Carboxylic acid anhydrides infrared absorption

Aldehydes Amides Carboxylic acid anhydrides Carboxylic acids Esters Ketones and functional groups, 56 infrared absorption frequencies, 519,... 

In infrared spectra, the carbonyl group is usually indicated by a single strong and sharp absorption. However, in the case of carboxylic acid anhydrides, R—C—O—C—R, two peaks are observed even though the two carbonyl groups are chemically equivalent. 

Infrared IR spectroscopy is quite useful in identifying carboxylic acid derivatives The, carbonyl stretching vibration is very strong and its position is sensitive to the nature of IKT the carbonyl group In general electron donation from the substituent decreases the double bond character of the bond between carbon and oxygen and decreases the stretch mg frequency Two distinct absorptions are observed for the symmetric and antisym metric stretching vibrations of the anhydride function... 

A suitable method for determining the anhydride group is titration with aqueous potassium hydroxide in pyridine after previous esterification of the carboxyl group with diazomethane. This esterification is carried out in diethyl ether methanol (9 + 1). After methylation, which takes about 10 minutes for 0.5 g of sample, the solvents are removed by evaporation and a portion of the derivatised polymer is dissolved in pyridine and titrated. In the infrared spectra of the resin before and after methylation it can be seen that the absorption band of the acid group at 5.84 i,m disappears and a carbonyl band of the ester at 5.74 p,m is formed. The acid content of the sample is found from the difference in titres of an immethylated and a methylated product. 

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