Methyl Acetate Infrared Spectrum

Repeated methylation of bagasse native lignin with dimethyl sulfate yielded a product with a 29-1 % methoxyl content. Four such methyl-ations did not change this value. However, the infrared spectrum of this derivative still revealed a small absorption band at 3400 cm1. Thus, it appeared that at least one hydroxyl group could not be methylated. Treatment of this methylated product with acetic anhydride and pyridine gave an acetate, which did not exhibit an absorption band at 3400 cm-1. Consequently, a tertiary hydroxyl group, which can be acetylated but not methylated, seems to be present in this lignin. 

The acetolysis of S-acetyl-l,2-0-isopropylidene-3,5-di-0-methyl-6-thio-a-D-glucofiiranose with acetic acid—acetic anhydride—sulfuric acid gave a small yield of the crystalline septanose triacetate (271). The septanose structure of 271 was established by the absence of the thiol band in its infrared spectrum, and by the signals for three O-acetyl and two O-methyl groups found in its nuclear magnetic resonance spectrum. 

Esters are carboxylic acid derivatives, and the spectrum of ethyl acetate is shown in Figure 14.19D. The carbonyl absorption does not distinguish this compound from an aldehyde or a ketone, but there is the C-O absorption at about 1200 cm 1. Because this is in the fingerprint region, however, its position can be difficult to identify. This is clearly the case for methyl pentanoate, where the C-0 absorption can easily be missed or misidentified. Based only on the infrared, it may be difficult to distinguish an ester from an aliphatic aldehyde or ketone. If the formula is known, however (from mass spectrometry), the identification is easier because an ester has two oxygen atoms, whereas the aldehyde or ketone has only one. 

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