Infrared Spectra of Carbohydrates LESTER P. KUEIN Ballistic Research Laboratories, tberdeen P r o r i n g Ground, .Ild.
The infrared absorption curtes for a iiumber of sugars and their deritatijes and for some cellulose derib ati*es ha+e been obtained. .inomeric fornis o f \ arious gl) cosides and sugar esters arc readilj distinguished h! their infrared cur\ es. as are the various isomeric sugars and sugar alcohols. Infrared .pectroscop) is an excellent tool for the identification o f sugars and for the icleritification of important functioiial g r o ~ in ~ pcarl,oh! tlrnte materials incliitling rellulose. The poqsihilit? of using this tool for the cIitJtititati\e dPtermiti.itior1 o f nitrate groups in nitr*orellulow11'1- 1)ec.n -hnlt 1 1 .
I
S THOSE fields of clieinisrry
i l l \vliic.li one deals with a tluniber of cnnipounds of similar c~lieniicaland physical pmpertics, infrared spectroscopy is a very useful tool for analysis ant! identification. Examples may be founcl i n many papers in the recent Literature, particularly in the fields of synthetic rubber and petroleum chemistry. The carbohydrate chemist has the difficult job of identifying the constituents of mixtures of chemically similw sugars or of determining certain functional groups present i n small numbers in polymeric materials such as starch or cellulose. 'L'INw considerations motivated the study of the infrared spectra of carbohydrates and appraisal of their usefulness as a tool for the m r bohydrate chemist. The spectra of a large number of carbohydrates and their dei,i\xtives have been obtained with a Baird infrared spectrornct,er In most cases the substances were purified by recrystallization until the melting point was constant. Substances which did not crystallize were measured by squeezing the viscous liquid bct~vecw two rock salt or silver chloride plates. The technique prepare the samples of the crystalline substances was t o them in a solvent such as water or methanol, put about 2 mi. of the solution on a silver chloride window, and evaporate to dryness in a vacuum oven. Samples prepared in this manner frequently give curves containing very broad and poorly resolved bands. hluch sharper and better resolved bands were obtained when the solid sample was finely ground in mineral oil by rubbing n.ith :i muller on a ground-glass plate.
A comparison of the curves obtained by these t K o nicthodd of preparing the sample is shonn in curve 1 (Figure 1). The mincxral oil has strong carbon-hydrogen bands a t about 3.5 and 7 microns, but in the regions more interesting to the carbohydrate chemist, around 6 and beyond 2.2 microns, it is transparent. Each curve i p marked with a symbol which indicates how the sample was p r ~ pared; G indicates that i t was ground in mineral oil and E intlicates that it was evaporated on the window from solution. The letter in brackets indicates the solvent used: W, water; C, chloroform; 8 , methanol; and P,pyridine. Each curve is iiunil)c.wtl and the index for the curves is given in Table I. Curve 2 of glucose is typical of the sugar spectrum. T h e is a very strong band a t about 3.1 microns due to the oxygen-hydrogen stretching frequency of the hydrogen-bonded hydroxyl groups, and in the region of 9 to 10 microns there are several closely spaced bands 15-hich are probably due to carbon-carbon and carbon-oxygen vibrations. The effect upon the spectrum oE various functional groups which the carbohydrate chemist f i t quently encounters is illustrated in the nest four curves. Glucuronic acid, curve 3, shows a very strong band a t 5.6 to 5.8 microns, which is due to the C-0 of the carboxyl group. Closer esamination shows that this is actually two bands and it may be that the 5.6 band is due to a lactone carbonyl and that the 5.8 band is due to the carbonyl in a free carboxyl group. Glucal triacetate, curve 4, has no unesterified hydroxyls; hence the osygenhydrogen band a t 3.1 microns is missing. The carbonyl band due to the acetate groups again shows up strongly and a new band a t 6.05 microns appears, which is due to the ethylene linkage. The strong band a t 8 to 8.2 microns is characteristic of esters.
276
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