Carbohydrate-alkali metal alcoholate

In anhydrous, alcoholic media at 25°, both the hydroxide and the cyanide of alkali metals react with nonacidic carbohydrates, to give colorless, amorphous, hygroscopic precipitates that are preponderantly mono-(alkali metal) alcoholate." Under the proper conditions of concentration, most of the metal alcoholates combine with an additional molecule of carbohydrate per molecule, to give products whose molar ratios of carbohydrate to alkali metal are greater than 1 1. A small proportion of the hydroxide adduct or the cyanide adduct accompanies the alcoholate as the latter is... 

The general structure of alkali metal alcoholates of polyhydroxy compounds is probably very similar to those proposed by Martell and Calvin1 for the alkali metal chelates of o-salicylaldehyde (see Figs. 9 and 10). 148 1M>IM Unfortunately, because of the highly amorphous nature of nearly all pf the alcoholates and adducts formed by the interaction of metal hydroxides with carbohydrates, x-ray diffraction studies have failed to furnish information regarding the precise location of the metal in these complexes. 

As far as propargyl thioethers are concerned, the substrates in this section follow all the principles discussed for propargyl ethers and propargylamines in the two preceding sections. For alkyl propargyl thioethers typical bases used are sodium amide in liquid ammonia, alcoholate or alkali metal hydroxide [178, 186-189, 191, 287-291], and again some derivatives of carbohydrates have been used successfully [292, 293], If an ester group is also present in the molecule, the reaction can be accompanied by a hydrolysis to the carboxylate [294]. 

In homogeneous, aqueous solution, alkali metal hydroxides react with carbohydrates to produce negatively charged carbohydrate species. Although the general feeling among chemists is that these species are free alcoholate anions, the possibility that they are composed, at least partially, of carbohydrate-hydroxide ion adducts cannot be dismissed. 

Table VT contains a comprehensive list of known alkali metal hydroxide adducts. Complexes prepared by the interaction of carbohydrates or acetates of carbohydrates with alkali metal hydroxide in anhydrous alcoholic...

Reducing sugars are decomposed by sodium hydroxide in boiling butanol. Treatment of reducing sugars with alcoholic alkali metal hydroxide must, therefore, be conducted at room temperature, in order to avoid this decomposition of the carbohydrate. 

The ability of a metal alcoholate to accommodate an additional molecule of carbohydrate increases with increasing ionic radius " Li < Na < K < Cs. The difference in stoichiometry between lithium and sodium is much greater than that between either sodium and potassium, or potassium and cesium. The coordination number of an alkali metal is known to increase with increasing ionic radius. Brewer148 reported that the maximum number of donor groups oriented about an alkali metal cation is four for lithium, and as many as six for sodium, potassium, rubidium, or cesium. A greater surface area would allow accommodation of more than one carbohydrate moiety but, in addition, solvent molecules are more strongly attached to cations of smaller radius, and these may not be readily displaced by carbohydrate molecules. 

Electrophoresis does not show the presence of uncharged species, such as undissociated metal alcoholate or carbohydrate-metal hydroxide adducts. These species are probably present in alcoholic solutions, but their concentration has not yet been ascertained. Their presence is suggested by the relatively low mobility of carbohydrates in alcoholic solutions of alkali metal hydroxide. In aqueous media, where greater dissociation of ion pairs should occur, the mobility is extremely high. The possible existence of free carbohydrate-hydroxide ion species cannot be disregarded, because of the hydrogen-bonding properties of the hydroxide ion. 

The method of Gaver147 for nonreducing carbohydrates (refluxing the carbohydrate in alcoholic alkali metal hydroxide solution without concomitant azeotropic distillation) might give results similar to those obtained by the Wolfrom method.8 -" However, the presence of water and hydroxide ion in the final reaction mixture would probably cause a. small fraction of the product to be the hydroxide adduct. 

IV. Alcoholates from Reactions, in Liquid Ammonia, of Carbohydrates with Alkali Metals, Alkaline-earth Metals, and Alkali Metal Amides... 

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