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Saturday 20 June 2015

Glyceraldehyde

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DL-glyceraldehyde and D-glyceraldehyde were studied using Infrared spectroscopy together with ¹³C and ¹H nuclear magnetic resonance. In the case of commercial DL-glyceraldehyde 1% or less of the free carbonyl band according to the intensity in the solid state was found. This is in agreement with the information from both ¹³C and ¹H nuclear magnetic resonance. In contrast D-glyceraldehyde, shows 10 ± 0.2% of the free carbonyl as determined from the intensity of the carbonyl band at 1731 cm^–1 we also observed an enol band at 1643 cm^–1.⁵This information is in agreement with the information from ¹³C and ¹H nuclear magnetic resonance (in ¹H resonance there is a doublet centered at 9.61 ppm for an aldehyde proton and in ¹³C resonance a signal appear at 204.6 ppm for the carbonyl function).

Determination of the hemiacetal dimeric structure of DL- and D-glyceraldehyde
First the solid DL-glyceraldehyde was studied. The ¹³C NMR of this compound is rather simple, it shows only three main signals at 92.4 ppm for 2-C(2,5) (Figure 1). A signal at 78.9 ppm for carbon C(3,6) in a dioxane ring and finally a signal at 60.5 ppm for a hydroxymethylene moiety. The system must have high symmetry since it shows only three signals for six carbon atoms. The ¹H NMR of this compound in DMSO-d₆ is also relatively simple. It shows only one doublet (6.72 and 6.70 ppm) for two protons at the oxygen atom of two hydroxyl groups in two dioxane carbons (C1 in glyceraldehyde), which in turn have one proton each at 4.5 ppm showing also couplings with two symmetrically placed protons at about 3.2 ppm on the dioxane ring (C2 in glyceraldehyde). The OH proton in the hydroxymethylene groups gives a triplet and is coupled to a gem AB system of the hydrogens at the hydroxymethylenes which show a multiplet between 3.6 and 3.3, also coupled with the protons at 3.2 ppm. Both NMR spectra (¹³C and ¹H) show slight modification which include the appearance of additional peaks, indicating stability between 20 and 80 ºC.

The data above correspond to a highly symmetrical substituted 1,4-dioxane dimeric hemiacetal. The structure includes very strong intramolecular hydrogen bonds since the ¹H NMR spectra show only slight modifications between 20 and 80 ºC. This structure has specific stereochemistry and during its formation two new asymmetric centers are induced in a natural regio-specific synthesis. Commercial DL-glyceraldehyde is thus not an epimeric mixture, but produces an epimeric hemicetal by spontaneous dimerization.
In the case of the commercial D-glyceraldehyde, the ¹³C shows a large number of peaks instead of the simple patterns for the DL-glyceraldehyde. One of the peaks is at 204.6 ppm and is assigned to the carbon atom in a carbonyl function of glyceraldehydes. Other 16 peaks are seen even after eliminating small peaks, which may be attributed to free glyceraldehydes hydrates. The large number of peaks may be attributed to different combinations of dimeric stereoisomers and other different conformers and hydrates (see below). The region between 104.5 and 90.2 ppm may be assigned to the presence of carbon atoms of hemicetal type and shows 18 signals. This may be assigned to the three main stereoisomers in different conformations. We also have to remember that one of the differences between DL- and D-glyceraldehyde is the level of hydration which in the last case amounts to about 0.17 mol of water per mol of the aldehyde and in the first case is almost absent. Between 83.5 and 77.5 ppm there are six main signals which correspond to carbon at position 3 and 6 of the dioxane ring (C2 of the glyceraldehyde) this agree with three main stereoisomers, this particular atom does not have hydroxyl or hydroxymethylene groups and will not change very much because of differences in hydrogen bonding. Finally, between 73.9 and 59.4 ppm we find 24 peaks which correspond to the hydroxymethylene carbon. This carbon shows the highest possibility of forming different conformations and combinations of hydrogen bonding including those with a molecule of water. The ¹H NMR is also very complex showing a signals for an aldehyde group at 9.620 and 9.617 ppm its integration is barely noticeable. Between 6.55 and 6.34 ppm there is a series of doublets corresponding to the OH group at C-2 and C-4 of a dioxane ring; six of them are most noticeable, further doublets are seen between 6.06 and 5.79 and between 5.61 and 5.41 ppm. Between 3.98 and 3.13 ppm there is a complex multiplet signal. The signal appear in the same region as for DL-glyceraldehyde but the pattern is much more complicated because of the presence of many dimeric hemicetal isomers and conformers as well as by the presence of different hydrates.

D,L-Glyceraldehyde (C3 H6 O3)