular weights which is particularly applicable to carbohydrates soluble in water.

The principle of Raoult’s discovery may be stated as follows: The depression of the freezing point of a liquid, caused by the presence of a dissolved liquid or solid, is proportionate to the absolute amount of substance dissolved and inversely proportionate to its molecular weight.

The following formulas may be used in computing results:

C = observed depression of freezing point:

P = weight of anhydrous substance in 100 grams:

C --- = A = depression produced by one gram substance in 100 grams: P

K = depression produced by dissolving in 100 cubic centimeters a number of grams of the substance corresponding to its molecular weight:

M = molecular weight:

C Then we have, K = ---- × M. P

K is a quantity varying with the nature of the solvent but with the same solvent remaining sensibly constant for numerous groups of compounds.

The value of

C A ---- P

can be determined by experiment. The molecular weight can therefore be calculated from the formula

K M = ----. A