The proton magnetic resonance spectra of the aldopentoses and aldohexoses were determined in deuterium oxide solution. The signals of the anomerio protons of the two furanose and the two pyranose forms were located; the identification of these signals is discussed. The proportions of the four forms in equilibrium solution were determined.
The equilibrium compositions of 1-deoxy-D-fructose, 1-deoxy-L-sorbose, 1-deoxy-L-psicose, D-threopentulose, D-arabino- and L-xylo-3-hexulose and coriose in aqueous solution have been determined by 13C N.M.R. spectroscopy. In all but the last case, considerable amounts of the acyclic keto form are present at equilibrium. The hydrated keto form was not detected; a model compound, l-deoxy- 3,4,5,6-tetra-O-methyl-D-fructose, was found not to be hydrated in aqueous solution.
The proportion of the pyranose and furanose forms in aqueous solutions of deoxyaldoses has been determined by p.m.r. spectroscopy. Removal of a hydroxyl group from C 2 or C 3 of an aldose causes an increase in the furanose content, particularly if the group was cis to another group. The strongest unfavourable interaction in the furanose ring appears to be the one between the side-chain and a neighbouring cis hydroxyl group. I n the preceding paper1 we reported the determination, by p.m.r, spectroscopy, of the proportions of the two pyranose and the two furanose forms in aqueous solutions of aldoses. The present paper describes the results of similar determinations on a number of deoxyaldoses; the purpose of the research was to determine the effect of the removal of an hydroxyl group on the tautomeric equilibria. The results show that the effect may be very small or rather large, depending on the position from which the hydroxyl group was removed, and on the configuration of the molecule. Accordingly, three different types of deoxyaldoses will be discussed: the 6-deoxy-, the 2-deoxy-, and the 3-deoxy-aldohexoses. The techniques and methods used in this investigation were those described in the preceding paper.1 6-Deoxy-D-glucose, 6-deoxy-L-mannose (L-rhamnose), and 6-deoxy-L-galactose (L-fucose) were investigated. In the anomeric region the p.m.r. spectrum of each of these sugars was closely similar to that of the parent sugar1 in respect of chemical shifts, spacing of peaks, and proportion of components in equilibrium (see Tables 1 and 2). This is to be expected since the presence of a hydroxyl group on a carbon atom which is not in the ring would have little effect on the stability and on the conformation of the ring. No furanose signals were detected in the spectra of 6-deoxy-D-glucose and -L-mannose; in the spectrum of 6-deoxy-L-galactose the furanose forms are visible, but, as in the spectrum of D-galactose,l the signals overlap with * P a r t 11, Aust. J. Chem., 1972, 25, 1695.
Nuclear magnetic resonance measurements for the system chloroform+ pyridine, in the temperature range 21-7-6O.O0C, have been used to estimate the standard enthalpy of association as A H = -101 kJ mole-1. Calorimetric measurements of the heat of mixing have been made at 25°C for binary mixtures of chloroform with pyridine and with cyclohexane. Attempts have been made to relate the spectroscopic and calorimetric results.As part of a study of the thermodynamic properties of solutions in which hydrogen bonds are present, the association of pyridine with a series of alcohols 1 has been investigated. In such a solution, however, there are two competing equilibria : the association of an alcohol molecule with a pyridine molecule to form a complex, and the self-association of the alcohol molecules to form dimers or higher polymers. If one is interested in determining the equilibrium constant for the former reaction, then one has to make allowances for the self-association. In an attempt to find two components which could form a complex, but which would self-associate to a negligible extent, it was decided to study chloroform with pyridine. Although the C-H bond will not normally take part in hydrogen bonding, it will do so in a molecule like chloroform. Chloroform molecules do self-associate,2* 3 but the equilibrium constant for this is much less than that for an alcohol.This paper describes two approaches to the problem of determining the hydrogen bond energy between chloroform and pyridine. The first is spectroscopic and the second calorimetric. Since this work was commenced, Berkeley and Hanna have published4 their n.m.r. results for the same system, but over a lower and more restricted temperature range. EXPERIMENTAL MATERIALSBenzene, carbon tetrachloride, chloroform and pyridine were purified by standard procedures, in which the drying agents were phosphorus pentoxide for the first three and sodium hydroxide 7 for the last.
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